Re: Potentially unbounded allocations in seq_read?

Tvrtko Ursulin
2013-12-11T17:48:50Z

On Wed, 2013-12-11 at 17:04 +0000, Tvrtko Ursulin wrote:
> Hi all,
>
> It seems that the buffer allocation in seq_read can double in size
> indefinitely, at least I've seen that in practice with /proc/<pid>/smaps
> (attempting to double m->size to 4M on a read of 1000 bytes). This
> produces an ugly WARN_ON_ONCE, which should perhaps be avoided? (given
> that it can be triggered by userspace at will)
>
> From the top comment in seq_file.c one would think that it is a
> fundamental limitation of the current code that everything which will be
> read (even if in chunks) needs to be in the kernel side buffer at the
> same time?

Oh-oh, seems that m->size is doubled on every read. So if app is reading
with a buffer smaller than data available, it can do nine reads before
it hits a >MAX_ORDER allocation. Not good. :)

Regards,

Tvrtko