[PATCH 3.12 122/142] fuse: break infinite loop in fuse_fill_write_pages()

From: Jiri Slaby
Date: Wed Feb 24 2016 - 05:24:11 EST

From: Roman Gushchin <klamm@xxxxxxxxxxxxxx>

3.12-stable review patch. If anyone has any objections, please let me know.


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

Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Maxim Patlasov <mpatlasov@xxxxxxxxxxxxx>
Cc: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
Signed-off-by: Roman Gushchin <klamm@xxxxxxxxxxxxxx>
Signed-off-by: Miklos Szeredi <miklos@xxxxxxxxxx>
Fixes: ea9b9907b82a ("fuse: implement perform_write")
Signed-off-by: Jiri Slaby <jslaby@xxxxxxx>
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 d08c108065e1..8ef52e12cd57 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -988,6 +988,7 @@ static ssize_t fuse_fill_write_pages(struct fuse_req *req,
tmp = iov_iter_copy_from_user_atomic(page, ii, offset, bytes);

+ iov_iter_advance(ii, tmp);
if (!tmp) {
@@ -1000,7 +1001,6 @@ static ssize_t fuse_fill_write_pages(struct fuse_req *req,
req->page_descs[req->num_pages].length = tmp;

- iov_iter_advance(ii, tmp);
count += tmp;
pos += tmp;
offset += tmp;