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>
Conflicts:
	fs/fuse/file.c

Change-Id: Id37193373294dd43191469389cfe68ca1736a54b

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index 585f3f18c7585723112130404fc7f1177f22f46c..063ce05a30344cf75ba8966761925a09e9292383 100644 (file)
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -1031,6 +1031,7 @@ static ssize_t fuse_fill_write_pages(struct fuse_req *req,
 
                mark_page_accessed(page);
 
+               iov_iter_advance(ii, tmp);
                if (!tmp) {
                        unlock_page(page);
                        page_cache_release(page);
@@ -1043,7 +1044,6 @@ static ssize_t fuse_fill_write_pages(struct fuse_req *req,
                req->page_descs[req->num_pages].length = tmp;
                req->num_pages++;
 
-               iov_iter_advance(ii, tmp);
                count += tmp;
                pos += tmp;
                offset += tmp;