FAILED: patch "[PATCH] smb: client: require a full NFS mode SID before reading mode" failed to apply to 5.15-stable tree

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x 2757ad3e4b6f9e0fed4c7739594e702abc5cab21
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026042429-hanky-simplify-54ff@gregkh' --subject-prefix 'PATCH 5.15.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 2757ad3e4b6f9e0fed4c7739594e702abc5cab21 Mon Sep 17 00:00:00 2001
From: Michael Bommarito <[email protected]>
Date: Mon, 20 Apr 2026 09:50:58 -0400
Subject: [PATCH] smb: client: require a full NFS mode SID before reading mode
bits

parse_dacl() treats an ACE SID matching sid_unix_NFS_mode as an NFS mode SID and reads sid.sub_auth[2] to recover the mode bits.

That assumes the ACE carries three subauthorities, but compare_sids() only compares min(a, b) subauthorities. A malicious server can return an ACE with num_subauth = 2 and sub_auth[] = {88, 3}, which still matches sid_unix_NFS_mode and then drives the sub_auth[2] read four bytes past the end of the ACE.

Require num_subauth >= 3 before treating the ACE as an NFS mode SID. This keeps the fix local to the special-SID mode path without changing compare_sids() semantics for the rest of cifsacl.

index cb4060ba5e31..4ec204d2c774 100644
@@ -879,6 +879,7 @@ static void parse_dacl(struct smb_acl *pdacl, char *end_of_acl,
 			dump_ace(ppace[i], end_of_dacl);
 #endif
 			if (mode_from_special_sid &&
+			    ppace[i]->sid.num_subauth >= 3 &&
 			    (compare_sids(&(ppace[i]->sid),
 					  &sid_unix_NFS_mode) == 0)) {
 				/*

From: Michael Bommarito <[email protected]>

[ Upstream commit 2757ad3e4b6f9e0fed4c7739594e702abc5cab21 ]

parse_dacl() treats an ACE SID matching sid_unix_NFS_mode as an NFS mode SID and reads sid.sub_auth[2] to recover the mode bits.

That assumes the ACE carries three subauthorities, but compare_sids() only compares min(a, b) subauthorities. A malicious server can return an ACE with num_subauth = 2 and sub_auth[] = {88, 3}, which still matches sid_unix_NFS_mode and then drives the sub_auth[2] read four bytes past the end of the ACE.

Require num_subauth >= 3 before treating the ACE as an NFS mode SID. This keeps the fix local to the special-SID mode path without changing compare_sids() semantics for the rest of cifsacl.

---
 fs/cifs/cifsacl.c | 1 +
 1 file changed, 1 insertion(+)

index bf861fef2f0c3..0a7fbd9fcdb3a 100644
@@ -807,6 +807,7 @@ static void parse_dacl(struct cifs_acl *pdacl, char *end_of_acl,
 			dump_ace(ppace[i], end_of_acl);
 #endif
 			if (mode_from_special_sid &&
+			    ppace[i]->sid.num_subauth >= 3 &&
 			    (compare_sids(&(ppace[i]->sid),
 					  &sid_unix_NFS_mode) == 0)) {
 				/*

2.53.0