[PATCH v2 2/2] btrfs: Add extra check for sub_stripes to avoid hostile 0 division attack.

[Qu Wenruo <quwenruo@cn.fujitsu.com>]

Although only RAID10 use sub_stripes, a hostile attack can modify chunk
tree and just add RAID10 bit to a single chunk.
Then btrfs_map_block will trigger a 0 division in kernel and destroy
everything.

Just add extra check when reading chunk from disk.

Reported-by: Lukas Lueg <lukas.lueg@gmail.com>
Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
---
v2:
   Return -EIO, and add kernel message output.
---
 fs/btrfs/volumes.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 8222f6f..fdcecf7 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -6061,6 +6061,17 @@ static int read_one_chunk(struct btrfs_root *root, struct btrfs_key *key,
 	map->stripe_len = btrfs_chunk_stripe_len(leaf, chunk);
 	map->type = btrfs_chunk_type(leaf, chunk);
 	map->sub_stripes = btrfs_chunk_sub_stripes(leaf, chunk);
+
+	/* Add extra check to avoid hostile 0 division attack */
+	if (map->type & BTRFS_BLOCK_GROUP_RAID10 &&
+	    map->sub_stripes == 0) {
+		btrfs_warn(root->fs_info,
+			   "RAID10 chunk found but with no sub stripe for bytenr: %llu\n",
+			   logical);
+		free_extent_map(em);
+		return -EIO;
+	}
+
 	for (i = 0; i < num_stripes; i++) {
 		map->stripes[i].physical =
 			btrfs_stripe_offset_nr(leaf, chunk, i);
-- 
2.3.5