Re: scsi: use-after-free in bio_copy_from_iter

[Johannes Thumshirn <jthumshirn@suse.de>]

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.

-- 
Johannes Thumshirn                                          Storage
jthumshirn@suse.de                                +49 911 74053 689
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