On Tue, Nov 24, 2015 at 10:33:59PM +0000, Ben Hutchings wrote:
> 3.2.74-rc1 review patch. If anyone has any objections, please let me know.
>
> ------------------
>
> From: Filipe Manana <fdmanana@xxxxxxxx>
>
> commit f1cd1f0b7d1b5d4aaa5711e8f4e4898b0045cb6d upstream.
>
> 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.
>
> Signed-off-by: Filipe Manana <fdmanana@xxxxxxxx>
> [bwh: Backported to 3.2: s/found_key\.type/btrfs_key_type(\&found_key)/]
Actually, in my backport to 3.16 I decided to keep the usage of
'found_key.type' instead, as the usage of btrfs_key_type() has been
dropped with commit 962a298f3511 ("btrfs: kill the key type accessor
helpers").
Cheers,
--
Luís
> Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
> ---
> fs/btrfs/xattr.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> --- a/fs/btrfs/xattr.c
> +++ b/fs/btrfs/xattr.c
> @@ -259,8 +259,10 @@ ssize_t btrfs_listxattr(struct dentry *d
> /* check to make sure this item is what we want */
> if (found_key.objectid != key.objectid)
> break;
> - if (btrfs_key_type(&found_key) != BTRFS_XATTR_ITEM_KEY)
> + if (btrfs_key_type(&found_key) > BTRFS_XATTR_ITEM_KEY)
> break;
> + if (btrfs_key_type(&found_key) < BTRFS_XATTR_ITEM_KEY)
> + goto next;
>
> di = btrfs_item_ptr(leaf, slot, struct btrfs_dir_item);
> if (verify_dir_item(root, leaf, di))
>
> --
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