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Date: Thu, 22 Mar 2007 09:29:05 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Alexey Dobriyan <adobriyan@sw.ru>
Cc: alan@redhat.com, linux-kernel@vger.kernel.org, devel@openvz.org
Subject: Re: [PATCH] Protect tty drivers list a little
Message-Id: <20070322092905.02404ed5.akpm@linux-foundation.org>
In-Reply-To: <20070322112542.GA6814@localhost.sw.ru>
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On Thu, 22 Mar 2007 14:25:42 +0300 Alexey Dobriyan <adobriyan@sw.ru> wrote:

> Additions and removal from tty_drivers list were just done as well as
> iterating on it for /proc/tty/drivers generation.
> 
> testing: modprobe/rmmod loop of simple module which does nothing but
> tty_register_driver() vs cat /proc/tty/drivers loop
> 
> BUG: unable to handle kernel paging request at virtual address 6b6b6b6b
>  printing eip:
> c01cefa7
> *pde = 00000000
> Oops: 0000 [#1]
> PREEMPT
> last sysfs file: devices/pci0000:00/0000:00:1d.7/usb5/5-0:1.0/bInterfaceProtocol
> Modules linked in: ohci_hcd af_packet e1000 ehci_hcd uhci_hcd usbcore xfs
> CPU:    0
> EIP:    0060:[<c01cefa7>]    Not tainted VLI
> EFLAGS: 00010297   (2.6.21-rc4-mm1 #4)
> EIP is at vsnprintf+0x3a4/0x5fc
> eax: 6b6b6b6b   ebx: f6cb50f2   ecx: 6b6b6b6b   edx: fffffffe
> esi: c0354700   edi: f6cb6000   ebp: 6b6b6b6b   esp: f31f5e68
> ds: 007b   es: 007b   fs: 00d8  gs: 0033  ss: 0068
> Process cat (pid: 31864, ti=f31f4000 task=c1998030 task.ti=f31f4000)
> Stack: 00000000 c0103f20 c013003a c0103f20 00000000 f6cb50da 0000000a 00000f0e
>        f6cb50f2 00000010 00000014 ffffffff ffffffff 00000007 c0354753 f6cb50f2
>        f73e39dc f73e39dc 00000001 c0175416 f31f5ed8 f31f5ed4 0ee00000 f32090bc
> Call Trace:
>  [<c0103f20>] restore_nocheck+0x12/0x15
>  [<c013003a>] mark_held_locks+0x6d/0x86
>  [<c0103f20>] restore_nocheck+0x12/0x15
>  [<c0175416>] seq_printf+0x2e/0x52
>  [<c0192895>] show_tty_range+0x35/0x1f3
>  [<c0175416>] seq_printf+0x2e/0x52
>  [<c0192add>] show_tty_driver+0x8a/0x1d9
>  [<c01758f6>] seq_read+0x70/0x2ba
>  [<c0175886>] seq_read+0x0/0x2ba
>  [<c018d8e6>] proc_reg_read+0x63/0x9f
>  [<c015e764>] vfs_read+0x7d/0xb5
>  [<c018d883>] proc_reg_read+0x0/0x9f
>  [<c015eab1>] sys_read+0x41/0x6a
>  [<c0103e4e>] sysenter_past_esp+0x5f/0x99
>  =======================
> Code: 00 8b 4d 04 e9 44 ff ff ff 8d 4d 04 89 4c 24 50 8b 6d 00 81 fd ff 0f 00 00 b8 a4 c1 35 c0 0f 46 e8 8b 54 24 2c 89 e9 89 c8 eb 06 <80> 38 00 74 07 40 4a 83 fa ff 75 f4 29 c8 89 c6 8b 44 24 28 89
> EIP: [<c01cefa7>] vsnprintf+0x3a4/0x5fc SS:ESP 0068:f31f5e68
> 

nice.

> ---
> 
>  drivers/char/tty_io.c      |    9 ++++++++-
>  fs/proc/proc_tty.c         |    3 +++
>  include/linux/tty_driver.h |    2 ++
>  3 files changed, 13 insertions(+), 1 deletion(-)
> 
> --- a/drivers/char/tty_io.c
> +++ b/drivers/char/tty_io.c
> @@ -127,6 +127,7 @@ EXPORT_SYMBOL(tty_std_termios);
>     into this file */
>     
>  LIST_HEAD(tty_drivers);			/* linked list of tty drivers */
> +DEFINE_SPINLOCK(tty_drivers_lock);
>  
>  /* Mutex to protect creating and releasing a tty. This is shared with
>     vt.c for deeply disgusting hack reasons */
> @@ -1086,13 +1087,16 @@ static struct tty_driver *get_tty_driver
>  {
>  	struct tty_driver *p;
>  
> +	spin_lock(&tty_drivers_lock);
>  	list_for_each_entry(p, &tty_drivers, tty_drivers) {
>  		dev_t base = MKDEV(p->major, p->minor_start);
>  		if (device < base || device >= base + p->num)
>  			continue;
> +		spin_unlock(&tty_drivers_lock);
>  		*index = device - base;
>  		return p;
>  	}
> +	spin_unlock(&tty_drivers_lock);
>  	return NULL;
>  }

The locking in here is kinda meaningless: we drop the lock and return an
unrefcounted something which really should have been covered by that lock. 
Or refcounted.

The reason is that get_tty_driver() and its return value are already covered
by tty_mutex.

So can we use tty_mutex to fix this race rather than adding a new lock?