Re: [PATCH] drm/panfrost: Avoid user size passed to kvmalloc()

[Steven Price <steven.price@arm.com>]

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