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SubjectRe: usercopy whitelist woe in scsi_sense_cache
On Tue, Apr 17, 2018 at 1:20 PM, Kees Cook <keescook@chromium.org> wrote:
> On Tue, Apr 17, 2018 at 1:03 PM, Kees Cook <keescook@chromium.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

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