[PATCH 6.18 140/198] ksmbd: require 3 sub-authorities before reading sub_auth[2]

Greg Kroah-Hartman
2026-04-20T16:03:18Z

From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

commit 53370cf9090777774e07fd9a8ebce67c6cc333ab upstream.

parse_dacl() compares each ACE SID against sid_unix_NFS_mode and on
match reads sid.sub_auth[2] as the file mode. If sid_unix_NFS_mode is
the prefix S-1-5-88-3 with num_subauth = 2 then compare_sids() compares
only min(num_subauth, 2) sub-authorities so a client SID with
num_subauth = 2 and sub_auth = {88, 3} will match.

If num_subauth = 2 and the ACE is placed at the very end of the security
descriptor, sub_auth[2] will be 4 bytes past end_of_acl. The
out-of-band bytes will then be masked to the low 9 bits and applied as
the file's POSIX mode, probably not something that is good to have
happen.

Fix this up by forcing the SID to actually carry a third sub-authority
before reading it at all.

Cc: Namjae Jeon <linkinjeon@kernel.org>
Cc: Steve French <smfrench@gmail.com>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Tom Talpey <tom@talpey.com>
Cc: linux-cifs@vger.kernel.org
Cc: <stable@kernel.org>
Assisted-by: gregkh_clanker_t1000
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
fs/smb/server/smbacl.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

--- a/fs/smb/server/smbacl.c
+++ b/fs/smb/server/smbacl.c
@@ -451,7 +451,8 @@ static void parse_dacl(struct mnt_idmap
ppace[i]->access_req =
smb_map_generic_desired_access(ppace[i]->access_req);

- if (!(compare_sids(&ppace[i]->sid, &sid_unix_NFS_mode))) {
+ if (ppace[i]->sid.num_subauth >= 3 &&
+ !(compare_sids(&ppace[i]->sid, &sid_unix_NFS_mode))) {
fattr->cf_mode =
le32_to_cpu(ppace[i]->sid.sub_auth[2]);
break;