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Date:	Thu, 7 Aug 2014 13:22:01 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	Pavel Emelyanov <xemul@...allels.com>,
	LKML <linux-kernel@...r.kernel.org>, Jiri Slaby <jslaby@...e.cz>
Subject: Re: [PATCH] tty: Fix potential use after free in release_one_tty

On Thu, Aug 07, 2014 at 01:34:28PM +0400, Cyrill Gorcunov wrote:
> On Thu, Aug 07, 2014 at 01:18:12PM +0400, Cyrill Gorcunov wrote:
> > > mod = driver->owner;
> > > tty_driver_kref_put(driver);
> > > module_put(mod);
> > > 
> > > Check the upstream whether the same issue exists there.
> > 
> > Same in tty.git
> > 
> > static void release_one_tty(struct work_struct *work)
> > {
> ---
> From: Cyrill Gorcunov <gorcunov@...il.com>
> Subject: tty: Fix potential use after free in release_one_tty
> 
> In case if we're releasing the last tty reference the following
> call sequence is possible
> 
> tty_driver_kref_put
>   destruct_tty_driver
>     kfree(driver);
> 
> where @driver is used in next module_put call, which leads to
> 
>  | [ 285.964007] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
>  | [ 285.964007] Workqueue: events release_one_tty
>  | [ 285.964007] task: ffff8800cc7ea5f0 ti: ffff8800cb800000 task.ti: ffff8800cb800000
>  | [ 285.964007] RIP: 0010:[<ffffffff810aeaf5>] [<ffffffff810aeaf5>] module_put+0x24/0xf4
>  | [ 285.964007] RSP: 0018:ffff8800cb801d48 EFLAGS: 00010213
>  | [ 285.964007] RAX: ffff8800cb801fd8 RBX: ffff8800ca3429d0 RCX: ffff8800cb1db400
>  | [ 285.964007] RDX: 0000000000000000 RSI: ffffffff817349c1 RDI: 0000000000000001
>  | [ 285.964007] RBP: ffff8800cb801d60 R08: ffff8800cd632b40 R09: 0000000000000000
>  | [ 285.964007] R10: 00000000ffffffff R11: ffff88011f40a000 R12: 6b6b6b6b6b6b6b6b
>  | [ 285.964007] R13: ffff8800ca342520 R14: 0000000000000000 R15: ffff88011f5d8200
>  | [ 285.964007] FS: 0000000000000000(0000) GS:ffff88011f400000(0000) knlGS:0000000000000000
>  | [ 285.964007] CS: 0010 DS: 0000 ES: 0000 CR0: 000000008005003b
>  | [ 285.964007] CR2: 00007faf5229d090 CR3: 0000000001c0b000 CR4: 00000000000006f0
>  | [ 285.964007] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>  | [ 285.964007] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
>  | [ 285.964007] Stack:
>  | [ 285.964007] ffff8800ca3429d0 ffff8800ca342a30 ffff8800ca342520 ffff8800cb801d88
>  | [ 285.964007] ffffffff8146554a ffff8800cc77cc78 ffff8800ca3429d0 ffff88011f5d3800
>  | [ 285.964007] ffff8800cb801e08 ffffffff810683c1 ffffffff810682ff 0000000000000046
>  | [ 285.964007] Call Trace:
>  | [ 285.964007] [<ffffffff8146554a>] release_one_tty+0x54/0xa3
>  | [ 285.964007] [<ffffffff810683c1>] process_one_work+0x223/0x404
>  | [ 285.964007] [<ffffffff810682ff>] ? process_one_work+0x161/0x404
>  | [ 285.964007] [<ffffffff81068971>] worker_thread+0x136/0x205
>  | [ 285.964007] [<ffffffff8106883b>] ? rescuer_thread+0x26a/0x26a
>  | [ 285.964007] [<ffffffff8106e5bf>] kthread+0xa2/0xaa
>  | [ 285.964007] [<ffffffff810a4586>] ? trace_hardirqs_on_caller+0x16/0x1eb
>  | [ 285.964007] [<ffffffff8106e51d>] ? __kthread_parkme+0x65/0x65
>  | [ 285.964007] [<ffffffff8173f59c>] ret_from_fork+0x7c/0xb0
>  | [ 285.964007] [<ffffffff8106e51d>] ? __kthread_parkme+0x65/0x65
>  | [ 285.964007] Code: 09 00 5b 41 5c 5d c3 0f 1f 44 00 00 55 48 85 ff 48 89 e5 41 55 41 54 49 89 fc 53 0f 84 d3 00
>  | 00 00 bf 01 00 00 00 e8 d0 a1 fc ff <49> 8b 84 24 50 02 00 00 65 48 ff 40 08 4c 8b 6d 08 0f 1f 44 00
> 
> so simply keep a local reference to the module owner and
> use it later.
> 
> (Note I hit the bug on custom kernel but I think vanilla one suffers
>  the same problem).
> 
> CC: Pavel Emelyanov <xemul@...allels.com>
> CC: Jiri Slaby <jslaby@...e.cz>
> CC: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Signed-off-by: Cyrill Gorcunov <gorcunov@...nvz.org>
> ---
>  drivers/tty/tty_io.c |    3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> Index: linux-2.6.git/drivers/tty/tty_io.c
> ===================================================================
> --- linux-2.6.git.orig/drivers/tty/tty_io.c
> +++ linux-2.6.git/drivers/tty/tty_io.c
> @@ -1559,13 +1559,14 @@ static void release_one_tty(struct work_
>  	struct tty_struct *tty =
>  		container_of(work, struct tty_struct, hangup_work);
>  	struct tty_driver *driver = tty->driver;
> +	struct module *owner = driver->owner;
>  
>  	if (tty->ops->cleanup)
>  		tty->ops->cleanup(tty);
>  
>  	tty->magic = 0;
>  	tty_driver_kref_put(driver);
> -	module_put(driver->owner);
> +	module_put(owner);
>  
>  	spin_lock(&tty_files_lock);
>  	list_del_init(&tty->tty_files);

Looks good, thanks, can you resend it in a format I can apply it in (one
that doesn't require me to edit it by hand)?  Then I'll be glad to queue
it up after 3.17-rc1 is out.

thanks,

greg k-h
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