linux-kernel - Re: usercopy whitelist woe in scsi_sense

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Date:   Tue, 17 Apr 2018 13:25:49 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Oleksandr Natalenko <oleksandr@...alenko.name>,
        Jens Axboe <axboe@...nel.dk>,
        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
Subject: Re: usercopy whitelist woe in scsi_sense_cache

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. ;)

-Kees

-- 
Kees Cook
Pixel Security