Re: [fuse-devel] [PATCH] fuse: break infinite loop in fuse_fill_write_pages()

[Maxim Patlasov]

On 10/02/2015 06:58 PM, Konstantin Khlebnikov wrote:
> On Sat, Oct 3, 2015 at 1:04 AM, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> > On Fri, 2 Oct 2015 12:27:45 -0700 Maxim Patlasov <mpatlasov@xxxxxxxxxxxxx> wrote:
> >
> > > On 10/02/2015 04:21 AM, Konstantin Khlebnikov wrote:
> > > > Bump. Add more peopple in CC.
> > > >
> > > > On Mon, Sep 21, 2015 at 1:02 PM, Roman Gushchin <klamm@xxxxxxxxxxxxxx> wrote:
> > > > > I got a report about unkillable task eating CPU. Thge further
> > > > > investigation shows, that the problem is in the fuse_fill_write_pages()
> > > > > function. If iov's first segment has zero length, we get an infinite
> > > > > loop, because we never reach iov_iter_advance() call.
> > > iov_iter_copy_from_user_atomic() eventually calls iterate_iovec(). The
> > > latter silently consumes zero-length iov. So I don't think "iov's first
> > > segment has zero length" can cause infinite loop.
> > I'm suspecting it got stuck because local variable `bytes' is zero, so
> > the code does `goto again' repeatedly.
> >
> > Or maybe not.   A more complete description of the bug would help.
> I suspect here is the same scenario like in 124d3b7041f:
> Zero-length segmend is followed by segment with invalid address:
> iov_iter_fault_in_readable() checks only first segment (zero-length)
> iov_iter_copy_from_user_atomic() skips it, fails at second and
> returns zero -> goto again without skipping zero-length segment.
>
> Patch calls iov_iter_advance() before goto again: we'll skip zero-length
> segment at second iteraction and iov_iter_fault_in_readable() will detect
> invalid address.

Makes sense to me. The patch looks fine.

Thanks,
Maxim
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