Subject: Re: [PATCH] drm/panfrost: Avoid user size passed to kvmalloc()
To:     Dan Carpenter <dan.carpenter@oracle.com>
Cc:     Rob Herring <robh@kernel.org>, Daniel Vetter <daniel@ffwll.ch>,
        David Airlie <airlied@linux.ie>,
        Tomeu Vizoso <tomeu.vizoso@collabora.com>,
        Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>,
        Boris Brezillon <boris.brezillon@collabora.com>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
References: <20211216161603.983711-1-steven.price@arm.com>
 <CAL_JsqKZBsJxy8h5EQf0wwfioWS-Kx9i=0cQ7p4FHckEXstGiw@mail.gmail.com>
 <4c564c0d-7702-9dfe-910f-969fe130aba8@arm.com> <20211217091046.GG1978@kadam>
 <4e56a7a8-c48c-49dc-6535-730ad871d1e1@arm.com> <20211217092817.GH1978@kadam>
From:   Steven Price <steven.price@arm.com>
Message-ID: <68709772-b515-f4ab-676f-2bf1f510dfe6@arm.com>
Date:   Fri, 17 Dec 2021 09:48:13 +0000
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On 17/12/2021 09:28, Dan Carpenter wrote:
> On Fri, Dec 17, 2021 at 09:16:19AM +0000, Steven Price wrote:
>> On 17/12/2021 09:10, Dan Carpenter wrote:
>>> On Fri, Dec 17, 2021 at 08:55:50AM +0000, Steven Price wrote:
>>>> However this one is harder to fix without setting an arbitrary cap on
>>>> the number of BOs during a sumbit. I'm not sure how other drivers handle
>>>> this - the ones I've looked at so far all have the same issue. There's
>>>> obviously the list that Dan already sent, but e.g. msm has the same bug
>>>> in msm_gem_submit.c:submit_create() with an amusing bug where the check
>>>> for (sz > SIZE_MAX) will never hit, although the call is to kzalloc() so
>>>> large allocations are going to fail anyway.
>>>
>>> sz is u64 and SIZE_MAX is ULONG_MAX so the (sz > SIZE_MAX) condition
>>> does work to prevent an integer overflow on 32bit systems.  But it's not
>>> beautiful.
>>
>> sz is the result of struct_size() which returns a size_t, and SIZE_MAX
>> in case of an overflow.
> 
> Correct.
> 
>> However the check is *greater than* SIZE_MAX
>> which will never occur even on 32 bit systems.
> 
> You've missed a part.  We add ((u64)nr_cmds * sizeof(submit->cmd[0]))
> to SIZE_MAX.  If nr_cmds is zero then, whatever, the kzmalloc() will
> fail.  No big deal.  But if it's non-zero then "sz" is larger than
> SIZE_MAX and we allocate a smaller buffer than expected leading to
> memory corruption.

Ah, my bracket matching is obviously off today - somehow I hadn't
noticed that the second line wasn't part of the call to struct_size().

> Btw, it turns out that I had a hand in writing that check so hooray for
> me.  :)  #HoorayForMe

Indeed hooray for you! ;) Although it still all seems like a very
round-a-bout way of enforcing an arbitrary maximum on the size!

Thanks,

Steve