[PATCH] smb: client: fix slab-out-of-bounds write in id_mode_to_cifs_acl()

[Dominik Maier <dmnk@google.com>]

In id_mode_to_cifs_acl(), while the chown branch reserves
sizeof(struct smb_ntsd) + 2 * sizeof(struct smb_sid) (plus
sizeof(struct smb_acl) and the worst-case ACE space after
commit 0ee150794c75 ("smb: client: fix heap overflow in DACL owner/group
rewrite")), the chmod branch only adds 2 * sizeof(struct smb_ace) or
5 * sizeof(struct smb_ace) to secdesclen (or 1 * sizeof(struct smb_ace)
on posix).

However, the chmod branch also invokes build_sec_desc() ->
copy_sec_desc(), which unconditionally writes the Owner SID at
pnntsd + sidsoffset and the Group SID at
pnntsd + sidsoffset + sizeof(struct smb_sid) (2 * 68 = 136 bytes total),
and set_chmod_dacl() always writes a struct smb_acl header (8 bytes)
even when the source descriptor has dacloffset == 0. When a security
descriptor has compact or overlapping Owner/Group SIDs (or
dacloffset == 0), calling chmod() under-allocates pnntsd and writes past
the end of the slab buffer.

Reserve sizeof(struct smb_acl) + 2 * sizeof(struct smb_sid) (plus
sizeof(struct smb_ntsd) on the posix path) in the chmod branch of
id_mode_to_cifs_acl().

Fixes: bc3e9dd9d104 ("cifs: Change SIDs in ACEs while transferring file ownership.")
Assisted-by: ARTIST LLM
Signed-off-by: Dominik Maier <dmnk@google.com>
---
 fs/smb/client/cifsacl.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/fs/smb/client/cifsacl.c b/fs/smb/client/cifsacl.c
index d1a92bb4d9a5..e24d9582f262 100644
--- a/fs/smb/client/cifsacl.c
+++ b/fs/smb/client/cifsacl.c
@@ -1826,12 +1826,15 @@ id_mode_to_cifs_acl(struct inode *inode, const char *path, __u64 *pnmode,
 
 	/* Potentially, five new ACEs can be added to the ACL for U,G,O mapping */
 	if (pnmode && *pnmode != NO_CHANGE_64) { /* chmod */
+		nsecdesclen = sizeof(struct smb_acl) +
+			      (sizeof(struct smb_sid) * 2);
 		if (posix)
-			nsecdesclen = 1 * sizeof(struct smb_ace);
+			nsecdesclen += sizeof(struct smb_ntsd) +
+				       sizeof(struct smb_ace);
 		else if (mode_from_sid)
-			nsecdesclen = secdesclen + (2 * sizeof(struct smb_ace));
+			nsecdesclen += secdesclen + (2 * sizeof(struct smb_ace));
 		else /* cifsacl */
-			nsecdesclen = secdesclen + (5 * sizeof(struct smb_ace));
+			nsecdesclen += secdesclen + (5 * sizeof(struct smb_ace));
 	} else { /* chown */
 		/* When ownership changes, changes new owner sid length could be different */
 		nsecdesclen = sizeof(struct smb_ntsd) + (sizeof(struct smb_sid) * 2);
-- 
2.56.0.257.gf7497c7d27-goog