linux-kernel - Re: [PATCH] drm/radeon: fix NULL pointer dereference in UMS mode in radeon_cs_parser

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Date:	Wed, 23 Jan 2013 12:21:29 -0500
From:	Ilija Hadzic <ilijahadzic@...il.com>
To:	Herton Ronaldo Krzesinski <herton.krzesinski@...onical.com>
Cc:	Shuah Khan <shuah.khan@...com>,
	Greg KH <gregkh@...uxfoundation.org>, jglisse@...hat.com,
	dri-devel@...ts.freedesktop.org,
	LKML <linux-kernel@...r.kernel.org>,
	"Deucher, Alexander" <alexander.deucher@....com>,
	shuahkhan@...il.com, airlied@...hat.com
Subject: Re: [PATCH] drm/radeon: fix NULL pointer dereference in UMS mode in radeon_cs_parser_fini()

On Wed, Jan 23, 2013 at 11:07 AM, Herton Ronaldo Krzesinski
<herton.krzesinski@...onical.com> wrote:
> On Thu, Jan 17, 2013 at 10:09:42AM -0700, Shuah Khan wrote:
>> On Wed, 2013-01-16 at 21:06 -0600, Ilija Hadzic wrote:
>> > Actually, the code path affected by your patch is not executed in UMS mode
>> > at all. Notice that radeon_cs_parser_fini is only called from
>> > radeon_cs_ioctl which is a KMS-only ioctl (see radeon_kms.c).
>> >
>> > The equivalent of the fix you are trying to do is in
>> > a6b7e1a02b77ab8fe8775d20a88c53d8ba55482e (function patched by that one is
>> > the one used by legacy-CS ioctl), which you should go together
>> > with ff4bd0827764e10a428a9d39e6814c5478863f94 if you are backporting UMS
>> > fixes to 3.7. Both are needed to prevent kernel crashes in UMS mode.
>> >
>> > -- Ilija
>>
>> Thanks. I will take a look at a6b7e1a02b77ab8fe8775d20a88c53d8ba55482e.
>> I sent back-ported ff4bd0827764e10a428a9d39e6814c5478863f94 patch to
>> stable and I will back-port and send
>> a6b7e1a02b77ab8fe8775d20a88c53d8ba55482e as well.
>
> While at it, also looks like commit
> 25d8999780f8c1f53928f4a24a09c01550423109 could also be added to stables.
> But while looking at it, while the patch itself is correct, I noted that
> there is a possibility of double free: if either of the
> p->chunks[p->chunk_ib_idx].kpage[] items are non NULL, we will free it in
> radeon_cs_parser_init and also when calling one of the fini functions
> (radeon_cs_parser_fini or r600_cs_parser_fini). So either we need to set
> kpage[n] to NULL after kfree, or just return the error.
>

Yes you are right. The error-handling path should force both kpage[]
pointers to NULL before returning -ENOMEM. That would fix the double
free.

Regarding, inclusion of 25d8999780f8c1f53928f4a24a09c01550423109 into
stable (and possible follow-up patch to fix potential double-kfree),
does this pass the "no fixes for theoretical problems in stable"
criterion ?

It is an obvious bug, but it happens so rarely that I am not surprised
that it has never happened: You need an (old) AGP card *and* you need
to run out of kmalloc memory.

-- Ilija
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