tty contention resulting from tty_open_by_device export

tty contention resulting from tty_open_by_device export

[Okash Khawaja]

Hi,

The commit 12e84c71b7d4ee (tty: export tty_open_by_driver) exports
tty_open_by_device to allow tty to be opened from inside kernel which
works fine except that it doesn't handle contention with user space or
another kernel-space open of the same tty. For example, opening a tty
from user space while it is kernel opened results in failure and a
kernel log message about mismatch between tty->count and tty's file open
count.

I suggest we make kernel access to tty exclusive so that if a user
process or kernel opens a kernel opened tty, it gets -EBUSY. Below is
a patch which does this by adding TTY_KOPENED flag to tty->flags. When
this flag is set, tty_open_by_driver returns -EBUSY. Instead of
overlaoding tty_open_by_driver for both kernel and user space, this
patch creates a separate function tty_kopen which closely follows
tty_open_by_driver.

I am far from an expert on tty and this patch might contain the wrong
approach. But it should convey what I mean.

Thanks,
Okash

---
 drivers/staging/speakup/spk_ttyio.c |    2 -
 drivers/tty/tty_io.c                |   56 ++++++++++++++++++++++++++++++++++--
 include/linux/tty.h                 |    7 +---
 3 files changed, 58 insertions(+), 7 deletions(-)

--- a/drivers/staging/speakup/spk_ttyio.c
+++ b/drivers/staging/speakup/spk_ttyio.c
@@ -164,7 +164,7 @@ static int spk_ttyio_initialise_ldisc(st
 	if (ret)
 		return ret;
 
-	tty = tty_open_by_driver(dev, NULL, NULL);
+	tty = tty_kopen(dev);
 	if (IS_ERR(tty))
 		return PTR_ERR(tty);
 
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -1786,6 +1786,53 @@ static struct tty_driver *tty_lookup_dri
 }
 
 /**
+ *	tty_kopen	-	open a tty device for kernel
+ *	@device: dev_t of device to open
+ *
+ *	Opens tty exclusively for kernel. Performs the driver lookup,
+ *	makes sure it's not already opened and performs the first-time
+ *	tty initialization.
+ *
+ *	Returns the locked initialized &tty_struct
+ *
+ *	Claims the global tty_mutex to serialize:
+ *	  - concurrent first-time tty initialization
+ *	  - concurrent tty driver removal w/ lookup
+ *	  - concurrent tty removal from driver table
+ */
+struct tty_struct *tty_kopen(dev_t device)
+{
+        struct tty_struct *tty;
+        struct tty_driver *driver = NULL;
+        int index = -1;
+
+        mutex_lock(&tty_mutex);
+        driver = tty_lookup_driver(device, NULL, &index);
+        if (IS_ERR(driver)) {
+                mutex_unlock(&tty_mutex);
+                return ERR_CAST(driver);
+        }
+
+        /* check whether we're reopening an existing tty */
+        tty = tty_driver_lookup_tty(driver, NULL, index);
+        if (IS_ERR(tty))
+                goto out;
+
+        if (tty) {
+                tty_kref_put(tty);  /* drop kref from tty_driver_lookup_tty() */
+                tty = ERR_PTR(-EBUSY);
+        } else { /* Returns with the tty_lock held for now */
+                tty = tty_init_dev(driver, index);
+                set_bit(TTY_KOPENED, &tty->flags);
+        }
+out:
+        mutex_unlock(&tty_mutex);
+        tty_driver_kref_put(driver);
+        return tty;
+}
+EXPORT_SYMBOL(tty_kopen);
+
+/**
  *	tty_open_by_driver	-	open a tty device
  *	@device: dev_t of device to open
  *	@inode: inode of device file
@@ -1801,7 +1848,7 @@ static struct tty_driver *tty_lookup_dri
  *	  - concurrent tty driver removal w/ lookup
  *	  - concurrent tty removal from driver table
  */
-struct tty_struct *tty_open_by_driver(dev_t device, struct inode *inode,
+static struct tty_struct *tty_open_by_driver(dev_t device, struct inode *inode,
 					     struct file *filp)
 {
 	struct tty_struct *tty;
@@ -1824,6 +1871,12 @@ struct tty_struct *tty_open_by_driver(de
 	}
 
 	if (tty) {
+		if (test_bit(TTY_KOPENED, &tty->flags)) {
+			tty_kref_put(tty);
+			mutex_unlock(&tty_mutex);
+			tty = ERR_PTR(-EBUSY);
+			goto out;
+		}
 		mutex_unlock(&tty_mutex);
 		retval = tty_lock_interruptible(tty);
 		tty_kref_put(tty);  /* drop kref from tty_driver_lookup_tty() */
@@ -1846,7 +1899,6 @@ out:
 	tty_driver_kref_put(driver);
 	return tty;
 }
-EXPORT_SYMBOL_GPL(tty_open_by_driver);
 
 /**
  *	tty_open		-	open a tty device
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -362,6 +362,7 @@ struct tty_file_private {
 #define TTY_NO_WRITE_SPLIT 	17	/* Preserve write boundaries to driver */
 #define TTY_HUPPED 		18	/* Post driver->hangup() */
 #define TTY_LDISC_HALTED	22	/* Line discipline is halted */
+#define TTY_KOPENED             23      /* TTY exclusively opened by kernel */
 
 /* Values for tty->flow_change */
 #define TTY_THROTTLE_SAFE 1
@@ -399,8 +400,7 @@ extern struct tty_struct *get_current_tt
 /* tty_io.c */
 extern int __init tty_init(void);
 extern const char *tty_name(const struct tty_struct *tty);
-extern struct tty_struct *tty_open_by_driver(dev_t device, struct inode *inode,
-		struct file *filp);
+extern struct tty_struct *tty_kopen(dev_t device);
 extern int tty_dev_name_to_number(const char *name, dev_t *number);
 #else
 static inline void tty_kref_put(struct tty_struct *tty)
@@ -422,8 +422,7 @@ static inline int __init tty_init(void)
 { return 0; }
 static inline const char *tty_name(const struct tty_struct *tty)
 { return "(none)"; }
-static inline struct tty_struct *tty_open_by_driver(dev_t device,
-		struct inode *inode, struct file *filp)
+static inline struct tty_struct *tty_kopen(dev_t device)
 { return NULL; }
 static inline int tty_dev_name_to_number(const char *name, dev_t *number)
 { return -ENOTSUPP; }

Re: tty contention resulting from tty_open_by_device export

[Okash Khawaja]

On Sat, Jul 08, 2017 at 10:38:03AM +0200, Greg Kroah-Hartman wrote:

> > +
> > +        if (tty) {
> > +                tty_kref_put(tty);  /* drop kref from tty_driver_lookup_tty() */
> 
> Put the comment above the line?
> 
Sure

> > +                tty = ERR_PTR(-EBUSY);
> > +        } else { /* Returns with the tty_lock held for now */
> 
> I don't understand this comment.
> 
This is same comment in tty_open_by_driver. Basically, tty_init_dev
returns tty locked so that it doesn't dissapear from underneath the
caller. I am not sure about the "for now" part of the comment. I'll
remove it.

> > +                tty = tty_init_dev(driver, index);
> > +                set_bit(TTY_KOPENED, &tty->flags);
> > +        }
> > +out:
> > +        mutex_unlock(&tty_mutex);
> > +        tty_driver_kref_put(driver);
> > +        return tty;
> > +}
> > +EXPORT_SYMBOL(tty_kopen);
> 
> EXPORT_SYMBOL_GPL()?  :)
> 
Of course, will update
> >  /**
> >   *	tty_open		-	open a tty device
> > --- a/include/linux/tty.h
> > +++ b/include/linux/tty.h
> > @@ -362,6 +362,7 @@ struct tty_file_private {
> >  #define TTY_NO_WRITE_SPLIT 	17	/* Preserve write boundaries to driver */
> >  #define TTY_HUPPED 		18	/* Post driver->hangup() */
> >  #define TTY_LDISC_HALTED	22	/* Line discipline is halted */
> > +#define TTY_KOPENED             23      /* TTY exclusively opened by kernel */
> 
> Minor nit, please use tabs here.
> 
Will do

> Overall, the idea looks sane to me.  Keeping userspace from opening a
> tty that the kernel has opened internally makes sense, hopefully
> userspace doesn't get too confused when that happens.  I don't think we
> normally return -EBUSY from an open call, have you seen what happens
> with apps when you do this (like minicom?)
> 
Yes, returning -EBUSY is a change in the interface. I will test against
applications like minicom before resubmitting this.

Thanks,
Okash

[patch 0/3] Re: tty contention resulting from tty_open_by_device export

[Okash Khawaja]

On Sat, Jul 08, 2017 at 10:38:03AM +0200, Greg Kroah-Hartman wrote:
> Overall, the idea looks sane to me.  Keeping userspace from opening a
> tty that the kernel has opened internally makes sense, hopefully
> userspace doesn't get too confused when that happens.  I don't think we
> normally return -EBUSY from an open call, have you seen what happens
> with apps when you do this (like minicom?)
>
I tested this wil minincom, picocom and commands like "echo foo >
/dev/ttyS0". They all correctly report "Device or resource busy".

I have addressed all the comments you made. I have also split the patch
into three. Following is summary of each.

Patch 1: introduces the tty_kopen function and checks for TTY_KOPENED
Patch 2: updates speakup code to use tty_kopen instead of
        tty_open_by_driver
Patch 3: reverses the export of tty_open_by_driver

Thanks,
Okash

[patch 1/3] tty: resolve tty contention between kernel and user space

[Okash Khawaja]

[-- Attachment #1: 17_add_tty_kopen --]
[-- Type: text/plain, Size: 4378 bytes --]

The commit 12e84c71b7d4ee (tty: export tty_open_by_driver) exports
tty_open_by_device to allow tty to be opened from inside kernel which
works fine except that it doesn't handle contention with user space or
another kernel-space open of the same tty. For example, opening a tty
from user space while it is kernel opened results in failure and a
kernel log message about mismatch between tty->count and tty's file
open count.

This patch makes kernel access to tty exclusive, so that if a user
process or kernel opens a kernel opened tty, it gets -EBUSY. It does
this by adding TTY_KOPENED flag to tty->flags. When this flag is set,
tty_open_by_driver returns -EBUSY. Instead of overlaoding
tty_open_by_driver for both kernel and user space, this
patch creates a separate function tty_kopen which closely follows
tty_open_by_driver.

Returning -EBUSY on tty open is a change in the interface. I have
tested this with minicom, picocom and commands like "echo foo >
/dev/ttyS0". They all correctly report "Device or resource busy" when
the tty is already kernel opened.

Signed-off-by: Okash Khawaja <okash.khawaja@gmail.com>

---
 drivers/tty/tty_io.c |   54 +++++++++++++++++++++++++++++++++++++++++++++++++++
 include/linux/tty.h  |    4 +++
 2 files changed, 58 insertions(+)

--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -1786,6 +1786,54 @@ static struct tty_driver *tty_lookup_dri
 }
 
 /**
+ *	tty_kopen	-	open a tty device for kernel
+ *	@device: dev_t of device to open
+ *
+ *	Opens tty exclusively for kernel. Performs the driver lookup,
+ *	makes sure it's not already opened and performs the first-time
+ *	tty initialization.
+ *
+ *	Returns the locked initialized &tty_struct
+ *
+ *	Claims the global tty_mutex to serialize:
+ *	  - concurrent first-time tty initialization
+ *	  - concurrent tty driver removal w/ lookup
+ *	  - concurrent tty removal from driver table
+ */
+struct tty_struct *tty_kopen(dev_t device)
+{
+	struct tty_struct *tty;
+	struct tty_driver *driver = NULL;
+	int index = -1;
+
+	mutex_lock(&tty_mutex);
+	driver = tty_lookup_driver(device, NULL, &index);
+	if (IS_ERR(driver)) {
+		mutex_unlock(&tty_mutex);
+		return ERR_CAST(driver);
+	}
+
+	/* check whether we're reopening an existing tty */
+	tty = tty_driver_lookup_tty(driver, NULL, index);
+	if (IS_ERR(tty))
+		goto out;
+
+	if (tty) {
+		/* drop kref from tty_driver_lookup_tty() */
+		tty_kref_put(tty);
+		tty = ERR_PTR(-EBUSY);
+	} else { /* tty_init_dev returns tty with the tty_lock held */
+		tty = tty_init_dev(driver, index);
+		set_bit(TTY_KOPENED, &tty->flags);
+	}
+out:
+	mutex_unlock(&tty_mutex);
+	tty_driver_kref_put(driver);
+	return tty;
+}
+EXPORT_SYMBOL_GPL(tty_kopen);
+
+/**
  *	tty_open_by_driver	-	open a tty device
  *	@device: dev_t of device to open
  *	@inode: inode of device file
@@ -1824,6 +1872,12 @@ struct tty_struct *tty_open_by_driver(de
 	}
 
 	if (tty) {
+		if (test_bit(TTY_KOPENED, &tty->flags)) {
+			tty_kref_put(tty);
+			mutex_unlock(&tty_mutex);
+			tty = ERR_PTR(-EBUSY);
+			goto out;
+		}
 		mutex_unlock(&tty_mutex);
 		retval = tty_lock_interruptible(tty);
 		tty_kref_put(tty);  /* drop kref from tty_driver_lookup_tty() */
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -362,6 +362,7 @@ struct tty_file_private {
 #define TTY_NO_WRITE_SPLIT 	17	/* Preserve write boundaries to driver */
 #define TTY_HUPPED 		18	/* Post driver->hangup() */
 #define TTY_LDISC_HALTED	22	/* Line discipline is halted */
+#define TTY_KOPENED		23	/* TTY exclusively opened by kernel */
 
 /* Values for tty->flow_change */
 #define TTY_THROTTLE_SAFE 1
@@ -401,6 +402,7 @@ extern int __init tty_init(void);
 extern const char *tty_name(const struct tty_struct *tty);
 extern struct tty_struct *tty_open_by_driver(dev_t device, struct inode *inode,
 		struct file *filp);
+extern struct tty_struct *tty_kopen(dev_t device);
 extern int tty_dev_name_to_number(const char *name, dev_t *number);
 #else
 static inline void tty_kref_put(struct tty_struct *tty)
@@ -425,6 +427,8 @@ static inline const char *tty_name(const
 static inline struct tty_struct *tty_open_by_driver(dev_t device,
 		struct inode *inode, struct file *filp)
 { return NULL; }
+static inline struct tty_struct *tty_kopen(dev_t device)
+{ return NULL; }
 static inline int tty_dev_name_to_number(const char *name, dev_t *number)
 { return -ENOTSUPP; }
 #endif

Re: [patch 1/3] tty: resolve tty contention between kernel and user space

[Okash Khawaja]

Hi,

On Sun, Jul 09, 2017 at 06:04:17PM +0300, Andy Shevchenko wrote:
 
> Above
> 1. take mutex
> 2. take reference
> 
> Here:
> 1. give mutex back
> 2. give reference back
> 
> I think we usually see symmetrical  calls, i.e.
> 
> 1. give reference back
> 2. give mutex back
> 
> So, what did I miss?

After we hold the kref, driver won't disappear from underneath us so we
don't need tty_mutex just for decrementing the refcount.

Thanks,
Okash

Re: [patch 1/3] tty: resolve tty contention between kernel and user space

[Okash Khawaja]

On Sun, Jul 09, 2017 at 06:04:17PM +0300, Andy Shevchenko wrote:
> On Sun, Jul 9, 2017 at 2:41 PM, Okash Khawaja <okash.khawaja@gmail.com> wrote:
> 
> > +struct tty_struct *tty_kopen(dev_t device)
> > +{
> > +       struct tty_struct *tty;
> > +       struct tty_driver *driver = NULL;
> > +       int index = -1;
> > +
> > +       mutex_lock(&tty_mutex);
> > +       driver = tty_lookup_driver(device, NULL, &index);
> > +       if (IS_ERR(driver)) {
> 
> > +               mutex_unlock(&tty_mutex);
> > +               return ERR_CAST(driver);
> 
> Hmm... perhaps
> 
> tty = ERR_CAST(driver);
> goto out_unlock;
> 
> See below for further details.
> 
Sorry missed this one out. Since tty_lookup_driver has failed, we don't
need to down the refcount on driver. So we can return here, without
going to out_unlock.

Thanks,
Okash

Re: [patch 1/3] tty: resolve tty contention between kernel and user space

[Okash Khawaja]

On Mon, Jul 10, 2017 at 06:21:37PM +0300, Andy Shevchenko wrote:
> On Mon, Jul 10, 2017 at 11:31 AM, Okash Khawaja <okash.khawaja@gmail.com> wrote:
> > On Sun, Jul 09, 2017 at 06:04:17PM +0300, Andy Shevchenko wrote:
> >> On Sun, Jul 9, 2017 at 2:41 PM, Okash Khawaja <okash.khawaja@gmail.com> wrote:
> >>
> >> > +struct tty_struct *tty_kopen(dev_t device)
> >> > +{
> >> > +       struct tty_struct *tty;
> >> > +       struct tty_driver *driver = NULL;
> >> > +       int index = -1;
> >> > +
> >> > +       mutex_lock(&tty_mutex);
> >> > +       driver = tty_lookup_driver(device, NULL, &index);
> >> > +       if (IS_ERR(driver)) {
> >>
> >> > +               mutex_unlock(&tty_mutex);
> >> > +               return ERR_CAST(driver);
> >>
> >> Hmm... perhaps
> >>
> >> tty = ERR_CAST(driver);
> >> goto out_unlock;
> >>
> >> See below for further details.
> >>
> > Sorry missed this one out. Since tty_lookup_driver has failed, we don't
> > need to down the refcount on driver. So we can return here, without
> > going to out_unlock.
> 
> Yeah, and my point is to use goto with the symmetric giveups of lock
> and reference.
Ah okay I see your point. Sure, I don't mind either way. However, this
code closely follows tty_open_by_driver, so I have tried to keep the
same pattern for sake of consistency :)

Thanks,
Okash

[patch 2/3] staging: speakup: use tty_kopen instead of tty_open_by_driver

[Okash Khawaja]

[-- Attachment #1: 18_use_tty_kopen --]
[-- Type: text/plain, Size: 509 bytes --]

This patch replaces call to tty_open_by_driver with a tty_kopen.

Signed-off-by: Okash Khawaja <okash.khawaja@gmail.com>

---
 drivers/staging/speakup/spk_ttyio.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/drivers/staging/speakup/spk_ttyio.c
+++ b/drivers/staging/speakup/spk_ttyio.c
@@ -164,7 +164,7 @@ static int spk_ttyio_initialise_ldisc(st
 	if (ret)
 		return ret;
 
-	tty = tty_open_by_driver(dev, NULL, NULL);
+	tty = tty_kopen(dev);
 	if (IS_ERR(tty))
 		return PTR_ERR(tty);
 

Re: [patch 2/3] staging: speakup: use tty_kopen instead of tty_open_by_driver

[Okash Khawaja]

On Sun, Jul 09, 2017 at 01:50:55PM +0200, Greg Kroah-Hartman wrote:
> On Sun, Jul 09, 2017 at 12:41:55PM +0100, Okash Khawaja wrote:
> > This patch replaces call to tty_open_by_driver with a tty_kopen.
> > 
> > Signed-off-by: Okash Khawaja <okash.khawaja@gmail.com>
> > 
> > ---
> >  drivers/staging/speakup/spk_ttyio.c |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > --- a/drivers/staging/speakup/spk_ttyio.c
> > +++ b/drivers/staging/speakup/spk_ttyio.c
> > @@ -164,7 +164,7 @@ static int spk_ttyio_initialise_ldisc(st
> >  	if (ret)
> >  		return ret;
> >  
> > -	tty = tty_open_by_driver(dev, NULL, NULL);
> > +	tty = tty_kopen(dev);
> >  	if (IS_ERR(tty))
> 
> Hm, the "no tty layer" return value for this will be NULL.  I doubt
> that's really a big deal, but perhaps just have that return
> PTR_ERR(-ENODEV) or something?
Good point, thanks. Will send a follow up patch

Okash

[patch 3/3] tty: undo export of tty_open_by_driver

[Okash Khawaja]

[-- Attachment #1: 19_undo_export_of_tty_open_by_driver --]
[-- Type: text/plain, Size: 1768 bytes --]

Since we have tty_kopen, we no longer need to export tty_open_by_driver.
This patch makes this function static.

Signed-off-by: Okash Khawaja <okash.khawaja@gmail.com>

---
 drivers/tty/tty_io.c |    3 +--
 include/linux/tty.h  |    5 -----
 2 files changed, 1 insertion(+), 7 deletions(-)

--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -1849,7 +1849,7 @@ EXPORT_SYMBOL_GPL(tty_kopen);
  *	  - concurrent tty driver removal w/ lookup
  *	  - concurrent tty removal from driver table
  */
-struct tty_struct *tty_open_by_driver(dev_t device, struct inode *inode,
+static struct tty_struct *tty_open_by_driver(dev_t device, struct inode *inode,
 					     struct file *filp)
 {
 	struct tty_struct *tty;
@@ -1900,7 +1900,6 @@ out:
 	tty_driver_kref_put(driver);
 	return tty;
 }
-EXPORT_SYMBOL_GPL(tty_open_by_driver);
 
 /**
  *	tty_open		-	open a tty device
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -400,8 +400,6 @@ extern struct tty_struct *get_current_tt
 /* tty_io.c */
 extern int __init tty_init(void);
 extern const char *tty_name(const struct tty_struct *tty);
-extern struct tty_struct *tty_open_by_driver(dev_t device, struct inode *inode,
-		struct file *filp);
 extern struct tty_struct *tty_kopen(dev_t device);
 extern int tty_dev_name_to_number(const char *name, dev_t *number);
 #else
@@ -424,9 +422,6 @@ static inline int __init tty_init(void)
 { return 0; }
 static inline const char *tty_name(const struct tty_struct *tty)
 { return "(none)"; }
-static inline struct tty_struct *tty_open_by_driver(dev_t device,
-		struct inode *inode, struct file *filp)
-{ return NULL; }
 static inline struct tty_struct *tty_kopen(dev_t device)
 { return NULL; }
 static inline int tty_dev_name_to_number(const char *name, dev_t *number)

[patch 4/3] tty: make tty_kopen return ENODEV in case of no TTY

[Okash Khawaja]

When TTY is not built, tty_kopen should return an error. This way
calling code remains consistent, regardless of whether tty is built or
not. This patch returns -ENODEV when there is no tty.

Signed-off-by: Okash Khawaja <okash.khawaja@gmail.com>

---
 include/linux/tty.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -423,7 +423,7 @@ static inline int __init tty_init(void)
 static inline const char *tty_name(const struct tty_struct *tty)
 { return "(none)"; }
 static inline struct tty_struct *tty_kopen(dev_t device)
-{ return NULL; }
+{ return ERR_PTR(-ENODEV); }
 static inline int tty_dev_name_to_number(const char *name, dev_t *number)
 { return -ENOTSUPP; }
 #endif

Re: [patch 0/3] Re: tty contention resulting from tty_open_by_device export

[Okash Khawaja]

On Mon, Jul 10, 2017 at 12:52:33PM +0100, Alan Cox wrote:
> On Sun, 09 Jul 2017 12:41:53 +0100
> Okash Khawaja <okash.khawaja@gmail.com> wrote:
> 
> > On Sat, Jul 08, 2017 at 10:38:03AM +0200, Greg Kroah-Hartman wrote:
> > > Overall, the idea looks sane to me.  Keeping userspace from opening a
> > > tty that the kernel has opened internally makes sense, hopefully
> > > userspace doesn't get too confused when that happens.  I don't think we
> > > normally return -EBUSY from an open call, have you seen what happens
> > > with apps when you do this (like minicom?)
> > >  
> > I tested this wil minincom, picocom and commands like "echo foo >
> > /dev/ttyS0". They all correctly report "Device or resource busy".
> > 
> > I have addressed all the comments you made. I have also split the patch
> > into three. Following is summary of each.
> 
> If the tty counts are being misreported then it would be better to fix
> the code to actually manage the counts properly. The core tty code is
> telling you that the tty is not in a valid state. While this is of
> itself a good API to have, the underlying reference miscounting ought
> IMHO to be fixed as well.
When opening from kernel, we don't use file pointer. The count mismatch
is between tty->count and #fd's. So opening from kernel leads to #fd's
being less than tty->count. I thought this difference is relevant to
user-space opening of tty, and not to kernel opening of tty. Can you
suggest how to address this mismatch?

> 
> Also you don't need a new TTY_KOPENED flag as far as I can see. All tty's
> have a usage count and active bit flags already. Use those.
Ah may be I didn't notice the active bit. Is it one of the #defines in
tty.h? Can usage count and active bit be used to differentiate between
whether the tty was opened by kernel or user?

Thanks,
Okash

Re: [patch 0/3] Re: tty contention resulting from tty_open_by_device export

[Okash Khawaja]

On Mon, Jul 10, 2017 at 01:33:07PM +0100, Okash Khawaja wrote:
> > If the tty counts are being misreported then it would be better to fix
> > the code to actually manage the counts properly. The core tty code is
> > telling you that the tty is not in a valid state. While this is of
> > itself a good API to have, the underlying reference miscounting ought
> > IMHO to be fixed as well.
> When opening from kernel, we don't use file pointer. The count mismatch
> is between tty->count and #fd's. So opening from kernel leads to #fd's
> being less than tty->count. I thought this difference is relevant to
> user-space opening of tty, and not to kernel opening of tty. Can you
> suggest how to address this mismatch?
Idea is tty_kopen only ever returns a newly initialised tty - returned
by tty_init_dev. Since the access is exclusive, tty->count shouldn't
matter. When the caller is done with it, it calls tty_release_struct
which frees up the tty itself.

But yes, all the while a tty is kopened, its tty->count and #fds will be
unequal.

Thanks,
Okash

Re: [patch 0/3] Re: tty contention resulting from tty_open_by_device export

[Okash Khawaja]

On Wed, Jul 12, 2017 at 07:20:28PM +0100, Alan Cox wrote:
> 
> > When opening from kernel, we don't use file pointer. The count mismatch
> > is between tty->count and #fd's. So opening from kernel leads to #fd's
> > being less than tty->count. I thought this difference is relevant to
> > user-space opening of tty, and not to kernel opening of tty. Can you
> > suggest how to address this mismatch?
> 
> Your kernel reference is the same as having a file open reference so I
> think this actually needs addressing in the maths. In other words count
> the number of kernel references and also add that into the test for
> check_tty_count (kernel references + #fds == count).
> 
> I'd really like to keep this right because that check has a long history
> of catching really nasty race conditions in the tty code. The
> open/close/hangup code is really fragile so worth the debugability.

I see. Okay based this, check_tty_count can be easily updated to take
into account kernel references.

> 
> > Ah may be I didn't notice the active bit. Is it one of the #defines in
> > tty.h? Can usage count and active bit be used to differentiate between
> > whether the tty was opened by kernel or user?
> 
> It only tells you whether the port is currently active for some purpose,
> not which. If you still want to implement exclusivity between kernel and
> user then it needs another flag, but I think that flag should be in
> port->flags as it is a property of the physical interface.
> 
> (Take a look at tty_port_open for example)
Okay I can add TTY_PORT_KOPENED to port->flags and that should work too.

However, can you please help me understand this:
Our use case requires kernel access to tty_struct and accordingly
tty_kopen returns tty_struct. The exclusivity between user and kernel
space is also meant to prevent one side from opening tty_struct while
another has it opened. In all this, it is tty_struct and not tty_port
which is the key resource we are concerned with. So shouldn't the
exclusivity flag belong to tty_struct?

Adding a the flag to port->flags but controlling it from code for
opening and closing tty will also mean we have tty_port_kopened,
tty_port_set_kopen etc inside tty open/close code.

Am I viewing this problem incorrectly?

Thanks,
Okash

Re: [patch 0/3] Re: tty contention resulting from tty_open_by_device export

[Okash Khawaja]

> On 17 Jul 2017, at 13:31, Greg Kroah-Hartman <gregkh@linuxfoundation.org> wrote:
> 
>> On Thu, Jul 13, 2017 at 12:29:54PM +0100, Okash Khawaja wrote:
>>> On Wed, Jul 12, 2017 at 07:20:28PM +0100, Alan Cox wrote:
>>> 
>>>> When opening from kernel, we don't use file pointer. The count mismatch
>>>> is between tty->count and #fd's. So opening from kernel leads to #fd's
>>>> being less than tty->count. I thought this difference is relevant to
>>>> user-space opening of tty, and not to kernel opening of tty. Can you
>>>> suggest how to address this mismatch?
>>> 
>>> Your kernel reference is the same as having a file open reference so I
>>> think this actually needs addressing in the maths. In other words count
>>> the number of kernel references and also add that into the test for
>>> check_tty_count (kernel references + #fds == count).
>>> 
>>> I'd really like to keep this right because that check has a long history
>>> of catching really nasty race conditions in the tty code. The
>>> open/close/hangup code is really fragile so worth the debugability.
>> 
>> I see. Okay based this, check_tty_count can be easily updated to take
>> into account kernel references.
> 
> Ok, I'll drop this series from my "to-apply" queue and wait for you to
> redo it.

Sure. I can fix the tty->count mismatch based on Alan's suggestion. However I don't understand why the exclusivity flag should belong to tty_port and not tty_struct. It will be good to know why. 

Thanks,
Okash

Re: [patch 0/3] Re: tty contention resulting from tty_open_by_device export

[Okash Khawaja]

On Mon, Jul 17, 2017 at 11:04:38PM +0100, Alan Cox wrote:
> 
> > Sure. I can fix the tty->count mismatch based on Alan's suggestion. However I don't understand why the exclusivity flag should belong to tty_port and not tty_struct. It will be good to know why. 
> 
> We are trying to move all the flags that we can and structs into the
> tty_port, except any that are used internally within the struct tty
> level code. The main reason for this is to make the object lifetimes and
> locking simpler - because the tty_port lasts for the time the hardware is
> present.

I see. That does make sense. I have appended a version of the patch which
adds the flag to tty_port and uses that inside tty_kopen.

>From readability point of view however, I think adding the flag to
tty_struct looks more intuitive. At least for now - until we move
other things from tty_struct to tty_port.

The patch is untested but I can work on this if that fits in with the
plans for tty.

Thanks,
Okash


---
 drivers/tty/tty_io.c |   67 +++++++++++++++++++++++++++++++++++++++++++++++----
 include/linux/tty.h  |   17 ++++++++++++
 2 files changed, 79 insertions(+), 5 deletions(-)

--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -280,7 +280,7 @@ static int check_tty_count(struct tty_st
 {
 #ifdef CHECK_TTY_COUNT
        struct list_head *p;
-       int count = 0;
+       int count = 0, kopen_count = 0;

        spin_lock(&tty->files_lock);
        list_for_each(p, &tty->tty_files) {
@@ -291,10 +291,12 @@ static int check_tty_count(struct tty_st
            tty->driver->subtype == PTY_TYPE_SLAVE &&
            tty->link && tty->link->count)
                count++;
-       if (tty->count != count) {
-               tty_warn(tty, "%s: tty->count(%d) != #fd's(%d)\n",
-                        routine, tty->count, count);
-               return count;
+       if (tty_port_kopened(tty->port))
+               kopen_count++;
+       if (tty->count != (count + kopen_count)) {
+               tty_warn(tty, "%s: tty->count(%d) != (#fd's(%d) + #kopen's(%d))\n",
+                        routine, tty->count, count, kopen_count);
+               return (count + kopen_count);
        }
 #endif
        return 0;
@@ -1522,6 +1524,7 @@ static int tty_release_checks(struct tty
  */
 void tty_release_struct(struct tty_struct *tty, int idx)
 {
+       // TODO: reset TTY_PORT_KOPENED on tty->port
        /*
         * Ask the line discipline code to release its structures
         */
@@ -1786,6 +1789,54 @@ static struct tty_driver *tty_lookup_dri
 }

 /**
+ *     tty_kopen       -       open a tty device for kernel
+ *     @device: dev_t of device to open
+ *
+ *     Opens tty exclusively for kernel. Performs the driver lookup,
+ *     makes sure it's not already opened and performs the first-time
+ *     tty initialization.
+ *
+ *     Returns the locked initialized &tty_struct
+ *
+ *     Claims the global tty_mutex to serialize:
+ *       - concurrent first-time tty initialization
+ *       - concurrent tty driver removal w/ lookup
+ *       - concurrent tty removal from driver table
+ */
+struct tty_struct *tty_kopen(dev_t device)
+{
+       struct tty_struct *tty;
+       struct tty_driver *driver = NULL;
+       int index = -1;
+
+       mutex_lock(&tty_mutex);
+       driver = tty_lookup_driver(device, NULL, &index);
+       if (IS_ERR(driver)) {
+               mutex_unlock(&tty_mutex);
+               return ERR_CAST(driver);
+       }
+
+       /* check whether we're reopening an existing tty */
+       tty = tty_driver_lookup_tty(driver, NULL, index);
+       if (IS_ERR(tty))
+               goto out;
+
+       if (tty) {
+               /* drop kref from tty_driver_lookup_tty() */
+               tty_kref_put(tty);
+               tty = ERR_PTR(-EBUSY);
+       } else { /* tty_init_dev returns tty with the tty_lock held */
+               tty = tty_init_dev(driver, index);
+               tty_port_set_kopened(tty->port, 1);
+       }
+out:
+       mutex_unlock(&tty_mutex);
+       tty_driver_kref_put(driver);
+       return tty;
+}
+EXPORT_SYMBOL_GPL(tty_kopen);
+
+/**
  *     tty_open_by_driver      -       open a tty device
  *     @device: dev_t of device to open
  *     @inode: inode of device file
@@ -1824,6 +1875,12 @@ struct tty_struct *tty_open_by_driver(de
        }

        if (tty) {
+               if (tty_port_kopened(tty->port)) {
+                       tty_kref_put(tty);
+                       mutex_unlock(&tty_mutex);
+                       tty = ERR_PTR(-EBUSY);
+                       goto out;
+               }
                mutex_unlock(&tty_mutex);
                retval = tty_lock_interruptible(tty);
                tty_kref_put(tty);  /* drop kref from tty_driver_lookup_tty() */
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -261,6 +261,7 @@ struct tty_port {
  */
 #define TTY_PORT_CTS_FLOW      3       /* h/w flow control enabled */
 #define TTY_PORT_CHECK_CD      4       /* carrier detect enabled */
+#define TTY_PORT_KOPENED       5       /* device exclusively opened by kernel */

 /*
  * Where all of the state associated with a tty is kept while the tty
@@ -401,6 +402,7 @@ extern int __init tty_init(void);
 extern const char *tty_name(const struct tty_struct *tty);
 extern struct tty_struct *tty_open_by_driver(dev_t device, struct inode *inode,
                struct file *filp);
+extern struct tty_struct *tty_kopen(dev_t device);
 extern int tty_dev_name_to_number(const char *name, dev_t *number);
 #else
 static inline void tty_kref_put(struct tty_struct *tty)
@@ -425,6 +427,8 @@ static inline const char *tty_name(const
 static inline struct tty_struct *tty_open_by_driver(dev_t device,
                struct inode *inode, struct file *filp)
 { return NULL; }
+static inline struct tty_struct *tty_kopen(dev_t device)
+{ return NULL; }
 static inline int tty_dev_name_to_number(const char *name, dev_t *number)
 { return -ENOTSUPP; }
 #endif
@@ -652,6 +656,19 @@ static inline void tty_port_set_initiali
                clear_bit(TTY_PORT_INITIALIZED, &port->iflags);
 }

+static inline bool tty_port_kopened(struct tty_port *port)
+{
+       return test_bit(TTY_PORT_KOPENED, &port->iflags);
+}
+
+static inline void tty_port_set_kopened(struct tty_port *port, bool val)
+{
+       if (val)
+               set_bit(TTY_PORT_KOPENED, &port->iflags);
+       else
+               clear_bit(TTY_PORT_KOPENED, &port->iflags);
+}
+
 extern struct tty_struct *tty_port_tty_get(struct tty_port *port);
 extern void tty_port_tty_set(struct tty_port *port, struct tty_struct *tty);
 extern int tty_port_carrier_raised(struct tty_port *port);

Re: [patch 0/3] Re: tty contention resulting from tty_open_by_device export

[Okash Khawaja]

On Tue, Jul 18, 2017 at 03:26:37PM +0300, Dan Carpenter wrote:
> > +       if (tty) {
> > +               /* drop kref from tty_driver_lookup_tty() */
> > +               tty_kref_put(tty);
> > +               tty = ERR_PTR(-EBUSY);
> > +       } else { /* tty_init_dev returns tty with the tty_lock held */
> > +               tty = tty_init_dev(driver, index);
> > +               tty_port_set_kopened(tty->port, 1);
>                                        ^^^^^^^^^
> 
> tty_init_dev() can fail leading to an error pointer dereference here.

Thanks very much. I will check for error pointer here.

Okash

[patch v2 0/3] tty contention resulting from tty_open_by_driver export

[Okash Khawaja]

Hi,

I have reworked the previous patch set. These are the changes:

1. Patch 1 fixes tty->count mismatch reported by check_tty_count when a
tty is kopened.
2. Patch 1 incorporates patch 4 in the previous patch set - it returns
-ENODEV when tty is not configured.

Thanks,
Okash

[patch v2 1/3] tty: resolve tty contention between kernel and user space

[Okash Khawaja]

[-- Attachment #1: 19_add_tty_kopen --]
[-- Type: text/plain, Size: 5398 bytes --]

The commit 12e84c71b7d4ee (tty: export tty_open_by_driver) exports
tty_open_by_device to allow tty to be opened from inside kernel which
works fine except that it doesn't handle contention with user space or
another kernel-space open of the same tty. For example, opening a tty
from user space while it is kernel opened results in failure and a
kernel log message about mismatch between tty->count and tty's file
open count.

This patch makes kernel access to tty exclusive, so that if a user
process or kernel opens a kernel opened tty, it gets -EBUSY. It does
this by adding TTY_KOPENED flag to tty->flags. When this flag is set,
tty_open_by_driver returns -EBUSY. Instead of overlaoding
tty_open_by_driver for both kernel and user space, this
patch creates a separate function tty_kopen which closely follows
tty_open_by_driver.

To address the mismatch between tty->count and #fd's, this patch adds
#kopen's to the count before comparing it with tty->count. That way
check_tty_count reflects correct usage count. 

Returning -EBUSY on tty open is a change in the interface. I have
tested this with minicom, picocom and commands like "echo foo >
/dev/ttyS0". They all correctly report "Device or resource busy" when
the tty is already kernel opened.

Signed-off-by: Okash Khawaja <okash.khawaja@gmail.com>

---
 drivers/tty/tty_io.c |   66 +++++++++++++++++++++++++++++++++++++++++++++++----
 include/linux/tty.h  |    4 +++
 2 files changed, 65 insertions(+), 5 deletions(-)

--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -280,7 +280,7 @@ static int check_tty_count(struct tty_st
 {
 #ifdef CHECK_TTY_COUNT
 	struct list_head *p;
-	int count = 0;
+	int count = 0, kopen_count = 0;
 
 	spin_lock(&tty->files_lock);
 	list_for_each(p, &tty->tty_files) {
@@ -291,10 +291,12 @@ static int check_tty_count(struct tty_st
 	    tty->driver->subtype == PTY_TYPE_SLAVE &&
 	    tty->link && tty->link->count)
 		count++;
-	if (tty->count != count) {
-		tty_warn(tty, "%s: tty->count(%d) != #fd's(%d)\n",
-			 routine, tty->count, count);
-		return count;
+	if (test_bit(TTY_KOPENED, &tty->flags))
+		kopen_count++;
+	if (tty->count != (count + kopen_count)) {
+		tty_warn(tty, "%s: tty->count(%d) != (#fd's(%d) + #kopen's(%d)\n",
+			 routine, tty->count, count, kopen_count);
+		return (count + kopen_count);
 	}
 #endif
 	return 0;
@@ -1786,6 +1788,54 @@ static struct tty_driver *tty_lookup_dri
 }
 
 /**
+ *	tty_kopen	-	open a tty device for kernel
+ *	@device: dev_t of device to open
+ *
+ *	Opens tty exclusively for kernel. Performs the driver lookup,
+ *	makes sure it's not already opened and performs the first-time
+ *	tty initialization.
+ *
+ *	Returns the locked initialized &tty_struct
+ *
+ *	Claims the global tty_mutex to serialize:
+ *	  - concurrent first-time tty initialization
+ *	  - concurrent tty driver removal w/ lookup
+ *	  - concurrent tty removal from driver table
+ */
+struct tty_struct *tty_kopen(dev_t device)
+{
+	struct tty_struct *tty;
+	struct tty_driver *driver = NULL;
+	int index = -1;
+
+	mutex_lock(&tty_mutex);
+	driver = tty_lookup_driver(device, NULL, &index);
+	if (IS_ERR(driver)) {
+		mutex_unlock(&tty_mutex);
+		return ERR_CAST(driver);
+	}
+
+	/* check whether we're reopening an existing tty */
+	tty = tty_driver_lookup_tty(driver, NULL, index);
+	if (IS_ERR(tty))
+		goto out;
+
+	if (tty) {
+		/* drop kref from tty_driver_lookup_tty() */
+		tty_kref_put(tty);
+		tty = ERR_PTR(-EBUSY);
+	} else { /* tty_init_dev returns tty with the tty_lock held */
+		tty = tty_init_dev(driver, index);
+		set_bit(TTY_KOPENED, &tty->flags);
+	}
+out:
+	mutex_unlock(&tty_mutex);
+	tty_driver_kref_put(driver);
+	return tty;
+}
+EXPORT_SYMBOL_GPL(tty_kopen);
+
+/**
  *	tty_open_by_driver	-	open a tty device
  *	@device: dev_t of device to open
  *	@inode: inode of device file
@@ -1824,6 +1874,12 @@ struct tty_struct *tty_open_by_driver(de
 	}
 
 	if (tty) {
+		if (test_bit(TTY_KOPENED, &tty->flags)) {
+			tty_kref_put(tty);
+			mutex_unlock(&tty_mutex);
+			tty = ERR_PTR(-EBUSY);
+			goto out;
+		}
 		mutex_unlock(&tty_mutex);
 		retval = tty_lock_interruptible(tty);
 		tty_kref_put(tty);  /* drop kref from tty_driver_lookup_tty() */
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -362,6 +362,7 @@ struct tty_file_private {
 #define TTY_NO_WRITE_SPLIT 	17	/* Preserve write boundaries to driver */
 #define TTY_HUPPED 		18	/* Post driver->hangup() */
 #define TTY_LDISC_HALTED	22	/* Line discipline is halted */
+#define TTY_KOPENED		23	/* TTY exclusively opened by kernel */
 
 /* Values for tty->flow_change */
 #define TTY_THROTTLE_SAFE 1
@@ -401,6 +402,7 @@ extern int __init tty_init(void);
 extern const char *tty_name(const struct tty_struct *tty);
 extern struct tty_struct *tty_open_by_driver(dev_t device, struct inode *inode,
 		struct file *filp);
+extern struct tty_struct *tty_kopen(dev_t device);
 extern int tty_dev_name_to_number(const char *name, dev_t *number);
 #else
 static inline void tty_kref_put(struct tty_struct *tty)
@@ -425,6 +427,8 @@ static inline const char *tty_name(const
 static inline struct tty_struct *tty_open_by_driver(dev_t device,
 		struct inode *inode, struct file *filp)
 { return NULL; }
+static inline struct tty_struct *tty_kopen(dev_t device)
+{ return ERR_PTR(-ENODEV); }
 static inline int tty_dev_name_to_number(const char *name, dev_t *number)
 { return -ENOTSUPP; }
 #endif

[patch v2 2/3] staging: speakup: use tty_kopen instead of tty_open_by_driver

[Okash Khawaja]

[-- Attachment #1: 20_use_tty_kopen --]
[-- Type: text/plain, Size: 509 bytes --]

This patch replaces call to tty_open_by_driver with a tty_kopen.

Signed-off-by: Okash Khawaja <okash.khawaja@gmail.com>

---
 drivers/staging/speakup/spk_ttyio.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/drivers/staging/speakup/spk_ttyio.c
+++ b/drivers/staging/speakup/spk_ttyio.c
@@ -158,7 +158,7 @@ static int spk_ttyio_initialise_ldisc(st
 	if (ret)
 		return ret;
 
-	tty = tty_open_by_driver(dev, NULL, NULL);
+	tty = tty_kopen(dev);
 	if (IS_ERR(tty))
 		return PTR_ERR(tty);
 

[patch v2 3/3] tty: undo export of tty_open_by_driver

[Okash Khawaja]

[-- Attachment #1: 21_undo_export_of_tty_open_by_driver --]
[-- Type: text/plain, Size: 1780 bytes --]

Since we have tty_kopen, we no longer need to export tty_open_by_driver.
This patch makes this function static.

Signed-off-by: Okash Khawaja <okash.khawaja@gmail.com>

---
 drivers/tty/tty_io.c |    3 +--
 include/linux/tty.h  |    5 -----
 2 files changed, 1 insertion(+), 7 deletions(-)

--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -1851,7 +1851,7 @@ EXPORT_SYMBOL_GPL(tty_kopen);
  *	  - concurrent tty driver removal w/ lookup
  *	  - concurrent tty removal from driver table
  */
-struct tty_struct *tty_open_by_driver(dev_t device, struct inode *inode,
+static struct tty_struct *tty_open_by_driver(dev_t device, struct inode *inode,
 					     struct file *filp)
 {
 	struct tty_struct *tty;
@@ -1902,7 +1902,6 @@ out:
 	tty_driver_kref_put(driver);
 	return tty;
 }
-EXPORT_SYMBOL_GPL(tty_open_by_driver);
 
 /**
  *	tty_open		-	open a tty device
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -400,8 +400,6 @@ extern struct tty_struct *get_current_tt
 /* tty_io.c */
 extern int __init tty_init(void);
 extern const char *tty_name(const struct tty_struct *tty);
-extern struct tty_struct *tty_open_by_driver(dev_t device, struct inode *inode,
-		struct file *filp);
 extern struct tty_struct *tty_kopen(dev_t device);
 extern int tty_dev_name_to_number(const char *name, dev_t *number);
 #else
@@ -424,9 +422,6 @@ static inline int __init tty_init(void)
 { return 0; }
 static inline const char *tty_name(const struct tty_struct *tty)
 { return "(none)"; }
-static inline struct tty_struct *tty_open_by_driver(dev_t device,
-		struct inode *inode, struct file *filp)
-{ return NULL; }
 static inline struct tty_struct *tty_kopen(dev_t device)
 { return ERR_PTR(-ENODEV); }
 static inline int tty_dev_name_to_number(const char *name, dev_t *number)