linux-kernel - Re: [PATCH 1/2] drm/i915: fix integer overflow in i915_gem

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Date:	Fri, 6 Apr 2012 10:01:36 -0400
From:	Xi Wang <xi.wang@...il.com>
To:	Chris Wilson <chris@...is-wilson.co.uk>
Cc:	Keith Packard <keithp@...thp.com>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH 1/2] drm/i915: fix integer overflow in i915_gem_execbuffer2()

On Apr 6, 2012, at 9:54 AM, Chris Wilson wrote:

> On Fri, 6 Apr 2012 09:46:46 -0400, Xi Wang <xi.wang@...il.com> wrote:
>> On Apr 6, 2012, at 9:36 AM, Chris Wilson wrote:
>> 
>>> On Fri,  6 Apr 2012 08:58:18 -0400, Xi Wang <xi.wang@...il.com> wrote:
>>>> A large args->buffer_count from userspace may overflow the allocation
>>>> size, leading to out-of-bounds access.
>>>> 
>>>> Use kmalloc_array() to avoid that.
>>> 
>>> I can safely say that exec list larger than 4GiB is going to be an
>>> illegal operation and would rather the ioctl failed outright with
>>> EINVAL.
>> 
>> On 32-bit platform?
> 
> On any platform. The largest it can legally be is a few tens of megabytes.

IDGI.  First we come to i915_gem_execbuffer2() from ioctl:

  exec2_list = kmalloc(sizeof(*exec2_list)*args->buffer_count, ...);

args->buffer_count is passed from userspace so it can be any value.
Let it overflow the 32-bit multiplication and turn the call to:

  exec2_list = kmalloc(0, ...);

Then the subsequent call to i915_gem_do_execbuffer(..., exec2_list)
may read exec2_list, which is out of bounds.

- xi

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