[PATCH v2 1/4] ceph: bound xattr value length in __build_xattrs()

[Michael Bommarito <michael.bommarito@gmail.com>]

__build_xattrs() decodes the MDS-supplied xattr blob one attribute at a
time. For each attribute it reads a 32-bit name length, advances past the
name bytes, reads a 32-bit value length, records the value pointer, and
advances past the value bytes. The two length fields are read with
ceph_decode_32_safe(), but the value bytes themselves are advanced over
with a bare "p += len" and no ceph_decode_need() check that "len" bytes
remain in the blob.

For every attribute except the last, the next iteration's
ceph_decode_32_safe() on the following name length implicitly verifies
that the previous value did not run past the blob end. The final
attribute has no successor, so its decoded value length is never checked
against the blob bounds. A malicious or compromised metadata server can
set the last attribute's value length larger than the bytes actually
present in the blob.

The blob is a dedicated kvmalloc() allocation sized to the wire length
(ceph_buffer_new() in ceph_fill_inode()). __set_xattr() records the
oversized length in xattr->val_len verbatim, and a later getxattr(2) runs
memcpy(value, xattr->val, xattr->val_len) into a user-supplied buffer,
copying bytes past the end of the allocation back to user space.

Impact: a malicious metadata server discloses adjacent kernel heap bytes
to a local user via getxattr(2) on a CephFS file. Add the missing
ceph_decode_need() so an out-of-bounds value length on the final
attribute fails the decode and returns -EIO instead of being stored.

Fixes: 355da1eb7a1f ("ceph: inode operations")
Cc: stable@vger.kernel.org
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Assisted-by: Claude:claude-opus-4-8
Reviewed-by: Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>
---
v2:
- Add Reviewed-by from Viacheslav Dubeyko.

 fs/ceph/xattr.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/ceph/xattr.c b/fs/ceph/xattr.c
index e773be07f7674..d488bb8fc00ba 100644
--- a/fs/ceph/xattr.c
+++ b/fs/ceph/xattr.c
@@ -848,6 +848,7 @@ static int __build_xattrs(struct inode *inode)
 			name = p;
 			p += len;
 			ceph_decode_32_safe(&p, end, len, bad);
+			ceph_decode_need(&p, end, len, bad);
 			val = p;
 			p += len;
 
-- 
2.53.0