Re: scsi: use-after-free in bio_copy_from_iter

[Dmitry Vyukov]

On Tue, Dec 6, 2016 at 10:32 AM, Johannes Thumshirn <jthumshirn@xxxxxxx> wrote:
> On Mon, Dec 05, 2016 at 07:03:39PM +0000, Al Viro wrote:
>> On Mon, Dec 05, 2016 at 04:17:53PM +0100, Johannes Thumshirn wrote:
>> > 633         hp = &srp->header;
>> > [...]
>> > 646                 hp->dxferp = (char __user *)buf + cmd_size;
>>
>> > So the memory for hp->dxferp comes from:
>> > 633         hp = &srp->header;
>>
>> ????
>>
>> > >From my debug instrumentation I see that the dxferp ends up in the
>> > iovec_iter's kvec->iov_base and the faulting address is always dxferp + n *
>> > 4k with n in [1, 16] (and we're copying 16 4k pages from the iovec into the
>> > bio).
>>
>> _Address_ of hp->dxferp comes from that assignment; the value is 'buf'
>> argument of sg_write() + small offset.  In this case, it should point
>> inside a pipe buffer, which is, indeed, at a kernel address.  Who'd
>> allocated srp is irrelevant.
>
> Yes I realized that as well when I had enough distance between me and the
> code...
>
>>
>> And if you end up dereferencing more than one page worth there, you do have
>> a problem - pipe buffers are not going to be that large.  Could you slap
>>       WARN_ON((size_t)input_size > count);
>> right after the calculation of input_size in sg_write() and see if it triggers
>> on your reproducer?
>
> I did and it didn't trigger. What triggers is (as expected) a
>         WARN_ON((size_t)mxsize > count);
> We have count at 80 and mxsize (which ends in hp->dxfer_len) at 65499. But the
> 65499 bytes are the len of the data we're suppost to be copying in via the
> iov. I'm still rather confused what's happening here, sorry.


I think the critical piece here is some kind of race or timing
condition. Note that the test program executes all of
memfd_create/write/open/sendfile twice. Second time the calls race
with each other, but they also can race with the first execution of
the calls.