[PATCH 3/3] ksmbd: require minimum ACE size in smb_check_perm_dacl()

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

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>
---

fs/smb/server/smbacl.c | 17 +++++++++++++----
1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/fs/smb/server/smbacl.c b/fs/smb/server/smbacl.c
index c30d01877c41..d5943256c071 100644
--- a/fs/smb/server/smbacl.c
+++ b/fs/smb/server/smbacl.c
@@ -1341,10 +1341,13 @@ 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) +
+			    CIFS_SID_BASE_SIZE > aces_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);
@@ -1359,13 +1362,19 @@ 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) +
+		    CIFS_SID_BASE_SIZE > aces_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;
--
2.53.0