Re: Potentially unbounded allocations in seq_read?

Al Viro
2013-12-11T17:49:14Z

On Wed, Dec 11, 2013 at 05:04:41PM +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)

An entry in /proc/<pid>/smaps that did not fit into 2Mb? Seriously?
How in hell has that happened? If you can trigger that at will, please
post the reproducer.

> >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?
>
> If that is true then only way to fix it would be to completely re-design
> the seq_file interface, just silencing the allocation failure with
> __GFP_NOWARN perhaps as a temporary measure.
>
> As an alternative, since it does sound a bit pathological, perhaps users
> for seq_file who know can be printing out such huge amounts of text
> should just use a different (new?) facility?

If a seq_file user is attempting to spew a couple of megs of text in one
->show() call, there's definitely something misused. Either they ought
to use a different iterator (might be feasible if that monster entry is
produced by some kind of loop) or just not use seq_file at all.

I'm very surprised that /proc/*/smaps has managed to step into that,
though - show_pid_smap() shouldn't be able to do so, AFAICS...