[PATCH 3.16 097/305] Btrfs: fix null pointer dereference on compressed write path error

[Ben Hutchings]

3.16.63-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Filipe Manana <fdmanana@xxxxxxxx>

commit 3527a018c00e5dbada2f9d7ed5576437b6dd5cfb upstream.

At inode.c:compress_file_range(), under the "free_pages_out" label, we can
end up dereferencing the "pages" pointer when it has a NULL value. This
case happens when "start" has a value of 0 and we fail to allocate memory
for the "pages" pointer. When that happens we jump to the "cont" label and
then enter the "if (start == 0)" branch where we immediately call the
cow_file_range_inline() function. If that function returns 0 (success
creating an inline extent) or an error (like -ENOMEM for example) we jump
to the "free_pages_out" label and then access "pages[i]" leading to a NULL
pointer dereference, since "nr_pages" has a value greater than zero at
that point.

Fix this by setting "nr_pages" to 0 when we fail to allocate memory for
the "pages" pointer.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=201119
Fixes: 771ed689d2cd ("Btrfs: Optimize compressed writeback and reads")
Reviewed-by: Liu Bo <bo.liu@xxxxxxxxxxxxxxxxx>
Signed-off-by: Filipe Manana <fdmanana@xxxxxxxx>
Reviewed-by: David Sterba <dsterba@xxxxxxxx>
Signed-off-by: David Sterba <dsterba@xxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 fs/btrfs/inode.c | 1 +
 1 file changed, 1 insertion(+)

--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -452,6 +452,7 @@ again:
 		pages = kzalloc(sizeof(struct page *) * nr_pages, GFP_NOFS);
 		if (!pages) {
 			/* just bail out to the uncompressed code */
+			nr_pages = 0;
 			goto cont;
 		}