[PATCH v2 0/2] vt: fix some vt_ioctl races

[PATCH v2 0/2] vt: fix some vt_ioctl races

[Eric Biggers]

Fix VT_DISALLOCATE freeing an in-use virtual console, and fix a
use-after-free in vt_in_use().

Changed since v1:
    - Made the vc_data be freed via tty_port refcounting.
    - Added patch to fix a use-after-free in vt_in_use().

Eric Biggers (2):
  vt: vt_ioctl: fix VT_DISALLOCATE freeing in-use virtual console
  vt: vt_ioctl: fix use-after-free in vt_in_use()

 drivers/tty/vt/vt.c       | 14 +++++++++++++-
 drivers/tty/vt/vt_ioctl.c | 24 ++++++++++++------------
 2 files changed, 25 insertions(+), 13 deletions(-)

-- 
2.25.1

[PATCH v2 1/2] vt: vt_ioctl: fix VT_DISALLOCATE freeing in-use virtual console

[Eric Biggers]

From: Eric Biggers <ebiggers@google.com>

The VT_DISALLOCATE ioctl can free a virtual console while tty_release()
is still running, causing a use-after-free in con_shutdown().  This
occurs because VT_DISALLOCATE considers a virtual console's
'struct vc_data' to be unused as soon as the corresponding tty's
refcount hits 0.  But actually it may be still being closed.

Fix this by making vc_data be reference-counted via the embedded
'struct tty_port'.  A newly allocated virtual console has refcount 1.
Opening it for the first time increments the refcount to 2.  Closing it
for the last time decrements the refcount (in tty_operations::cleanup()
so that it happens late enough), as does VT_DISALLOCATE.

Reproducer:
	#include <fcntl.h>
	#include <linux/vt.h>
	#include <sys/ioctl.h>
	#include <unistd.h>

	int main()
	{
		if (fork()) {
			for (;;)
				close(open("/dev/tty5", O_RDWR));
		} else {
			int fd = open("/dev/tty10", O_RDWR);

			for (;;)
				ioctl(fd, VT_DISALLOCATE, 5);
		}
	}

KASAN report:
	BUG: KASAN: use-after-free in con_shutdown+0x76/0x80 drivers/tty/vt/vt.c:3278
	Write of size 8 at addr ffff88806a4ec108 by task syz_vt/129

	CPU: 0 PID: 129 Comm: syz_vt Not tainted 5.6.0-rc2 #11
	Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ?-20191223_100556-anatol 04/01/2014
	Call Trace:
	 [...]
	 con_shutdown+0x76/0x80 drivers/tty/vt/vt.c:3278
	 release_tty+0xa8/0x410 drivers/tty/tty_io.c:1514
	 tty_release_struct+0x34/0x50 drivers/tty/tty_io.c:1629
	 tty_release+0x984/0xed0 drivers/tty/tty_io.c:1789
	 [...]

	Allocated by task 129:
	 [...]
	 kzalloc include/linux/slab.h:669 [inline]
	 vc_allocate drivers/tty/vt/vt.c:1085 [inline]
	 vc_allocate+0x1ac/0x680 drivers/tty/vt/vt.c:1066
	 con_install+0x4d/0x3f0 drivers/tty/vt/vt.c:3229
	 tty_driver_install_tty drivers/tty/tty_io.c:1228 [inline]
	 tty_init_dev+0x94/0x350 drivers/tty/tty_io.c:1341
	 tty_open_by_driver drivers/tty/tty_io.c:1987 [inline]
	 tty_open+0x3ca/0xb30 drivers/tty/tty_io.c:2035
	 [...]

	Freed by task 130:
	 [...]
	 kfree+0xbf/0x1e0 mm/slab.c:3757
	 vt_disallocate drivers/tty/vt/vt_ioctl.c:300 [inline]
	 vt_ioctl+0x16dc/0x1e30 drivers/tty/vt/vt_ioctl.c:818
	 tty_ioctl+0x9db/0x11b0 drivers/tty/tty_io.c:2660
	 [...]

Fixes: 4001d7b7fc27 ("vt: push down the tty lock so we can see what is left to tackle")
Cc: <stable@vger.kernel.org> # v3.4+
Reported-by: syzbot+522643ab5729b0421998@syzkaller.appspotmail.com
Signed-off-by: Eric Biggers <ebiggers@google.com>
---
 drivers/tty/vt/vt.c       | 14 +++++++++++++-
 drivers/tty/vt/vt_ioctl.c | 12 ++++--------
 2 files changed, 17 insertions(+), 9 deletions(-)

diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index bbc26d73209a4..ec34f1f5f3bb5 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -1102,6 +1102,9 @@ int vc_allocate(unsigned int currcons)	/* return 0 on success */
 	tty_port_init(&vc->port);
 	INIT_WORK(&vc_cons[currcons].SAK_work, vc_SAK);
 
+	/* if this wasn't the case, we'd have to implement port->ops.destruct */
+	BUILD_BUG_ON(offsetof(struct vc_data, port) != 0);
+
 	visual_init(vc, currcons, 1);
 
 	if (!*vc->vc_uni_pagedir_loc)
@@ -3250,6 +3253,7 @@ static int con_install(struct tty_driver *driver, struct tty_struct *tty)
 
 	tty->driver_data = vc;
 	vc->port.tty = tty;
+	tty_port_get(&vc->port);
 
 	if (!tty->winsize.ws_row && !tty->winsize.ws_col) {
 		tty->winsize.ws_row = vc_cons[currcons].d->vc_rows;
@@ -3285,6 +3289,13 @@ static void con_shutdown(struct tty_struct *tty)
 	console_unlock();
 }
 
+static void con_cleanup(struct tty_struct *tty)
+{
+	struct vc_data *vc = tty->driver_data;
+
+	tty_port_put(&vc->port);
+}
+
 static int default_color           = 7; /* white */
 static int default_italic_color    = 2; // green (ASCII)
 static int default_underline_color = 3; // cyan (ASCII)
@@ -3410,7 +3421,8 @@ static const struct tty_operations con_ops = {
 	.throttle = con_throttle,
 	.unthrottle = con_unthrottle,
 	.resize = vt_resize,
-	.shutdown = con_shutdown
+	.shutdown = con_shutdown,
+	.cleanup = con_cleanup,
 };
 
 static struct cdev vc0_cdev;
diff --git a/drivers/tty/vt/vt_ioctl.c b/drivers/tty/vt/vt_ioctl.c
index 7297997fcf04c..f62f498f63c05 100644
--- a/drivers/tty/vt/vt_ioctl.c
+++ b/drivers/tty/vt/vt_ioctl.c
@@ -310,10 +310,8 @@ static int vt_disallocate(unsigned int vc_num)
 		vc = vc_deallocate(vc_num);
 	console_unlock();
 
-	if (vc && vc_num >= MIN_NR_CONSOLES) {
-		tty_port_destroy(&vc->port);
-		kfree(vc);
-	}
+	if (vc && vc_num >= MIN_NR_CONSOLES)
+		tty_port_put(&vc->port);
 
 	return ret;
 }
@@ -333,10 +331,8 @@ static void vt_disallocate_all(void)
 	console_unlock();
 
 	for (i = 1; i < MAX_NR_CONSOLES; i++) {
-		if (vc[i] && i >= MIN_NR_CONSOLES) {
-			tty_port_destroy(&vc[i]->port);
-			kfree(vc[i]);
-		}
+		if (vc[i] && i >= MIN_NR_CONSOLES)
+			tty_port_put(&vc[i]->port);
 	}
 }
 
-- 
2.25.1

[PATCH v2 2/2] vt: vt_ioctl: fix use-after-free in vt_in_use()

[Eric Biggers]

From: Eric Biggers <ebiggers@google.com>

vt_in_use() dereferences console_driver->ttys[i] without proper locking.
This is broken because the tty can be closed and freed concurrently.

We could fix this by using 'READ_ONCE(console_driver->ttys[i]) != NULL'
and skipping the check of tty_struct::count.  But, looking at
console_driver->ttys[i] isn't really appropriate anyway because even if
it is NULL the tty can still be in the process of being closed.

Instead, fix it by making vt_in_use() require console_lock() and check
whether the vt is allocated and has port refcount > 1.  This works since
following the patch "vt: vt_ioctl: fix VT_DISALLOCATE freeing in-use
virtual console" the port refcount is incremented while the vt is open.

Reproducer (very unreliable, but it worked for me after a few minutes):

	#include <fcntl.h>
	#include <linux/vt.h>

	int main()
	{
		int fd, nproc;
		struct vt_stat state;
		char ttyname[16];

		fd = open("/dev/tty10", O_RDONLY);
		for (nproc = 1; nproc < 8; nproc *= 2)
			fork();
		for (;;) {
			sprintf(ttyname, "/dev/tty%d", rand() % 8);
			close(open(ttyname, O_RDONLY));
			ioctl(fd, VT_GETSTATE, &state);
		}
	}

KASAN report:

	BUG: KASAN: use-after-free in vt_in_use drivers/tty/vt/vt_ioctl.c:48 [inline]
	BUG: KASAN: use-after-free in vt_ioctl+0x1ad3/0x1d70 drivers/tty/vt/vt_ioctl.c:657
	Read of size 4 at addr ffff888065722468 by task syz-vt2/132

	CPU: 0 PID: 132 Comm: syz-vt2 Not tainted 5.6.0-rc5-00130-g089b6d3654916 #13
	Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ?-20191223_100556-anatol 04/01/2014
	Call Trace:
	 [...]
	 vt_in_use drivers/tty/vt/vt_ioctl.c:48 [inline]
	 vt_ioctl+0x1ad3/0x1d70 drivers/tty/vt/vt_ioctl.c:657
	 tty_ioctl+0x9db/0x11b0 drivers/tty/tty_io.c:2660
	 [...]

	Allocated by task 136:
	 [...]
	 kzalloc include/linux/slab.h:669 [inline]
	 alloc_tty_struct+0x96/0x8a0 drivers/tty/tty_io.c:2982
	 tty_init_dev+0x23/0x350 drivers/tty/tty_io.c:1334
	 tty_open_by_driver drivers/tty/tty_io.c:1987 [inline]
	 tty_open+0x3ca/0xb30 drivers/tty/tty_io.c:2035
	 [...]

	Freed by task 41:
	 [...]
	 kfree+0xbf/0x200 mm/slab.c:3757
	 free_tty_struct+0x8d/0xb0 drivers/tty/tty_io.c:177
	 release_one_tty+0x22d/0x2f0 drivers/tty/tty_io.c:1468
	 process_one_work+0x7f1/0x14b0 kernel/workqueue.c:2264
	 worker_thread+0x8b/0xc80 kernel/workqueue.c:2410
	 [...]

Fixes: 4001d7b7fc27 ("vt: push down the tty lock so we can see what is left to tackle")
Cc: <stable@vger.kernel.org> # v3.4+
Signed-off-by: Eric Biggers <ebiggers@google.com>
---
 drivers/tty/vt/vt_ioctl.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/drivers/tty/vt/vt_ioctl.c b/drivers/tty/vt/vt_ioctl.c
index f62f498f63c05..ca0e57b1c0b43 100644
--- a/drivers/tty/vt/vt_ioctl.c
+++ b/drivers/tty/vt/vt_ioctl.c
@@ -43,9 +43,11 @@ bool vt_dont_switch;
 
 static inline bool vt_in_use(unsigned int i)
 {
-	extern struct tty_driver *console_driver;
+	const struct vc_data *vc = vc_cons[i].d;
 
-	return console_driver->ttys[i] && console_driver->ttys[i]->count;
+	WARN_CONSOLE_UNLOCKED();
+
+	return vc && kref_read(&vc->port.kref) > 1;
 }
 
 static inline bool vt_busy(int i)
@@ -643,15 +645,16 @@ int vt_ioctl(struct tty_struct *tty,
 		struct vt_stat __user *vtstat = up;
 		unsigned short state, mask;
 
-		/* Review: FIXME: Console lock ? */
 		if (put_user(fg_console + 1, &vtstat->v_active))
 			ret = -EFAULT;
 		else {
 			state = 1;	/* /dev/tty0 is always open */
+			console_lock();
 			for (i = 0, mask = 2; i < MAX_NR_CONSOLES && mask;
 							++i, mask <<= 1)
 				if (vt_in_use(i))
 					state |= mask;
+			console_unlock();
 			ret = put_user(state, &vtstat->v_state);
 		}
 		break;
@@ -661,10 +664,11 @@ int vt_ioctl(struct tty_struct *tty,
 	 * Returns the first available (non-opened) console.
 	 */
 	case VT_OPENQRY:
-		/* FIXME: locking ? - but then this is a stupid API */
+		console_lock();
 		for (i = 0; i < MAX_NR_CONSOLES; ++i)
 			if (!vt_in_use(i))
 				break;
+		console_unlock();
 		uival = i < MAX_NR_CONSOLES ? (i+1) : -1;
 		goto setint;		 
 
-- 
2.25.1

Re: [PATCH v2 1/2] vt: vt_ioctl: fix VT_DISALLOCATE freeing in-use virtual console

[Jiri Slaby]

On 18. 03. 20, 23:38, Eric Biggers wrote:
> --- a/drivers/tty/vt/vt.c
> +++ b/drivers/tty/vt/vt.c
> @@ -1102,6 +1102,9 @@ int vc_allocate(unsigned int currcons)	/* return 0 on success */
>  	tty_port_init(&vc->port);
>  	INIT_WORK(&vc_cons[currcons].SAK_work, vc_SAK);
>  
> +	/* if this wasn't the case, we'd have to implement port->ops.destruct */
> +	BUILD_BUG_ON(offsetof(struct vc_data, port) != 0);
> +

This is 3 lines, implementing destruct would be like 4-5 :)? Please
implement destruct instead.

Otherwise looks good.

thanks,
-- 
js
suse labs

Re: [PATCH v2 1/2] vt: vt_ioctl: fix VT_DISALLOCATE freeing in-use virtual console

[Eric Biggers]

On Thu, Mar 19, 2020 at 08:36:28AM +0100, Jiri Slaby wrote:
> On 18. 03. 20, 23:38, Eric Biggers wrote:
> > --- a/drivers/tty/vt/vt.c
> > +++ b/drivers/tty/vt/vt.c
> > @@ -1102,6 +1102,9 @@ int vc_allocate(unsigned int currcons)	/* return 0 on success */
> >  	tty_port_init(&vc->port);
> >  	INIT_WORK(&vc_cons[currcons].SAK_work, vc_SAK);
> >  
> > +	/* if this wasn't the case, we'd have to implement port->ops.destruct */
> > +	BUILD_BUG_ON(offsetof(struct vc_data, port) != 0);
> > +
> 
> This is 3 lines, implementing destruct would be like 4-5 :)? Please
> implement destruct instead.
> 
> Otherwise looks good.
> 

Actually implementing destruct would be 12 lines, see below.  Remember there is
no tty_port_operations defined yet so we'd have to define it just for destruct.

Do you still prefer it?

diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index ec34f1f5f3bb5..309a39197be0a 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -1075,6 +1075,17 @@ static void visual_deinit(struct vc_data *vc)
 	module_put(vc->vc_sw->owner);
 }
 
+static void vc_port_destruct(struct tty_port *port)
+{
+	struct vc_data *vc = container_of(port, struct vc_data, port);
+
+	kfree(vc);
+}
+
+static const struct tty_port_operations vc_port_ops = {
+	.destruct = vc_port_destruct,
+};
+
 int vc_allocate(unsigned int currcons)	/* return 0 on success */
 {
 	struct vt_notifier_param param;
@@ -1100,11 +1111,9 @@ int vc_allocate(unsigned int currcons)	/* return 0 on success */
 
 	vc_cons[currcons].d = vc;
 	tty_port_init(&vc->port);
+	vc->port.ops = &vc_port_ops;
 	INIT_WORK(&vc_cons[currcons].SAK_work, vc_SAK);
 
-	/* if this wasn't the case, we'd have to implement port->ops.destruct */
-	BUILD_BUG_ON(offsetof(struct vc_data, port) != 0);
-
 	visual_init(vc, currcons, 1);
 
 	if (!*vc->vc_uni_pagedir_loc)

Re: [PATCH v2 1/2] vt: vt_ioctl: fix VT_DISALLOCATE freeing in-use virtual console

[Greg Kroah-Hartman]

On Thu, Mar 19, 2020 at 10:10:49PM -0700, Eric Biggers wrote:
> On Thu, Mar 19, 2020 at 08:36:28AM +0100, Jiri Slaby wrote:
> > On 18. 03. 20, 23:38, Eric Biggers wrote:
> > > --- a/drivers/tty/vt/vt.c
> > > +++ b/drivers/tty/vt/vt.c
> > > @@ -1102,6 +1102,9 @@ int vc_allocate(unsigned int currcons)	/* return 0 on success */
> > >  	tty_port_init(&vc->port);
> > >  	INIT_WORK(&vc_cons[currcons].SAK_work, vc_SAK);
> > >  
> > > +	/* if this wasn't the case, we'd have to implement port->ops.destruct */
> > > +	BUILD_BUG_ON(offsetof(struct vc_data, port) != 0);
> > > +
> > 
> > This is 3 lines, implementing destruct would be like 4-5 :)? Please
> > implement destruct instead.
> > 
> > Otherwise looks good.
> > 
> 
> Actually implementing destruct would be 12 lines, see below.  Remember there is
> no tty_port_operations defined yet so we'd have to define it just for destruct.
> 
> Do you still prefer it?
> 
> diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
> index ec34f1f5f3bb5..309a39197be0a 100644
> --- a/drivers/tty/vt/vt.c
> +++ b/drivers/tty/vt/vt.c
> @@ -1075,6 +1075,17 @@ static void visual_deinit(struct vc_data *vc)
>  	module_put(vc->vc_sw->owner);
>  }
>  
> +static void vc_port_destruct(struct tty_port *port)
> +{
> +	struct vc_data *vc = container_of(port, struct vc_data, port);
> +
> +	kfree(vc);
> +}
> +
> +static const struct tty_port_operations vc_port_ops = {
> +	.destruct = vc_port_destruct,
> +};
> +
>  int vc_allocate(unsigned int currcons)	/* return 0 on success */
>  {
>  	struct vt_notifier_param param;
> @@ -1100,11 +1111,9 @@ int vc_allocate(unsigned int currcons)	/* return 0 on success */
>  
>  	vc_cons[currcons].d = vc;
>  	tty_port_init(&vc->port);
> +	vc->port.ops = &vc_port_ops;
>  	INIT_WORK(&vc_cons[currcons].SAK_work, vc_SAK);
>  
> -	/* if this wasn't the case, we'd have to implement port->ops.destruct */
> -	BUILD_BUG_ON(offsetof(struct vc_data, port) != 0);
> -
>  	visual_init(vc, currcons, 1);
>  
>  	if (!*vc->vc_uni_pagedir_loc)


Yes, this is good to have, thanks for doing this.

greg k-h

Re: [PATCH v2 2/2] vt: vt_ioctl: fix use-after-free in vt_in_use()

[Jiri Slaby]

On 18. 03. 20, 23:38, Eric Biggers wrote:
> --- a/drivers/tty/vt/vt_ioctl.c
> +++ b/drivers/tty/vt/vt_ioctl.c
> @@ -43,9 +43,11 @@ bool vt_dont_switch;
>  
>  static inline bool vt_in_use(unsigned int i)
>  {
> -	extern struct tty_driver *console_driver;
> +	const struct vc_data *vc = vc_cons[i].d;
>  
> -	return console_driver->ttys[i] && console_driver->ttys[i]->count;
> +	WARN_CONSOLE_UNLOCKED();
> +
> +	return vc && kref_read(&vc->port.kref) > 1;
>  }
>  
>  static inline bool vt_busy(int i)
> @@ -643,15 +645,16 @@ int vt_ioctl(struct tty_struct *tty,
>  		struct vt_stat __user *vtstat = up;
>  		unsigned short state, mask;
>  
> -		/* Review: FIXME: Console lock ? */
>  		if (put_user(fg_console + 1, &vtstat->v_active))
>  			ret = -EFAULT;
>  		else {
>  			state = 1;	/* /dev/tty0 is always open */
> +			console_lock();

Could you comment on this one and the lock below why you added it here?

To me, it seems, we should rather introduce a vt alloc/dealloc lock
protecting cases like this, not console lock. But not now, some time
later. So a comment would help when/once we/I get into it...

The interface (ie. the ioctls) also look weird and racy. Both of them.
Like the "OK, I give you this number, but it might not be correct by
now." kind of thing.

This let me think, who could use this? The answer is many 8-/. openpt,
systemd, sysvinit, didn't check others.

Perhaps we should provide openvt -- analogy of openpty and deprecate
VT_OPENQRY?

With VT_GETSTATE, the situation is more complicated:
sysvinit uses VT_GETSTATE only if TIOCGDEV is not available, so
VT_GETSTATE is actually unneeded there.

systemd uses it to find the current console (vtstat->v_active) and
systemd-logind uses it for spawning autovt on free consoles. That sort
of makes sense...

>  			for (i = 0, mask = 2; i < MAX_NR_CONSOLES && mask;
>  							++i, mask <<= 1)
>  				if (vt_in_use(i))
>  					state |= mask;
> +			console_unlock();
>  			ret = put_user(state, &vtstat->v_state);
>  		}
>  		break;
> @@ -661,10 +664,11 @@ int vt_ioctl(struct tty_struct *tty,
>  	 * Returns the first available (non-opened) console.
>  	 */
>  	case VT_OPENQRY:
> -		/* FIXME: locking ? - but then this is a stupid API */
> +		console_lock();
>  		for (i = 0; i < MAX_NR_CONSOLES; ++i)
>  			if (!vt_in_use(i))
>  				break;
> +		console_unlock();
>  		uival = i < MAX_NR_CONSOLES ? (i+1) : -1;
>  		goto setint;		 
>  
> 

thanks,
-- 
js
suse labs

Re: [PATCH v2 2/2] vt: vt_ioctl: fix use-after-free in vt_in_use()

[Eric Biggers]

On Fri, Mar 20, 2020 at 02:42:12PM +0100, Jiri Slaby wrote:
> On 18. 03. 20, 23:38, Eric Biggers wrote:
> > --- a/drivers/tty/vt/vt_ioctl.c
> > +++ b/drivers/tty/vt/vt_ioctl.c
> > @@ -43,9 +43,11 @@ bool vt_dont_switch;
> >  
> >  static inline bool vt_in_use(unsigned int i)
> >  {
> > -	extern struct tty_driver *console_driver;
> > +	const struct vc_data *vc = vc_cons[i].d;
> >  
> > -	return console_driver->ttys[i] && console_driver->ttys[i]->count;
> > +	WARN_CONSOLE_UNLOCKED();
> > +
> > +	return vc && kref_read(&vc->port.kref) > 1;
> >  }
> >  
> >  static inline bool vt_busy(int i)
> > @@ -643,15 +645,16 @@ int vt_ioctl(struct tty_struct *tty,
> >  		struct vt_stat __user *vtstat = up;
> >  		unsigned short state, mask;
> >  
> > -		/* Review: FIXME: Console lock ? */
> >  		if (put_user(fg_console + 1, &vtstat->v_active))
> >  			ret = -EFAULT;
> >  		else {
> >  			state = 1;	/* /dev/tty0 is always open */
> > +			console_lock();
> 
> Could you comment on this one and the lock below why you added it here?
> 
> To me, it seems, we should rather introduce a vt alloc/dealloc lock
> protecting cases like this, not console lock. But not now, some time
> later. So a comment would help when/once we/I get into it...

I think the locking I added to VT_GETSTATE and VT_OPENQRY is pretty
self-explanatory: it's needed because they call vt_in_use() which now requires
console_lock.  So I'm not sure what you'd like me to add there?

As for vt_in_use() itself, I already added WARN_CONSOLE_UNLOCKED() to it.
But I can add a comment to it too if it would be useful, like:

static inline bool vt_in_use(unsigned int i)
{
        const struct vc_data *vc = vc_cons[i].d;

        /*
         * console_lock must be held to prevent the vc from being deallocated
         * while we're checking whether it's in-use.
         */
        WARN_CONSOLE_UNLOCKED();

        return vc && kref_read(&vc->port.kref) > 1;
}


> The interface (ie. the ioctls) also look weird and racy. Both of them.
> Like the "OK, I give you this number, but it might not be correct by
> now." kind of thing.
> 
> This let me think, who could use this? The answer is many 8-/. openpt,
> systemd, sysvinit, didn't check others.
> 
> Perhaps we should provide openvt -- analogy of openpty and deprecate
> VT_OPENQRY?
> 
> With VT_GETSTATE, the situation is more complicated:
> sysvinit uses VT_GETSTATE only if TIOCGDEV is not available, so
> VT_GETSTATE is actually unneeded there.
> 
> systemd uses it to find the current console (vtstat->v_active) and
> systemd-logind uses it for spawning autovt on free consoles. That sort
> of makes sense...
> 

Yes, these are bad APIs.

Once I did remove a buggy ioctl elsewhere in the kernel rather than fixing it.
But you have to be very, very confident that nothing is using it.  That doesn't
seem to be the case for VT_GETSTATE and VT_OPENQRY as it's not hard to find code
using them.  E.g. here's another user of both of them:
https://android.googlesource.com/platform/system/core/+/ccecf1425412beb2bc3bb38d470293fdc244d6f1/toolbox/setconsole.c

So we're probably stuck with them for now.  If you'd like to explore adding a
better API and trying to get all users to use it, you're certainly welcome to.
But it would be orthogonal to fixing this bug.

- Eric

[PATCH v3 0/2] vt: fix some vt_ioctl races

[Eric Biggers]

Fix VT_DISALLOCATE freeing an in-use virtual console, and fix a
use-after-free in vt_in_use().

Changed since v2:
    - Implemented tty_port_operations::destruct().
    - Added comments regarding vt_in_use() locking.

Changed since v1:
    - Made the vc_data be freed via tty_port refcounting.
    - Added patch to fix a use-after-free in vt_in_use().

Eric Biggers (2):
  vt: vt_ioctl: fix VT_DISALLOCATE freeing in-use virtual console
  vt: vt_ioctl: fix use-after-free in vt_in_use()

 drivers/tty/vt/vt.c       | 23 ++++++++++++++++++++++-
 drivers/tty/vt/vt_ioctl.c | 28 ++++++++++++++++------------
 2 files changed, 38 insertions(+), 13 deletions(-)

-- 
2.25.2

[PATCH v3 1/2] vt: vt_ioctl: fix VT_DISALLOCATE freeing in-use virtual console

[Eric Biggers]

From: Eric Biggers <ebiggers@google.com>

The VT_DISALLOCATE ioctl can free a virtual console while tty_release()
is still running, causing a use-after-free in con_shutdown().  This
occurs because VT_DISALLOCATE considers a virtual console's
'struct vc_data' to be unused as soon as the corresponding tty's
refcount hits 0.  But actually it may be still being closed.

Fix this by making vc_data be reference-counted via the embedded
'struct tty_port'.  A newly allocated virtual console has refcount 1.
Opening it for the first time increments the refcount to 2.  Closing it
for the last time decrements the refcount (in tty_operations::cleanup()
so that it happens late enough), as does VT_DISALLOCATE.

Reproducer:
	#include <fcntl.h>
	#include <linux/vt.h>
	#include <sys/ioctl.h>
	#include <unistd.h>

	int main()
	{
		if (fork()) {
			for (;;)
				close(open("/dev/tty5", O_RDWR));
		} else {
			int fd = open("/dev/tty10", O_RDWR);

			for (;;)
				ioctl(fd, VT_DISALLOCATE, 5);
		}
	}

KASAN report:
	BUG: KASAN: use-after-free in con_shutdown+0x76/0x80 drivers/tty/vt/vt.c:3278
	Write of size 8 at addr ffff88806a4ec108 by task syz_vt/129

	CPU: 0 PID: 129 Comm: syz_vt Not tainted 5.6.0-rc2 #11
	Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ?-20191223_100556-anatol 04/01/2014
	Call Trace:
	 [...]
	 con_shutdown+0x76/0x80 drivers/tty/vt/vt.c:3278
	 release_tty+0xa8/0x410 drivers/tty/tty_io.c:1514
	 tty_release_struct+0x34/0x50 drivers/tty/tty_io.c:1629
	 tty_release+0x984/0xed0 drivers/tty/tty_io.c:1789
	 [...]

	Allocated by task 129:
	 [...]
	 kzalloc include/linux/slab.h:669 [inline]
	 vc_allocate drivers/tty/vt/vt.c:1085 [inline]
	 vc_allocate+0x1ac/0x680 drivers/tty/vt/vt.c:1066
	 con_install+0x4d/0x3f0 drivers/tty/vt/vt.c:3229
	 tty_driver_install_tty drivers/tty/tty_io.c:1228 [inline]
	 tty_init_dev+0x94/0x350 drivers/tty/tty_io.c:1341
	 tty_open_by_driver drivers/tty/tty_io.c:1987 [inline]
	 tty_open+0x3ca/0xb30 drivers/tty/tty_io.c:2035
	 [...]

	Freed by task 130:
	 [...]
	 kfree+0xbf/0x1e0 mm/slab.c:3757
	 vt_disallocate drivers/tty/vt/vt_ioctl.c:300 [inline]
	 vt_ioctl+0x16dc/0x1e30 drivers/tty/vt/vt_ioctl.c:818
	 tty_ioctl+0x9db/0x11b0 drivers/tty/tty_io.c:2660
	 [...]

Fixes: 4001d7b7fc27 ("vt: push down the tty lock so we can see what is left to tackle")
Cc: <stable@vger.kernel.org> # v3.4+
Reported-by: syzbot+522643ab5729b0421998@syzkaller.appspotmail.com
Signed-off-by: Eric Biggers <ebiggers@google.com>
---
 drivers/tty/vt/vt.c       | 23 ++++++++++++++++++++++-
 drivers/tty/vt/vt_ioctl.c | 12 ++++--------
 2 files changed, 26 insertions(+), 9 deletions(-)

diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index bbc26d73209a4..309a39197be0a 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -1075,6 +1075,17 @@ static void visual_deinit(struct vc_data *vc)
 	module_put(vc->vc_sw->owner);
 }
 
+static void vc_port_destruct(struct tty_port *port)
+{
+	struct vc_data *vc = container_of(port, struct vc_data, port);
+
+	kfree(vc);
+}
+
+static const struct tty_port_operations vc_port_ops = {
+	.destruct = vc_port_destruct,
+};
+
 int vc_allocate(unsigned int currcons)	/* return 0 on success */
 {
 	struct vt_notifier_param param;
@@ -1100,6 +1111,7 @@ int vc_allocate(unsigned int currcons)	/* return 0 on success */
 
 	vc_cons[currcons].d = vc;
 	tty_port_init(&vc->port);
+	vc->port.ops = &vc_port_ops;
 	INIT_WORK(&vc_cons[currcons].SAK_work, vc_SAK);
 
 	visual_init(vc, currcons, 1);
@@ -3250,6 +3262,7 @@ static int con_install(struct tty_driver *driver, struct tty_struct *tty)
 
 	tty->driver_data = vc;
 	vc->port.tty = tty;
+	tty_port_get(&vc->port);
 
 	if (!tty->winsize.ws_row && !tty->winsize.ws_col) {
 		tty->winsize.ws_row = vc_cons[currcons].d->vc_rows;
@@ -3285,6 +3298,13 @@ static void con_shutdown(struct tty_struct *tty)
 	console_unlock();
 }
 
+static void con_cleanup(struct tty_struct *tty)
+{
+	struct vc_data *vc = tty->driver_data;
+
+	tty_port_put(&vc->port);
+}
+
 static int default_color           = 7; /* white */
 static int default_italic_color    = 2; // green (ASCII)
 static int default_underline_color = 3; // cyan (ASCII)
@@ -3410,7 +3430,8 @@ static const struct tty_operations con_ops = {
 	.throttle = con_throttle,
 	.unthrottle = con_unthrottle,
 	.resize = vt_resize,
-	.shutdown = con_shutdown
+	.shutdown = con_shutdown,
+	.cleanup = con_cleanup,
 };
 
 static struct cdev vc0_cdev;
diff --git a/drivers/tty/vt/vt_ioctl.c b/drivers/tty/vt/vt_ioctl.c
index 7297997fcf04c..f62f498f63c05 100644
--- a/drivers/tty/vt/vt_ioctl.c
+++ b/drivers/tty/vt/vt_ioctl.c
@@ -310,10 +310,8 @@ static int vt_disallocate(unsigned int vc_num)
 		vc = vc_deallocate(vc_num);
 	console_unlock();
 
-	if (vc && vc_num >= MIN_NR_CONSOLES) {
-		tty_port_destroy(&vc->port);
-		kfree(vc);
-	}
+	if (vc && vc_num >= MIN_NR_CONSOLES)
+		tty_port_put(&vc->port);
 
 	return ret;
 }
@@ -333,10 +331,8 @@ static void vt_disallocate_all(void)
 	console_unlock();
 
 	for (i = 1; i < MAX_NR_CONSOLES; i++) {
-		if (vc[i] && i >= MIN_NR_CONSOLES) {
-			tty_port_destroy(&vc[i]->port);
-			kfree(vc[i]);
-		}
+		if (vc[i] && i >= MIN_NR_CONSOLES)
+			tty_port_put(&vc[i]->port);
 	}
 }
 
-- 
2.25.2

[PATCH v3 2/2] vt: vt_ioctl: fix use-after-free in vt_in_use()

[Eric Biggers]

From: Eric Biggers <ebiggers@google.com>

vt_in_use() dereferences console_driver->ttys[i] without proper locking.
This is broken because the tty can be closed and freed concurrently.

We could fix this by using 'READ_ONCE(console_driver->ttys[i]) != NULL'
and skipping the check of tty_struct::count.  But, looking at
console_driver->ttys[i] isn't really appropriate anyway because even if
it is NULL the tty can still be in the process of being closed.

Instead, fix it by making vt_in_use() require console_lock() and check
whether the vt is allocated and has port refcount > 1.  This works since
following the patch "vt: vt_ioctl: fix VT_DISALLOCATE freeing in-use
virtual console" the port refcount is incremented while the vt is open.

Reproducer (very unreliable, but it worked for me after a few minutes):

	#include <fcntl.h>
	#include <linux/vt.h>

	int main()
	{
		int fd, nproc;
		struct vt_stat state;
		char ttyname[16];

		fd = open("/dev/tty10", O_RDONLY);
		for (nproc = 1; nproc < 8; nproc *= 2)
			fork();
		for (;;) {
			sprintf(ttyname, "/dev/tty%d", rand() % 8);
			close(open(ttyname, O_RDONLY));
			ioctl(fd, VT_GETSTATE, &state);
		}
	}

KASAN report:

	BUG: KASAN: use-after-free in vt_in_use drivers/tty/vt/vt_ioctl.c:48 [inline]
	BUG: KASAN: use-after-free in vt_ioctl+0x1ad3/0x1d70 drivers/tty/vt/vt_ioctl.c:657
	Read of size 4 at addr ffff888065722468 by task syz-vt2/132

	CPU: 0 PID: 132 Comm: syz-vt2 Not tainted 5.6.0-rc5-00130-g089b6d3654916 #13
	Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ?-20191223_100556-anatol 04/01/2014
	Call Trace:
	 [...]
	 vt_in_use drivers/tty/vt/vt_ioctl.c:48 [inline]
	 vt_ioctl+0x1ad3/0x1d70 drivers/tty/vt/vt_ioctl.c:657
	 tty_ioctl+0x9db/0x11b0 drivers/tty/tty_io.c:2660
	 [...]

	Allocated by task 136:
	 [...]
	 kzalloc include/linux/slab.h:669 [inline]
	 alloc_tty_struct+0x96/0x8a0 drivers/tty/tty_io.c:2982
	 tty_init_dev+0x23/0x350 drivers/tty/tty_io.c:1334
	 tty_open_by_driver drivers/tty/tty_io.c:1987 [inline]
	 tty_open+0x3ca/0xb30 drivers/tty/tty_io.c:2035
	 [...]

	Freed by task 41:
	 [...]
	 kfree+0xbf/0x200 mm/slab.c:3757
	 free_tty_struct+0x8d/0xb0 drivers/tty/tty_io.c:177
	 release_one_tty+0x22d/0x2f0 drivers/tty/tty_io.c:1468
	 process_one_work+0x7f1/0x14b0 kernel/workqueue.c:2264
	 worker_thread+0x8b/0xc80 kernel/workqueue.c:2410
	 [...]

Fixes: 4001d7b7fc27 ("vt: push down the tty lock so we can see what is left to tackle")
Cc: <stable@vger.kernel.org> # v3.4+
Signed-off-by: Eric Biggers <ebiggers@google.com>
---
 drivers/tty/vt/vt_ioctl.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/drivers/tty/vt/vt_ioctl.c b/drivers/tty/vt/vt_ioctl.c
index f62f498f63c05..daf61c28ba766 100644
--- a/drivers/tty/vt/vt_ioctl.c
+++ b/drivers/tty/vt/vt_ioctl.c
@@ -43,9 +43,15 @@ bool vt_dont_switch;
 
 static inline bool vt_in_use(unsigned int i)
 {
-	extern struct tty_driver *console_driver;
+	const struct vc_data *vc = vc_cons[i].d;
 
-	return console_driver->ttys[i] && console_driver->ttys[i]->count;
+	/*
+	 * console_lock must be held to prevent the vc from being deallocated
+	 * while we're checking whether it's in-use.
+	 */
+	WARN_CONSOLE_UNLOCKED();
+
+	return vc && kref_read(&vc->port.kref) > 1;
 }
 
 static inline bool vt_busy(int i)
@@ -643,15 +649,16 @@ int vt_ioctl(struct tty_struct *tty,
 		struct vt_stat __user *vtstat = up;
 		unsigned short state, mask;
 
-		/* Review: FIXME: Console lock ? */
 		if (put_user(fg_console + 1, &vtstat->v_active))
 			ret = -EFAULT;
 		else {
 			state = 1;	/* /dev/tty0 is always open */
+			console_lock(); /* required by vt_in_use() */
 			for (i = 0, mask = 2; i < MAX_NR_CONSOLES && mask;
 							++i, mask <<= 1)
 				if (vt_in_use(i))
 					state |= mask;
+			console_unlock();
 			ret = put_user(state, &vtstat->v_state);
 		}
 		break;
@@ -661,10 +668,11 @@ int vt_ioctl(struct tty_struct *tty,
 	 * Returns the first available (non-opened) console.
 	 */
 	case VT_OPENQRY:
-		/* FIXME: locking ? - but then this is a stupid API */
+		console_lock(); /* required by vt_in_use() */
 		for (i = 0; i < MAX_NR_CONSOLES; ++i)
 			if (!vt_in_use(i))
 				break;
+		console_unlock();
 		uival = i < MAX_NR_CONSOLES ? (i+1) : -1;
 		goto setint;		 
 
-- 
2.25.2

Re: [PATCH v3 0/2] vt: fix some vt_ioctl races

[Greg Kroah-Hartman]

On Sat, Mar 21, 2020 at 08:43:03PM -0700, Eric Biggers wrote:
> Fix VT_DISALLOCATE freeing an in-use virtual console, and fix a
> use-after-free in vt_in_use().
> 
> Changed since v2:
>     - Implemented tty_port_operations::destruct().
>     - Added comments regarding vt_in_use() locking.
> 
> Changed since v1:
>     - Made the vc_data be freed via tty_port refcounting.
>     - Added patch to fix a use-after-free in vt_in_use().
> 
> Eric Biggers (2):
>   vt: vt_ioctl: fix VT_DISALLOCATE freeing in-use virtual console
>   vt: vt_ioctl: fix use-after-free in vt_in_use()
> 
>  drivers/tty/vt/vt.c       | 23 ++++++++++++++++++++++-
>  drivers/tty/vt/vt_ioctl.c | 28 ++++++++++++++++------------
>  2 files changed, 38 insertions(+), 13 deletions(-)


Jiri, any objection to these?

thanks,

greg k-h

Re: [PATCH v3 1/2] vt: vt_ioctl: fix VT_DISALLOCATE freeing in-use virtual console

[Jiri Slaby]

On 22. 03. 20, 4:43, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.com>
> 
> The VT_DISALLOCATE ioctl can free a virtual console while tty_release()
> is still running, causing a use-after-free in con_shutdown().  This
> occurs because VT_DISALLOCATE considers a virtual console's
> 'struct vc_data' to be unused as soon as the corresponding tty's
> refcount hits 0.  But actually it may be still being closed.
> 
> Fix this by making vc_data be reference-counted via the embedded
> 'struct tty_port'.  A newly allocated virtual console has refcount 1.
> Opening it for the first time increments the refcount to 2.  Closing it
> for the last time decrements the refcount (in tty_operations::cleanup()
> so that it happens late enough), as does VT_DISALLOCATE.
> 
> Reproducer:
> 	#include <fcntl.h>
> 	#include <linux/vt.h>
> 	#include <sys/ioctl.h>
> 	#include <unistd.h>
> 
> 	int main()
> 	{
> 		if (fork()) {
> 			for (;;)
> 				close(open("/dev/tty5", O_RDWR));
> 		} else {
> 			int fd = open("/dev/tty10", O_RDWR);
> 
> 			for (;;)
> 				ioctl(fd, VT_DISALLOCATE, 5);
> 		}
> 	}
> 
> KASAN report:
> 	BUG: KASAN: use-after-free in con_shutdown+0x76/0x80 drivers/tty/vt/vt.c:3278
> 	Write of size 8 at addr ffff88806a4ec108 by task syz_vt/129
> 
> 	CPU: 0 PID: 129 Comm: syz_vt Not tainted 5.6.0-rc2 #11
> 	Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ?-20191223_100556-anatol 04/01/2014
> 	Call Trace:
> 	 [...]
> 	 con_shutdown+0x76/0x80 drivers/tty/vt/vt.c:3278
> 	 release_tty+0xa8/0x410 drivers/tty/tty_io.c:1514
> 	 tty_release_struct+0x34/0x50 drivers/tty/tty_io.c:1629
> 	 tty_release+0x984/0xed0 drivers/tty/tty_io.c:1789
> 	 [...]
> 
> 	Allocated by task 129:
> 	 [...]
> 	 kzalloc include/linux/slab.h:669 [inline]
> 	 vc_allocate drivers/tty/vt/vt.c:1085 [inline]
> 	 vc_allocate+0x1ac/0x680 drivers/tty/vt/vt.c:1066
> 	 con_install+0x4d/0x3f0 drivers/tty/vt/vt.c:3229
> 	 tty_driver_install_tty drivers/tty/tty_io.c:1228 [inline]
> 	 tty_init_dev+0x94/0x350 drivers/tty/tty_io.c:1341
> 	 tty_open_by_driver drivers/tty/tty_io.c:1987 [inline]
> 	 tty_open+0x3ca/0xb30 drivers/tty/tty_io.c:2035
> 	 [...]
> 
> 	Freed by task 130:
> 	 [...]
> 	 kfree+0xbf/0x1e0 mm/slab.c:3757
> 	 vt_disallocate drivers/tty/vt/vt_ioctl.c:300 [inline]
> 	 vt_ioctl+0x16dc/0x1e30 drivers/tty/vt/vt_ioctl.c:818
> 	 tty_ioctl+0x9db/0x11b0 drivers/tty/tty_io.c:2660
> 	 [...]
> 
> Fixes: 4001d7b7fc27 ("vt: push down the tty lock so we can see what is left to tackle")
> Cc: <stable@vger.kernel.org> # v3.4+
> Reported-by: syzbot+522643ab5729b0421998@syzkaller.appspotmail.com
> Signed-off-by: Eric Biggers <ebiggers@google.com>

Acked-by: Jiri Slaby <jslaby@suse.cz>

> ---
>  drivers/tty/vt/vt.c       | 23 ++++++++++++++++++++++-
>  drivers/tty/vt/vt_ioctl.c | 12 ++++--------
>  2 files changed, 26 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
> index bbc26d73209a4..309a39197be0a 100644
> --- a/drivers/tty/vt/vt.c
> +++ b/drivers/tty/vt/vt.c
> @@ -1075,6 +1075,17 @@ static void visual_deinit(struct vc_data *vc)
>  	module_put(vc->vc_sw->owner);
>  }
>  
> +static void vc_port_destruct(struct tty_port *port)
> +{
> +	struct vc_data *vc = container_of(port, struct vc_data, port);
> +
> +	kfree(vc);
> +}
> +
> +static const struct tty_port_operations vc_port_ops = {
> +	.destruct = vc_port_destruct,
> +};
> +
>  int vc_allocate(unsigned int currcons)	/* return 0 on success */
>  {
>  	struct vt_notifier_param param;
> @@ -1100,6 +1111,7 @@ int vc_allocate(unsigned int currcons)	/* return 0 on success */
>  
>  	vc_cons[currcons].d = vc;
>  	tty_port_init(&vc->port);
> +	vc->port.ops = &vc_port_ops;
>  	INIT_WORK(&vc_cons[currcons].SAK_work, vc_SAK);
>  
>  	visual_init(vc, currcons, 1);
> @@ -3250,6 +3262,7 @@ static int con_install(struct tty_driver *driver, struct tty_struct *tty)
>  
>  	tty->driver_data = vc;
>  	vc->port.tty = tty;
> +	tty_port_get(&vc->port);
>  
>  	if (!tty->winsize.ws_row && !tty->winsize.ws_col) {
>  		tty->winsize.ws_row = vc_cons[currcons].d->vc_rows;
> @@ -3285,6 +3298,13 @@ static void con_shutdown(struct tty_struct *tty)
>  	console_unlock();
>  }
>  
> +static void con_cleanup(struct tty_struct *tty)
> +{
> +	struct vc_data *vc = tty->driver_data;
> +
> +	tty_port_put(&vc->port);
> +}
> +
>  static int default_color           = 7; /* white */
>  static int default_italic_color    = 2; // green (ASCII)
>  static int default_underline_color = 3; // cyan (ASCII)
> @@ -3410,7 +3430,8 @@ static const struct tty_operations con_ops = {
>  	.throttle = con_throttle,
>  	.unthrottle = con_unthrottle,
>  	.resize = vt_resize,
> -	.shutdown = con_shutdown
> +	.shutdown = con_shutdown,
> +	.cleanup = con_cleanup,
>  };
>  
>  static struct cdev vc0_cdev;
> diff --git a/drivers/tty/vt/vt_ioctl.c b/drivers/tty/vt/vt_ioctl.c
> index 7297997fcf04c..f62f498f63c05 100644
> --- a/drivers/tty/vt/vt_ioctl.c
> +++ b/drivers/tty/vt/vt_ioctl.c
> @@ -310,10 +310,8 @@ static int vt_disallocate(unsigned int vc_num)
>  		vc = vc_deallocate(vc_num);
>  	console_unlock();
>  
> -	if (vc && vc_num >= MIN_NR_CONSOLES) {
> -		tty_port_destroy(&vc->port);
> -		kfree(vc);
> -	}
> +	if (vc && vc_num >= MIN_NR_CONSOLES)
> +		tty_port_put(&vc->port);
>  
>  	return ret;
>  }
> @@ -333,10 +331,8 @@ static void vt_disallocate_all(void)
>  	console_unlock();
>  
>  	for (i = 1; i < MAX_NR_CONSOLES; i++) {
> -		if (vc[i] && i >= MIN_NR_CONSOLES) {
> -			tty_port_destroy(&vc[i]->port);
> -			kfree(vc[i]);
> -		}
> +		if (vc[i] && i >= MIN_NR_CONSOLES)
> +			tty_port_put(&vc[i]->port);
>  	}
>  }
>  
> 


-- 
js
suse labs

Re: [PATCH v3 2/2] vt: vt_ioctl: fix use-after-free in vt_in_use()

[Jiri Slaby]

On 22. 03. 20, 4:43, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.com>
> 
> vt_in_use() dereferences console_driver->ttys[i] without proper locking.
> This is broken because the tty can be closed and freed concurrently.
> 
> We could fix this by using 'READ_ONCE(console_driver->ttys[i]) != NULL'
> and skipping the check of tty_struct::count.  But, looking at
> console_driver->ttys[i] isn't really appropriate anyway because even if
> it is NULL the tty can still be in the process of being closed.
> 
> Instead, fix it by making vt_in_use() require console_lock() and check
> whether the vt is allocated and has port refcount > 1.  This works since
> following the patch "vt: vt_ioctl: fix VT_DISALLOCATE freeing in-use
> virtual console" the port refcount is incremented while the vt is open.
> 
> Reproducer (very unreliable, but it worked for me after a few minutes):
> 
> 	#include <fcntl.h>
> 	#include <linux/vt.h>
> 
> 	int main()
> 	{
> 		int fd, nproc;
> 		struct vt_stat state;
> 		char ttyname[16];
> 
> 		fd = open("/dev/tty10", O_RDONLY);
> 		for (nproc = 1; nproc < 8; nproc *= 2)
> 			fork();
> 		for (;;) {
> 			sprintf(ttyname, "/dev/tty%d", rand() % 8);
> 			close(open(ttyname, O_RDONLY));
> 			ioctl(fd, VT_GETSTATE, &state);
> 		}
> 	}
> 
> KASAN report:
> 
> 	BUG: KASAN: use-after-free in vt_in_use drivers/tty/vt/vt_ioctl.c:48 [inline]
> 	BUG: KASAN: use-after-free in vt_ioctl+0x1ad3/0x1d70 drivers/tty/vt/vt_ioctl.c:657
> 	Read of size 4 at addr ffff888065722468 by task syz-vt2/132
> 
> 	CPU: 0 PID: 132 Comm: syz-vt2 Not tainted 5.6.0-rc5-00130-g089b6d3654916 #13
> 	Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ?-20191223_100556-anatol 04/01/2014
> 	Call Trace:
> 	 [...]
> 	 vt_in_use drivers/tty/vt/vt_ioctl.c:48 [inline]
> 	 vt_ioctl+0x1ad3/0x1d70 drivers/tty/vt/vt_ioctl.c:657
> 	 tty_ioctl+0x9db/0x11b0 drivers/tty/tty_io.c:2660
> 	 [...]
> 
> 	Allocated by task 136:
> 	 [...]
> 	 kzalloc include/linux/slab.h:669 [inline]
> 	 alloc_tty_struct+0x96/0x8a0 drivers/tty/tty_io.c:2982
> 	 tty_init_dev+0x23/0x350 drivers/tty/tty_io.c:1334
> 	 tty_open_by_driver drivers/tty/tty_io.c:1987 [inline]
> 	 tty_open+0x3ca/0xb30 drivers/tty/tty_io.c:2035
> 	 [...]
> 
> 	Freed by task 41:
> 	 [...]
> 	 kfree+0xbf/0x200 mm/slab.c:3757
> 	 free_tty_struct+0x8d/0xb0 drivers/tty/tty_io.c:177
> 	 release_one_tty+0x22d/0x2f0 drivers/tty/tty_io.c:1468
> 	 process_one_work+0x7f1/0x14b0 kernel/workqueue.c:2264
> 	 worker_thread+0x8b/0xc80 kernel/workqueue.c:2410
> 	 [...]
> 
> Fixes: 4001d7b7fc27 ("vt: push down the tty lock so we can see what is left to tackle")
> Cc: <stable@vger.kernel.org> # v3.4+
> Signed-off-by: Eric Biggers <ebiggers@google.com>

I cannot think of anything better with the current poor code state, so:

Acked-by: Jiri Slaby <jslaby@suse.cz>

thanks,
-- 
-- 
js
suse labs