Re: [PATCH] blk-mq: Clear out elevator private data

[Kees Cook]

On Tue, Apr 17, 2018 at 2:45 PM, Jens Axboe <axboe@xxxxxxxxx> wrote:
> On 4/17/18 3:42 PM, Kees Cook wrote:
>> Some elevators may not correctly check rq->rq_flags & RQF_ELVPRIV, and
>> may attempt to read rq->elv fields. When requests got reused, this
>> caused BFQ to think it already had a bfqq (rq->elv.priv[1]) allocated.
>> This could lead to odd behaviors like having the sense buffer address
>> slowly start incrementing. This eventually tripped HARDENED_USERCOPY
>> and KASAN.
>>
>> This patch wipes all of rq->elv instead of just rq->elv.icq. While
>> it shouldn't technically be needed, this ends up being a robustness
>> improvement that should lead to at least finding bugs in elevators faster.
>
> Comments from the other email still apply, we should not need to do this
> full memset() for every request. From a quick look, BFQ needs to straighten
> out its usage of prepare request and interactions with insert_request.

Sure, understood. I would point out, FWIW, that memset() gets unrolled
by the compiler and this is just two more XORs in the same cacheline
(the two words following icq). (And there is SO much more being
cleared during alloc, it didn't seem like hardly any extra cost vs the
robustness it provided.)

-Kees

-- 
Kees Cook
Pixel Security