From f1cd1f0b7d1b5d4aaa5711e8f4e4898b0045cb6d Mon Sep 17 00:00:00 2001 From: Filipe Manana Date: Mon, 9 Nov 2015 18:06:38 +0000 Subject: [PATCH] Btrfs: fix race when listing an inode's xattrs When listing a inode's xattrs we have a time window where we race against a concurrent operation for adding a new hard link for our inode that makes us not return any xattr to user space. In order for this to happen, the first xattr of our inode needs to be at slot 0 of a leaf and the previous leaf must still have room for an inode ref (or extref) item, and this can happen because an inode's listxattrs callback does not lock the inode's i_mutex (nor does the VFS does it for us), but adding a hard link to an inode makes the VFS lock the inode's i_mutex before calling the inode's link callback. If we have the following leafs: Leaf X (has N items) Leaf Y [ ... (257 INODE_ITEM 0) (257 INODE_REF 256) ] [ (257 XATTR_ITEM 12345), ... ] slot N - 2 slot N - 1 slot 0 The race illustrated by the following sequence diagram is possible: CPU 1 CPU 2 btrfs_listxattr() searches for key (257 XATTR_ITEM 0) gets path with path->nodes[0] == leaf X and path->slots[0] == N because path->slots[0] is >= btrfs_header_nritems(leaf X), it calls btrfs_next_leaf() btrfs_next_leaf() releases the path adds key (257 INODE_REF 666) to the end of leaf X (slot N), and leaf X now has N + 1 items searches for the key (257 INODE_REF 256), with path->keep_locks == 1, because that is the last key it saw in leaf X before releasing the path ends up at leaf X again and it verifies that the key (257 INODE_REF 256) is no longer the last key in leaf X, so it returns with path->nodes[0] == leaf X and path->slots[0] == N, pointing to the new item with key (257 INODE_REF 666) btrfs_listxattr's loop iteration sees that the type of the key pointed by the path is different from the type BTRFS_XATTR_ITEM_KEY and so it breaks the loop and stops looking for more xattr items --> the application doesn't get any xattr listed for our inode So fix this by breaking the loop only if the key's type is greater than BTRFS_XATTR_ITEM_KEY and skip the current key if its type is smaller. Cc: stable@vger.kernel.org Signed-off-by: Filipe Manana --- fs/btrfs/xattr.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) diff --git a/fs/btrfs/xattr.c b/fs/btrfs/xattr.c index 6f518c90e1c1..1fcd7b6e7564 100644 --- a/fs/btrfs/xattr.c +++ b/fs/btrfs/xattr.c @@ -313,8 +313,10 @@ ssize_t btrfs_listxattr(struct dentry *dentry, char *buffer, size_t size) /* check to make sure this item is what we want */ if (found_key.objectid != key.objectid) break; - if (found_key.type != BTRFS_XATTR_ITEM_KEY) + if (found_key.type > BTRFS_XATTR_ITEM_KEY) break; + if (found_key.type < BTRFS_XATTR_ITEM_KEY) + goto next; di = btrfs_item_ptr(leaf, slot, struct btrfs_dir_item); if (verify_dir_item(root, leaf, di))