[PATCH v2] Btrfs: fix null pointer dereference on compressed write path error

[fdmanana@kernel.org]

From: Filipe Manana <fdmanana@suse.com>

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")
Signed-off-by: Filipe Manana <fdmanana@suse.com>
---

V2: Updated changelog.

 fs/btrfs/inode.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 66c6c4103d2f..d6b61b1facdd 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -503,6 +503,7 @@ static noinline void compress_file_range(struct inode *inode,
 		pages = kcalloc(nr_pages, sizeof(struct page *), GFP_NOFS);
 		if (!pages) {
 			/* just bail out to the uncompressed code */
+			nr_pages = 0;
 			goto cont;
 		}
 
-- 
2.11.0