[CVE-2018-13094][T][SRU][PATCH] xfs: don't call xfs_da_shrink_inode with NULL bp

[Po-Hsu Lin]

From: Eric Sandeen <sandeen at sandeen.net>

CVE-2018-13094

xfs_attr3_leaf_create may have errored out before instantiating a buffer,
for example if the blkno is out of range.  In that case there is no work
to do to remove it, and in fact xfs_da_shrink_inode will lead to an oops
if we try.

This also seems to fix a flaw where the original error from
xfs_attr3_leaf_create gets overwritten in the cleanup case, and it
removes a pointless assignment to bp which isn't used after this.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=199969
Reported-by: Xu, Wen <wen.xu at gatech.edu>
Tested-by: Xu, Wen <wen.xu at gatech.edu>
Signed-off-by: Eric Sandeen <sandeen at redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong at oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong at oracle.com>
(backported from commit bb3d48dcf86a97dc25fe9fc2c11938e19cb4399a)
Signed-off-by: Po-Hsu Lin <po-hsu.lin at canonical.com>
---
 fs/xfs/xfs_attr_leaf.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/fs/xfs/xfs_attr_leaf.c b/fs/xfs/xfs_attr_leaf.c
index 6bad3ef..7619a93 100644
--- a/fs/xfs/xfs_attr_leaf.c
+++ b/fs/xfs/xfs_attr_leaf.c
@@ -699,9 +699,8 @@ xfs_attr_shortform_to_leaf(xfs_da_args_t *args)
 	ASSERT(blkno == 0);
 	error = xfs_attr3_leaf_create(args, blkno, &bp);
 	if (error) {
-		error = xfs_da_shrink_inode(args, 0, bp);
-		bp = NULL;
-		if (error)
+		/* xfs_attr3_leaf_create may not have instantiated a block */
+		if (bp && (xfs_da_shrink_inode(args, 0, bp) != 0))
 			goto out;
 		xfs_idata_realloc(dp, size, XFS_ATTR_FORK);	/* try to put */
 		memcpy(ifp->if_u1.if_data, tmpbuffer, size);	/* it back */
-- 
2.7.4