[3.16.y-ckt stable] Patch "fuse: break infinite loop in fuse_fill_write_pages()" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    fuse: break infinite loop in fuse_fill_write_pages()

to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.16.y-queue

This patch is scheduled to be released in version 3.16.7-ckt23.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.16.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

---8<------------------------------------------------------------

>From e595f1d352d3531f5295b45ce7feab29d483ec69 Mon Sep 17 00:00:00 2001
From: Roman Gushchin <klamm at yandex-team.ru>
Date: Mon, 12 Oct 2015 16:33:44 +0300
Subject: fuse: break infinite loop in fuse_fill_write_pages()

commit 3ca8138f014a913f98e6ef40e939868e1e9ea876 upstream.

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 at linux-foundation.org>
Cc: Maxim Patlasov <mpatlasov at parallels.com>
Cc: Konstantin Khlebnikov <khlebnikov at yandex-team.ru>
Signed-off-by: Roman Gushchin <klamm at yandex-team.ru>
Signed-off-by: Miklos Szeredi <miklos at szeredi.hu>
Fixes: ea9b9907b82a ("fuse: implement perform_write")
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 fs/fuse/file.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index caa8d95b24e8..e2a2c14a90ee 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -1088,6 +1088,7 @@ static ssize_t fuse_fill_write_pages(struct fuse_req *req,
 		tmp = iov_iter_copy_from_user_atomic(page, ii, offset, bytes);
 		flush_dcache_page(page);

+		iov_iter_advance(ii, tmp);
 		if (!tmp) {
 			unlock_page(page);
 			page_cache_release(page);
@@ -1100,7 +1101,6 @@ 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;