[PATCH 6.12 19/35] ksmbd: require minimum ACE size in smb_check_perm_dacl()

Greg Kroah-Hartman
2026-04-24T13:43:50Z

From: Michael Bommarito <michael.bommarito@gmail.com>

commit d07b26f39246a82399661936dd0c853983cfade7 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
fs/smb/server/smbacl.c | 17 +++++++++++++----
1 file changed, 13 insertions(+), 4 deletions(-)

--- a/fs/smb/server/smbacl.c
+++ b/fs/smb/server/smbacl.c
@@ -1342,10 +1342,13 @@ int smb_check_perm_dacl(struct ksmbd_con
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);
@@ -1363,13 +1366,19 @@ int smb_check_perm_dacl(struct ksmbd_con
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;