[PATCH v2] xfs: harden directory integrity checks some more

["Darrick J. Wong" <darrick.wong@oracle.com>]

From: Darrick J. Wong <darrick.wong@oracle.com>

If a malicious filesystem image contains a block+ format directory
wherein the directory inode's core.mode is set such that
S_ISDIR(core.mode) == 0, and if there are subdirectories of the
corrupted directory, an attempt to traverse up the directory tree will
crash the kernel in __xfs_dir3_data_check.  Running the online scrub's
parent checks will tend to do this.

The crash occurs because the directory inode's d_ops get set to
xfs_dir[23]_nondir_ops (it's not a directory) but the parent pointer
scrubber's indiscriminate call to xfs_readdir proceeds past the ASSERT
if we have non fatal asserts configured.

Fix the null pointer dereference crash in __xfs_dir3_data_check by
looking for S_ISDIR or wrong d_ops; and teach the parent scrubber
to bail out if it is fed a non-directory "parent".

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
v2: check for wrong d_ops, not individual function pointers
---
 fs/xfs/libxfs/xfs_dir2_data.c |    8 ++++++++
 fs/xfs/scrub/parent.c         |    2 +-
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/fs/xfs/libxfs/xfs_dir2_data.c b/fs/xfs/libxfs/xfs_dir2_data.c
index 3237812..853d9ab 100644
--- a/fs/xfs/libxfs/xfs_dir2_data.c
+++ b/fs/xfs/libxfs/xfs_dir2_data.c
@@ -73,6 +73,14 @@ __xfs_dir3_data_check(
 	 */
 	ops = xfs_dir_get_ops(mp, dp);
 
+	/*
+	 * If this isn't a directory, or we don't get handed the dir ops,
+	 * something is seriously wrong.  Bail out.
+	 */
+	if ((dp && !S_ISDIR(VFS_I(dp)->i_mode)) ||
+	    ops != xfs_dir_get_ops(mp, NULL))
+		return __this_address;
+
 	hdr = bp->b_addr;
 	p = (char *)ops->data_entry_p(hdr);
 
diff --git a/fs/xfs/scrub/parent.c b/fs/xfs/scrub/parent.c
index dd704fd..0d38514 100644
--- a/fs/xfs/scrub/parent.c
+++ b/fs/xfs/scrub/parent.c
@@ -171,7 +171,7 @@ xfs_scrub_parent_validate(
 	error = xfs_iget(mp, sc->tp, dnum, 0, 0, &dp);
 	if (!xfs_scrub_fblock_xref_process_error(sc, XFS_DATA_FORK, 0, &error))
 		goto out;
-	if (dp == sc->ip) {
+	if (dp == sc->ip || !S_ISDIR(VFS_I(dp)->i_mode)) {
 		xfs_scrub_fblock_set_corrupt(sc, XFS_DATA_FORK, 0);
 		goto out_rele;
 	}