[PATCH] btrfs: fix a overflowing boundary writing in csum_tree_block

[<rongqing.li@windriver.com>]

From: Li RongQing <roy.qing.li@gmail.com>

It is impossible that csum_size is larger than sizeof(long), but the codes
still add the handler for this condition, like allocate new memory, for
extension. If it becomes true someday, copying csum_size size memory to local
32bit variable found and val will overflow these two variables.

Fix it by returning the max 4 byte checksum.

Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
---
 fs/btrfs/disk-io.c |    2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index ce8b8b6..cfa2953 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -296,6 +296,8 @@ static int csum_tree_block(struct btrfs_root *root, struct extent_buffer *buf,
 		if (memcmp_extent_buffer(buf, result, 0, csum_size)) {
 			u32 val;
 			u32 found = 0;
+
+			csum_size = min_t(u16, csum_size, sizeof(u32));
 			memcpy(&found, result, csum_size);
 
 			read_extent_buffer(buf, &val, 0, csum_size);
-- 
1.7.10.4