FAILED: patch "[PATCH] ksmbd: require minimum ACE size in smb_check_perm_dacl()" failed to apply to 6.6-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 6.6-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 <stable@vger.kernel.org>.

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-6.6.y
git checkout FETCH_HEAD
git cherry-pick -x d07b26f39246a82399661936dd0c853983cfade7
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2026042426-growing-dime-ce6e@gregkh' --subject-prefix 'PATCH 6.6.y' HEAD^..

Possible dependencies:



thanks,

greg k-h

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

From d07b26f39246a82399661936dd0c853983cfade7 Mon Sep 17 00:00:00 2001
From: Michael Bommarito <michael.bommarito@gmail.com>
Date: Tue, 14 Apr 2026 15:15:33 -0400
Subject: [PATCH] ksmbd: require minimum ACE size in smb_check_perm_dacl()

Both ACE-walk loops in smb_check_perm_dacl() only guard against an
under-sized remaining buffer, not against an ACE whose declared
`ace->size` is smaller than the struct it claims to describe:

  if (offsetof(struct smb_ace, access_req) > aces_size)
      break;
  ace_size = le16_to_cpu(ace->size);
  if (ace_size > aces_size)
      break;

The first check only requires the 4-byte ACE header to be in bounds;
it does not require access_req (4 bytes at offset 4) to be readable.
An attacker who has set a crafted DACL on a file they own can declare
ace->size == 4 with aces_size == 4, pass both checks, and then

  granted |= le32_to_cpu(ace->access_req);               /* upper loop */
  compare_sids(&sid, &ace->sid);                         /* lower loop */

reads access_req at offset 4 (OOB by up to 4 bytes) and ace->sid at
offset 8 (OOB by up to CIFS_SID_BASE_SIZE + SID_MAX_SUB_AUTHORITIES
* 4 bytes).

Tighten both loops to require

  ace_size >= offsetof(struct smb_ace, sid) + CIFS_SID_BASE_SIZE

which is the smallest valid on-wire ACE layout (4-byte header +
4-byte access_req + 8-byte sid base with zero sub-auths).  Also
reject ACEs whose sid.num_subauth exceeds SID_MAX_SUB_AUTHORITIES
before letting compare_sids() dereference sub_auth[] entries.

parse_sec_desc() already enforces an equivalent check (lines 441-448);
smb_check_perm_dacl() simply grew weaker validation over time.

Reachability: authenticated SMB client with permission to set an ACL
on a file.  On a subsequent CREATE against that file, the kernel
walks the stored DACL via smb_check_perm_dacl() and triggers the
OOB read.  Not pre-auth, and the OOB read is not reflected to the
attacker, but KASAN reports and kernel state corruption are
possible.

Fixes: e2f34481b24d ("cifsd: add server-side procedures for SMB3")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-6
Assisted-by: Codex:gpt-5-4
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

diff --git a/fs/smb/server/smbacl.c b/fs/smb/server/smbacl.c
index 061a305bf9c8..bba26a0355bb 100644
--- a/fs/smb/server/smbacl.c
+++ b/fs/smb/server/smbacl.c
@@ -1342,10 +1342,13 @@ int smb_check_perm_dacl(struct ksmbd_conn *conn, const struct path *path,
 		ace = (struct smb_ace *)((char *)pdacl + sizeof(struct smb_acl));
 		aces_size = acl_size - sizeof(struct smb_acl);
 		for (i = 0; i < le16_to_cpu(pdacl->num_aces); i++) {
-			if (offsetof(struct smb_ace, access_req) > aces_size)
+			if (offsetof(struct smb_ace, sid) +
+			    aces_size < CIFS_SID_BASE_SIZE)
 				break;
 			ace_size = le16_to_cpu(ace->size);
-			if (ace_size > aces_size)
+			if (ace_size > aces_size ||
+			    ace_size < offsetof(struct smb_ace, sid) +
+				       CIFS_SID_BASE_SIZE)
 				break;
 			aces_size -= ace_size;
 			granted |= le32_to_cpu(ace->access_req);
@@ -1360,13 +1363,19 @@ int smb_check_perm_dacl(struct ksmbd_conn *conn, const struct path *path,
 	ace = (struct smb_ace *)((char *)pdacl + sizeof(struct smb_acl));
 	aces_size = acl_size - sizeof(struct smb_acl);
 	for (i = 0; i < le16_to_cpu(pdacl->num_aces); i++) {
-		if (offsetof(struct smb_ace, access_req) > aces_size)
+		if (offsetof(struct smb_ace, sid) +
+		    aces_size < CIFS_SID_BASE_SIZE)
 			break;
 		ace_size = le16_to_cpu(ace->size);
-		if (ace_size > aces_size)
+		if (ace_size > aces_size ||
+		    ace_size < offsetof(struct smb_ace, sid) +
+			       CIFS_SID_BASE_SIZE)
 			break;
 		aces_size -= ace_size;
 
+		if (ace->sid.num_subauth > SID_MAX_SUB_AUTHORITIES)
+			break;
+
 		if (!compare_sids(&sid, &ace->sid) ||
 		    !compare_sids(&sid_unix_NFS_mode, &ace->sid)) {
 			found = 1;