Re: scsi: use-after-free in bio_copy_from_iter

[Dmitry Vyukov <dvyukov@google.com>]

On Tue, Dec 6, 2016 at 4:38 PM, Johannes Thumshirn <jthumshirn@suse.de> wrote:
> On Tue, Dec 06, 2016 at 10:43:57AM +0100, Dmitry Vyukov wrote:
>> On Tue, Dec 6, 2016 at 10:32 AM, Johannes Thumshirn <jthumshirn@suse.de> 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.
>
> FWIW I've just run the reproducer once instead of looping it to check how it
> would normally behave and it bailes out at:
>
> 604         if (count < (SZ_SG_HEADER + 6))
> 605                 return -EIO;    /* The minimum scsi command length is 6 bytes. */
>
> That means, weren't going down the copy_form_iter() road at all. Usually, but
> sometimes we do. And then we try to copy 16 pages from the pipe buffer (is
> this correct?).
> The reproducer does: sendfile("/dev/sg0", memfd, offset_in_memfd, 0x10000);
>
> I don't see how we get there? Could it be random data from the mmap() we point
> the memfd to?
>
> This bug is confusing to be honest.


Where does this count come from? What address in the user program? Is
it 0x20012fxx?
One possibility for non-deterministically changing inputs is that this part:

  case 2:
    NONFAILING(*(uint32_t*)0x20012fd8 = (uint32_t)0x28);
    NONFAILING(*(uint32_t*)0x20012fdc = (uint32_t)0xffff);
    NONFAILING(*(uint64_t*)0x20012fe0 = (uint64_t)0x0);
    NONFAILING(*(uint64_t*)0x20012fe8 = (uint64_t)0xffffffffffff993f);
    NONFAILING(*(uint64_t*)0x20012ff0 = (uint64_t)0xa8b);
    NONFAILING(*(uint32_t*)0x20012ff8 = (uint32_t)0xff);
    r[9] = syscall(__NR_write, r[2], 0x20012fd8ul, 0x28ul, 0, 0,
                           0, 0, 0, 0);

runs concurrently with this part:

  case 0:
    r[0] =
        syscall(__NR_mmap, 0x20000000ul, 0x13000ul, 0x3ul,
                        0x32ul, 0xfffffffffffffffful, 0x0ul, 0, 0, 0);

So all of the input data to the write, or a subset of the input data,
can be zeros.