linux-kernel - Re: vfs: INFO: possible circular locking dependency detected

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Message-ID: <CA+1xoqfdM8ZRNB+vYqYo+-9N9rhy2CjXU=qYwCLu0=SYBu00xg@mail.gmail.com>
Date:	Wed, 9 May 2012 18:36:17 +0200
From:	Sasha Levin <levinsasha928@...il.com>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	linux-fsdevel@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: vfs: INFO: possible circular locking dependency detected

On Wed, May 9, 2012 at 6:28 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> On Wed, May 09, 2012 at 06:23:30PM +0200, Sasha Levin wrote:
>> On Wed, May 9, 2012 at 6:12 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
>> > On Wed, May 09, 2012 at 05:25:14PM +0200, Sasha Levin wrote:
>> >> Hi all,
>> >>
>> >> I've started seeing the following warning while fuzzing inside a KVM guest with the latest -next:
>> > ? ? ? ?It's not a realistic attack, fortunately, since you need root
>> > to get past open_exec() on any of those... ?Wait. ?How _did_ you get
>> > past open_exec(), anyway? ?MAY_EXEC is not supposed to be granted on
>> > anything that has no exec bits at all and AFAICS none of those files
>> > have them.
>>
>> You could chmod +x and run them, no?
>
> Can't.  proc_setattr() will give you -EPERM and refuse to do anything
> if you call it with ATTR_MODE in ->ia_valid.

If we look at /proc/irq/*/smp_affinity, which uses seq file ops, we can do this:

sh-4.2# ls -al /proc/irq/5/smp_affinity
-rw------- 1 root 0 0 May  9 16:35 /proc/irq/5/smp_affinity
sh-4.2# chmod +x /proc/irq/5/smp_affinity
sh-4.2# ls -al /proc/irq/5/smp_affinity
-rwx--x--x 1 root 0 0 May  9 16:35 /proc/irq/5/smp_affinity
sh-4.2# /proc/irq/5/smp_affinity
/proc/irq/5/smp_affinity: line 1: 1f: command not found

There are quite a lot of files under /proc that let me do that.
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