linux-kernel - Re: usercopy whitelist woe in scsi_sense

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Date:   Tue, 17 Apr 2018 14:28:46 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Kees Cook <keescook@...omium.org>,
        Oleksandr Natalenko <oleksandr@...alenko.name>,
        Bart Van Assche <bart.vanassche@....com>,
        Paolo Valente <paolo.valente@...aro.org>
Cc:     David Windsor <dave@...lcore.net>,
        "James E.J. Bottomley" <jejb@...ux.vnet.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        linux-scsi@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Christoph Hellwig <hch@....de>,
        Hannes Reinecke <hare@...e.com>,
        Johannes Thumshirn <jthumshirn@...e.de>,
        linux-block@...r.kernel.org,
        Paolo Valente <paolo.valente@...aro.org>
Subject: Re: usercopy whitelist woe in scsi_sense_cache

On 4/17/18 2:25 PM, Kees Cook wrote:
> On Tue, Apr 17, 2018 at 1:20 PM, Kees Cook <keescook@...omium.org> wrote:
>> On Tue, Apr 17, 2018 at 1:03 PM, Kees Cook <keescook@...omium.org> wrote:
>>> The above bfq_dispatch_request+0x99/0xad0 is still
>>> __bfq_dispatch_request at block/bfq-iosched.c:3902, just with KASAN
>>> removed. 0x99 is 153 decimal:
>>>
>>> (gdb) disass bfq_dispatch_request
>>> Dump of assembler code for function bfq_dispatch_request:
>>> ...
>>>    0xffffffff8134b2ad <+141>:   test   %rax,%rax
>>>    0xffffffff8134b2b0 <+144>:   je     0xffffffff8134b2bd
>>> <bfq_dispatch_request+157>
>>>    0xffffffff8134b2b2 <+146>:   addl   $0x1,0x100(%rax)
>>>    0xffffffff8134b2b9 <+153>:   addl   $0x1,0x3c(%rbx)
>>>    0xffffffff8134b2bd <+157>:   orl    $0x2,0x18(%r12)
>>>    0xffffffff8134b2c3 <+163>:   test   %ebp,%ebp
>>>    0xffffffff8134b2c5 <+165>:   je     0xffffffff8134b2ce
>>> <bfq_dispatch_request+174>
>>>    0xffffffff8134b2c7 <+167>:   mov    0x108(%r14),%rax
>>>    0xffffffff8134b2ce <+174>:   mov    %r15,%rdi
>>>    0xffffffff8134b2d1 <+177>:   callq  0xffffffff81706f90 <_raw_spin_unlock_irq>
>>>
>>> Just as a sanity-check, at +157 %r12 should be rq, rq_flags is 0x18
>>> offset from, $0x2 is RQF_STARTED, so that maps to "rq->rq_flags |=
>>> RQF_STARTED", the next C statement. I don't know what +146 is, though?
>>> An increment of something 256 bytes offset? There's a lot of inline
>>> fun and reordering happening here, so I'm ignoring that for the
>>> moment.
>>
>> No -- I'm reading this wrong. The RIP is the IP _after_ the trap, so
>> +146 is the offender.
>>
>> [   29.284746] watchpoint @ ffff95d41a0fe580 triggered
>> [   29.285349] sense before:ffff95d41f45f700 after:ffff95d41f45f701 (@ffff95d41a
>> 0fe580)
>> [   29.286176] elevator before:ffff95d419419c00 after:ffff95d419419c00
>> [   29.286847] elevator_data before:ffff95d419418c00 after:ffff95d419418c00
>> ...
>> [   29.295069] RIP: 0010:bfq_dispatch_request+0x99/0xbb0
>> [   29.295622] RSP: 0018:ffffb26e01707a40 EFLAGS: 00000002
>> [   29.296181] RAX: ffff95d41a0fe480 RBX: ffff95d419418c00 RCX: ffff95d419418c08
>>
>> RAX is ffff95d41a0fe480 and sense is stored at ffff95d41a0fe580,
>> exactly 0x100 away.
>>
>> WTF is this addl?
> 
> What are the chances? :P Two ++ statements in a row separate by a
> collapsed goto. FML. :)
> 
> ...
>                         bfqq->dispatched++;
>                         goto inc_in_driver_start_rq;
> ...
> inc_in_driver_start_rq:
>                 bfqd->rq_in_driver++;
> ...
> 
> And there's the 0x100 (256):
> 
> struct bfq_queue {
> ...
>         int                        dispatched;           /*   256     4 */
> 
> So bfqq is corrupted somewhere... I'll keep digging. I hope you're all
> enjoying my live debugging transcript. ;)

It has to be the latter bfqq->dispatched increment, as those are
transient (and bfqd is not).

Adding Paolo.

-- 
Jens Axboe