linux-kernel - Re: [PATCH] proc: fix null pointer deref in proc_pid

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Message-ID: <CAJn8CcEcy5CQHD4UhrOaBYU+yDnMQ5aTs1YGOqrOTB2JgM4=sw@mail.gmail.com>
Date:	Thu, 12 Jan 2012 10:45:30 +0800
From:	Xiaotian Feng <xtfeng@...il.com>
To:	Kees Cook <keescook@...omium.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Xiaotian Feng <dannyfeng@...cent.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Vasiliy Kulikov <segoon@...nwall.com>,
	Stephen Wilson <wilsons@...rt.ca>,
	David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH] proc: fix null pointer deref in proc_pid_permission()

On Thu, Jan 12, 2012 at 6:41 AM, Kees Cook <keescook@...omium.org> wrote:
> On Wed, Jan 11, 2012 at 1:58 PM, Andrew Morton
> <akpm@...ux-foundation.org> wrote:
>> On Wed, 11 Jan 2012 12:43:30 -0800
>> Kees Cook <keescook@...omium.org> wrote:
>>> On Wed, Jan 11, 2012 at 01:47:05PM -0500, Xiaotian Feng wrote:
>>> > get_proc_task() can fail to search the task and return NULL, put_task_struct()
>>> > will then bomb the kernel with following oops:
>>> >
>>> > [ 1870.574045] BUG: unable to handle kernel NULL pointer dereference at 0000000000000010
>>> > [ 1870.574065] IP: [<ffffffff81217d34>] proc_pid_permission+0x64/0xe0
>>> > [ 1870.574088] PGD 112075067 PUD 112814067 PMD 0
>>> > [ 1870.574106] Oops: 0002 [#1] PREEMPT SMP
>>> >
>>> > This is a regression introduced by commit 0499680a, kernel should
>>> > return -ESRCH if get_proc_task() failed.
>>>
>>> Nice catch!
>>>
>>> However since this error is returned to userspace, shouldn't this be
>>> -ENOENT instead?
>>
>> Failed get_proc_task() frequently results in -ESRCH.  And less
>> frequently results in -ENOENT.
>>
>> It seems odd that inode_operations.permission() would ever return
>> anything other than zero or -EPERM.
>
> Right, but won't this show up at ESRCH from open(2)? If this is used
> as-is, we just need to have the manpages updated.
>

You're right, some of get_proc_task() returns -ENOENT.  Maybe we should
return -ENOENT to avoid breaking userspace tools. Andrew?

> -Kees
>
> --
> Kees Cook
> ChromeOS Security
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