fuse: break infinite loop in fuse_fill_write_pages()
authorRoman Gushchin <klamm@yandex-team.ru>
Mon, 12 Oct 2015 13:33:44 +0000 (16:33 +0300)
committerMiklos Szeredi <miklos@szeredi.hu>
Tue, 10 Nov 2015 09:32:37 +0000 (10:32 +0100)
commit3ca8138f014a913f98e6ef40e939868e1e9ea876
treef8958826e39c9270e3786f773f9720114177770e
parent2c5816b4beccc8ba709144539f6fdd764f8fa49c
fuse: break infinite loop in fuse_fill_write_pages()

I got a report about unkillable task eating CPU. 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.

Fix this by calling iov_iter_advance() before repeating an attempt to
copy data from userspace.

A similar problem is described in 124d3b7041f ("fix writev regression:
pan hanging unkillable and un-straceable"). If 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.

Special thanks to Konstantin Khlebnikov, who helped a lot with the commit
description.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Maxim Patlasov <mpatlasov@parallels.com>
Cc: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: Roman Gushchin <klamm@yandex-team.ru>
Signed-off-by: Miklos Szeredi <miklos@szeredi.hu>
Fixes: ea9b9907b82a ("fuse: implement perform_write")
Cc: <stable@vger.kernel.org>
fs/fuse/file.c