fuse: break infinite loop in fuse_fill_write_pages()

This doesn't have a CVE ID yet.

debian/changelog

@@ -51,6 +51,9 @@ linux (4.3.4-1) UNRELEASED; urgency=medium
     - tcp: restore fastopen with no data in SYN packet
     - rhashtable: Fix walker list corruption
 
+  [ Ben Hutchings ]
+  * fuse: break infinite loop in fuse_fill_write_pages() (CVE-2015-XXXX)
+
  -- Ben Hutchings <ben@decadent.org.uk>  Sat, 23 Jan 2016 11:51:46 +0000
 
 linux (4.3.3-7) unstable; urgency=medium

debian/patches/bugfix/all/fuse-break-infinite-loop-in-fuse_fill_write_pages.patch

@@ -0,0 +1,56 @@
+From: Roman Gushchin <klamm@yandex-team.ru>
+Date: Mon, 12 Oct 2015 16:33:44 +0300
+Subject: fuse: break infinite loop in fuse_fill_write_pages()
+Origin: https://git.kernel.org/linus/3ca8138f014a913f98e6ef40e939868e1e9ea876
+
+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")
+Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
+---
+ fs/fuse/file.c | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+--- a/fs/fuse/file.c
++++ b/fs/fuse/file.c
+@@ -1049,6 +1049,7 @@ static ssize_t fuse_fill_write_pages(str
+ 		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);
+@@ -1061,7 +1062,6 @@ static ssize_t fuse_fill_write_pages(str
+ 		req->page_descs[req->num_pages].length = tmp;
+ 		req->num_pages++;
+ 
+-		iov_iter_advance(ii, tmp);
+ 		count += tmp;
+ 		pos += tmp;
+ 		offset += tmp;

debian/patches/series

@@ -140,3 +140,4 @@ bugfix/all/bcache-unregister-reboot-notifier-if-bcache-fails-to.patch
 bugfix/all/bcache-allows-use-of-register-in-udev-to-avoid-devic.patch
 bugfix/all/bcache-prevent-crash-on-changing-writeback_running.patch
 bugfix/all/bcache-change-refill_dirty-to-always-scan-entire-dis.patch
+bugfix/all/fuse-break-infinite-loop-in-fuse_fill_write_pages.patch