Re: Potentially unbounded allocations in seq_read?

Tvrtko Ursulin
2013-12-12T13:40:52Z

On Wed, 2013-12-11 at 18:07 +0000, Al Viro wrote:
> On Wed, Dec 11, 2013 at 05:59:57PM +0000, Tvrtko Ursulin wrote:
> > On Wed, 2013-12-11 at 17:49 +0000, Al Viro wrote:
> > > 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.
> >
> > Yeah, no, wrong assumption. It was not about the size but the number of
> > reads. For example:
> >
> > open("/proc/3131/smaps", O_RDONLY|O_LARGEFILE) = 3
> > fstat64(1, {st_mode=S_IFCHR|0666, st_rdev=makedev(1, 3), ...}) = 0
> > brk(0xf9019000) = 0xf9019000
> > read(3, "be483000-be4ab000 rw-s 00000000 "..., 4096) = 4054
> > write(1, "be483000-be4ab000 rw-s 00000000 "..., 4054) = 4054
> > read(3, "be5c3000-be5c5000 rw-s 10da3f000"..., 4096) = 3656
> > write(1, "be5c3000-be5c5000 rw-s 10da3f000"..., 3656) = 3656
> > read(3, "be6b1000-be6b2000 rw-s 10da48000"..., 4096) = 3661
> > write(1, "be6b1000-be6b2000 rw-s 10da48000"..., 3661) = 3661
> > read(3, "be6b9000-be6ba000 rw-s 10da38000"..., 4096) = 3599
> > write(1, "be6b9000-be6ba000 rw-s 10da38000"..., 3599) = 3599
> > read(3, "be6d6000-be7b6000 rw-s 10d889000"..., 4096) = 3599
> > write(1, "be6d6000-be7b6000 rw-s 10d889000"..., 3599) = 3599
> > read(3, "be884000-be885000 rw-s 10d85c000"..., 4096) = 3661
> > write(1, "be884000-be885000 rw-s 10d85c000"..., 3661) = 3661
> > read(3, "be88d000-be8de000 rw-p 00000000 "..., 4096) = 4007
> > write(1, "be88d000-be8de000 rw-p 00000000 "..., 4007) = 4007
> > read(3, "bea29000-bea4d000 r-xp 00000000 "..., 4096) = 4057
> > write(1, "bea29000-bea4d000 r-xp 00000000 "..., 4057) = 4057
> > read(3, "beab3000-bead0000 r--p 00000000 "..., 4096) = 2092
> > write(1, "beab3000-bead0000 r--p 00000000 "..., 2092) = 2092
> > read(3, 0xf9017030, 4096) = -1 ENOMEM (Out of memory)
>
> OK, so you've got ENOMEM somewhere, but where had it come from?
> The buffer from previous read() ought to have sufficed for this one,
> unless the next entry had been much longer than usual...

So this is the story... task_mmu.c:show_map_vma() calls seq_path. There
we have a d_path call which returns -ENAMETOOLONG and keeps doing so
even though the buffer grows to huge proportions. It is something on
tmpfs, don't know what.

But in the meantime, shouldn't seq_path be a bit more considerate on
this particular error and not mark the state as "could not fit" forever?
Perhaps it would make sense to limit it a bit?

Or even more so, on errors _other_ than -ENAMETOOLONG it will at the
moment mark the result as "need more space". That also sounds broken to
me.

Regards,

Tvrtko