FAILED: patch "[PATCH] ksmbd: deny access for an empty DACL instead of granting the" failed to apply to 6.18-stable tree

The patch below does not apply to the 6.18-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-6.18.y
git checkout FETCH_HEAD
git cherry-pick -x de0cffef897c1c232ed973f89928cd6c93d0b1e3
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026110336-whole-agile-fb2c@gregkh' --subject-prefix 'PATCH 6.18.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

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

From de0cffef897c1c232ed973f89928cd6c93d0b1e3 Mon Sep 17 00:00:00 2001
From: Aamir Ahmed <[email protected]>
Date: Wed, 16 Sep 2026 11:49:27 +0100
Subject: [PATCH] ksmbd: deny access for an empty DACL instead of granting the
request

smb_check_perm_dacl() returns the access mask it computes to the SMB2 CREATE handler, which uses it as the granted access for the open. For a security descriptor whose DACL is present but contains no ACEs, the function is supposed to deny access: an empty (non-NULL) DACL grants no access to anyone except the object owner's implicit READ_CONTROL and WRITE_DAC (MS-DTYP 2.4.5).

The empty-DACL branch only denies when both

- the DACL has no trailing bytes (pdacl_size == sizeof(struct smb_acl)), and
- the request contains bits beyond READ_CONTROL / WRITE_DAC.

In every other case it takes the shared "goto err_out" with rc still 0, which returns success without modifying *pdaccess, so the caller grants the full requested access. Three problems follow:

- The READ_CONTROL / WRITE_DAC carve-out models the owner's implicit rights, but the check runs before is_owner is computed, so those rights are handed to *every* caller, not just the owner. For READ_CONTROL the SMB2 CREATE path does not fall back to inode_permission(), so any user can read the security descriptor of an object whose empty DACL is meant to deny them.

- A request of exactly READ_CONTROL and/or WRITE_DAC is granted to any caller. WRITE_DAC lets the handle rewrite the DACL.

- A DACL that declares zero ACEs but carries trailing bytes (pdacl_size > sizeof(struct smb_acl)) skips the deny entirely and is granted whatever was requested.

On shares where the NT ACL is the authoritative access control -- e.g. "force user" shares, where every client maps to one POSIX identity so POSIX cannot distinguish users -- this is an access-control bypass.

Compute is_owner before the empty-DACL branch and deny access from an empty DACL unless the caller is the owner requesting no more than READ_CONTROL / WRITE_DAC.

index c959699ace56..664f01263611 100644
@@ -1502,21 +1502,27 @@ int smb_check_perm_dacl(struct ksmbd_conn *conn, const struct path *path,
 	if (pdacl_size > acl_size || pdacl_size < sizeof(struct smb_acl))
 		goto err_out;
 
-	if (!pdacl->num_aces) {
-		if (!(pdacl_size - sizeof(struct smb_acl)) &&
-		    *pdaccess & ~(FILE_READ_CONTROL_LE | FILE_WRITE_DAC_LE)) {
-			rc = -EACCES;
-			goto err_out;
-		}
-		goto err_out;
-	}
-
 	if (!uid)
 		sid_type = SIDUNIX_USER;
 	id_to_sid(uid, sid_type, &sid);
 	vfsuid = i_uid_into_vfsuid(idmap, d_inode(path->dentry));
 	is_owner = uid == from_kuid(&init_user_ns, vfsuid_into_kuid(vfsuid));
 
+	if (!pdacl->num_aces) {
+		/*
+		 * An empty (present, zero-ACE) DACL grants no access to
+		 * anyone except the object owner's implicit READ_CONTROL
+		 * and WRITE_DAC (MS-DTYP 2.4.5). Deny every other caller,
+		 * deny the owner any access beyond those two bits, and do
+		 * not let trailing bytes after a zero-ACE DACL become an
+		 * implicit grant.
+		 */
+		if (!is_owner ||
+		    (*pdaccess & ~(FILE_READ_CONTROL_LE | FILE_WRITE_DAC_LE)))
+			rc = -EACCES;
+		goto err_out;
+	}
+
 	if (*pdaccess & FILE_MAXIMAL_ACCESS_LE) {
 		ace = (struct smb_ace *)((char *)pdacl + sizeof(struct smb_acl));
 		aces_size = pdacl_size - sizeof(struct smb_acl);

From: Aamir Ahmed <[email protected]>

[ Upstream commit de0cffef897c1c232ed973f89928cd6c93d0b1e3 ]

smb_check_perm_dacl() returns the access mask it computes to the SMB2 CREATE handler, which uses it as the granted access for the open. For a security descriptor whose DACL is present but contains no ACEs, the function is supposed to deny access: an empty (non-NULL) DACL grants no access to anyone except the object owner's implicit READ_CONTROL and WRITE_DAC (MS-DTYP 2.4.5).

The empty-DACL branch only denies when both

- the DACL has no trailing bytes (pdacl_size == sizeof(struct smb_acl)), and
- the request contains bits beyond READ_CONTROL / WRITE_DAC.

In every other case it takes the shared "goto err_out" with rc still 0, which returns success without modifying *pdaccess, so the caller grants the full requested access. Three problems follow:

- The READ_CONTROL / WRITE_DAC carve-out models the owner's implicit rights, but the check runs before is_owner is computed, so those rights are handed to *every* caller, not just the owner. For READ_CONTROL the SMB2 CREATE path does not fall back to inode_permission(), so any user can read the security descriptor of an object whose empty DACL is meant to deny them.

- A request of exactly READ_CONTROL and/or WRITE_DAC is granted to any caller. WRITE_DAC lets the handle rewrite the DACL.

- A DACL that declares zero ACEs but carries trailing bytes (pdacl_size > sizeof(struct smb_acl)) skips the deny entirely and is granted whatever was requested.

On shares where the NT ACL is the authoritative access control -- e.g. "force user" shares, where every client maps to one POSIX identity so POSIX cannot distinguish users -- this is an access-control bypass.

Compute is_owner before the empty-DACL branch and deny access from an empty DACL unless the caller is the owner requesting no more than READ_CONTROL / WRITE_DAC.

---
 fs/smb/server/smbacl.c | 26 ++++++++++++++++++--------
 1 file changed, 18 insertions(+), 8 deletions(-)

index 49a35974dd296..33a607e709f18 100644
@@ -1469,6 +1469,8 @@ int smb_check_perm_dacl(struct ksmbd_conn *conn, const struct path *path,
 	struct posix_acl_entry *pa_entry;
 	unsigned int sid_type = SIDOWNER;
 	unsigned short ace_size;
+	bool is_owner;
+	vfsuid_t vfsuid;
 
 	ksmbd_debug(SMB, "check permission using windows acl\n");
 	pntsd_size = ksmbd_vfs_get_sd_xattr(conn, idmap,

@@ -1489,19 +1491,27 @@ int smb_check_perm_dacl(struct ksmbd_conn *conn, const struct path *path,
 	if (pdacl_size > acl_size || pdacl_size < sizeof(struct smb_acl))
 		goto err_out;
 
+	if (!uid)
+		sid_type = SIDUNIX_USER;
+	id_to_sid(uid, sid_type, &sid);
+	vfsuid = i_uid_into_vfsuid(idmap, d_inode(path->dentry));
+	is_owner = uid == from_kuid(&init_user_ns, vfsuid_into_kuid(vfsuid));
+
 	if (!pdacl->num_aces) {
-		if (!(pdacl_size - sizeof(struct smb_acl)) &&
-		    *pdaccess & ~(FILE_READ_CONTROL_LE | FILE_WRITE_DAC_LE)) {
+		/*
+		 * An empty (present, zero-ACE) DACL grants no access to
+		 * anyone except the object owner's implicit READ_CONTROL
+		 * and WRITE_DAC (MS-DTYP 2.4.5). Deny every other caller,
+		 * deny the owner any access beyond those two bits, and do
+		 * not let trailing bytes after a zero-ACE DACL become an
+		 * implicit grant.
+		 */
+		if (!is_owner ||
+		    (*pdaccess & ~(FILE_READ_CONTROL_LE | FILE_WRITE_DAC_LE)))
 			rc = -EACCES;
-			goto err_out;
-		}
 		goto err_out;
 	}
 
-	if (!uid)
-		sid_type = SIDUNIX_USER;
-	id_to_sid(uid, sid_type, &sid);
-
 	if (*pdaccess & FILE_MAXIMAL_ACCESS_LE) {
 		access_bits = READ_CONTROL | WRITE_DAC | FILE_READ_ATTRIBUTES |
 			DELETE;

2.53.0