[Devel] [PATCH RHEL8 COMMIT] ms/xfs: fix boundary test in xfs_attr_shortform_verify

[Konstantin Khorenko]

The commit is pushed to "branch-rh8-4.18.0-193.6.3.vz8.4.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh8-4.18.0-193.6.3.vz8.4.7
------>
commit 228b82a36a4cc5e1e015f15585b502e0d126bebd
Author: Vasily Averin <vvs at virtuozzo.com>
Date:   Wed Sep 9 13:41:57 2020 +0300

    ms/xfs: fix boundary test in xfs_attr_shortform_verify
    
    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:
    
     # touch file
     # setfattr -n root.a file
    
    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 mainline f4020438fab05364018c91f7e02ebdd192085933)
    Fixes CVE-2020-14385
    https://jira.sw.ru/browse/PSBM-107720
    Signed-off-by: Vasily Averin <vvs at virtuozzo.com>
    
    (cherry picked from commit 2bde8a6068ba41cad1394863e30a3ec6e124388d)
    Signed-off-by: Konstantin Khorenko <khorenko at virtuozzo.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 cbcc400f7634..7dbca5a14a90 100644
--- a/fs/xfs/libxfs/xfs_attr_leaf.c
+++ b/fs/xfs/libxfs/xfs_attr_leaf.c
@@ -949,8 +949,10 @@ 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. */