[SRU Focal, Focal/oem-5.6] xfs: fix boundary test in xfs_attr_shortform_verify

[Thadeu Lima de Souza Cascardo]

From: Eric Sandeen <sandeen at redhat.com>

The boundary test for the fixed-offset parts of xfs_attr_sf_entry in
xfs_attr_shortform_verify is off by one, because the variable array
at the end is defined as nameval[1] not nameval[].
Hence we need to subtract 1 from the calculation.

This can be shown by:

and verifications will fail when it's written to disk.

This only matters for a last attribute which has a single-byte name
and no value, otherwise the combination of namelen & valuelen will
push endp further out and this test won't fail.

Fixes: 1e1bbd8e7ee06 ("xfs: create structure verifier function for shortform xattrs")
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>
Reviewed-by: Christoph Hellwig <hch at lst.de>
(cherry picked from commit f4020438fab05364018c91f7e02ebdd192085933)
CVE-2020-14385
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo at canonical.com>
---
 fs/xfs/libxfs/xfs_attr_leaf.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/xfs/libxfs/xfs_attr_leaf.c b/fs/xfs/libxfs/xfs_attr_leaf.c
index fed537a4353d..54c412549edf 100644
--- a/fs/xfs/libxfs/xfs_attr_leaf.c
+++ b/fs/xfs/libxfs/xfs_attr_leaf.c
@@ -1010,8 +1010,10 @@ xfs_attr_shortform_verify(
 		 * struct xfs_attr_sf_entry has a variable length.
 		 * Check the fixed-offset parts of the structure are
 		 * within the data buffer.
+		 * xfs_attr_sf_entry is defined with a 1-byte variable
+		 * array at the end, so we must subtract that off.
 		 */
-		if (((char *)sfep + sizeof(*sfep)) >= endp)
+		if (((char *)sfep + sizeof(*sfep) - 1) >= endp)
 			return __this_address;
 
 		/* Don't allow names with known bad length. */
-- 
2.25.1