[PATCH 1/2] ksmbd: let the owner read and change the DACL when no ACE names it

[Ian Bridges <icb@fastmail.org>]

smb_check_perm_dacl() refuses the owner when neither its DACL lookup nor
the POSIX ACL fallback matches. The owner then cannot read or replace
the DACL over SMB. MS-DTYP 2.5.3.2 grants the owner READ_CONTROL and
WRITE_DAC unless an OWNER RIGHTS ACE applies.

An empty DACL allows the owner's explicit requests for these rights but
refuses MAXIMUM_ALLOWED. smb2_open() uses that check for the maximal
access reply (MS-SMB2 3.3.5.9.5), so even a READ_CONTROL open fails when
it asks for that reply.

Grant the owner's implicit rights on both paths, for explicit requests
and MAXIMUM_ALLOWED, unless an OWNER RIGHTS ACE applies.

Fixes: e2f34481b24d ("cifsd: add server-side procedures for SMB3")
Cc: stable@vger.kernel.org
Signed-off-by: Ian Bridges <icb@fastmail.org>
---

Notes:
    Reachable on a share with "vfs objects = acl_xattr", for example after
    the owner replaces a file's DACL with one that grants only another
    user.
    
    The empty-DACL check this patch changes came with de0cffef897c ("ksmbd:
    deny access for an empty DACL instead of granting the request"), merged
    for v7.4, so an older stable tree needs that commit first. The check
    before it refused MAXIMUM_ALLOWED on an empty DACL as well.
    
    Tested on ksmbd-for-next d13da43938d1 with KASAN. Only the owner's
    results change. The owner of a file whose only ACE grants another user
    can now open it for READ_CONTROL or WRITE_DAC, and is still refused
    READ_DATA. On an empty DACL, the owner's MAXIMUM_ALLOWED handle gets
    READ_CONTROL and WRITE_DAC when POSIX permits the open. A READ_CONTROL
    open with a maximal access context also succeeds. Its handle holds
    READ_CONTROL, and its reply reports both rights. Without POSIX read
    permission, MAXIMUM_ALLOWED still fails in smb2_open(). An ACE stored
    after a DACL whose ACE count is zero still grants nothing.
    
    A reproducer is available on request.
    
    This fix was developed with LLM assistance.

 fs/smb/server/smbacl.c | 34 ++++++++++++++++++++++++++++++----
 1 file changed, 30 insertions(+), 4 deletions(-)

diff --git a/fs/smb/server/smbacl.c b/fs/smb/server/smbacl.c
index 664f01263611..83974643aa88 100644
--- a/fs/smb/server/smbacl.c
+++ b/fs/smb/server/smbacl.c
@@ -1477,6 +1477,7 @@ int smb_check_perm_dacl(struct ksmbd_conn *conn, const struct path *path,
 	int i, found = 0;
 	unsigned int access_bits = 0, denied = 0;
 	struct smb_ace *others_ace = NULL;
+	bool or_ace = false;
 	struct posix_acl_entry *pa_entry;
 	unsigned int sid_type = SIDOWNER;
 	unsigned short ace_size;
@@ -1509,6 +1510,8 @@ int smb_check_perm_dacl(struct ksmbd_conn *conn, const struct path *path,
 	is_owner = uid == from_kuid(&init_user_ns, vfsuid_into_kuid(vfsuid));
 
 	if (!pdacl->num_aces) {
+		unsigned int allowed = 0;
+
 		/*
 		 * An empty (present, zero-ACE) DACL grants no access to
 		 * anyone except the object owner's implicit READ_CONTROL
@@ -1517,9 +1520,14 @@ int smb_check_perm_dacl(struct ksmbd_conn *conn, const struct path *path,
 		 * 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)))
+		if (is_owner)
+			allowed = READ_CONTROL | WRITE_DAC;
+		if (*pdaccess & FILE_MAXIMAL_ACCESS_LE)
+			requested |= allowed;
+		if (!allowed || requested & ~allowed)
 			rc = -EACCES;
+		else
+			*pdaccess = cpu_to_le32(requested);
 		goto err_out;
 	}
 
@@ -1604,6 +1612,9 @@ int smb_check_perm_dacl(struct ksmbd_conn *conn, const struct path *path,
 			found = 1;
 			break;
 		}
+		if (!compare_sids(&sid_owner_rights, &ace->sid) &&
+		    !(ace->flags & INHERIT_ONLY_ACE))
+			or_ace = true;
 		if (!compare_sids(&sid_everyone, &ace->sid) ||
 		    !compare_sids(&sid_authusers, &ace->sid))
 			others_ace = ace;
@@ -1645,8 +1656,23 @@ int smb_check_perm_dacl(struct ksmbd_conn *conn, const struct path *path,
 		if (others_ace) {
 			ace = others_ace;
 		} else {
-			ksmbd_debug(SMB, "Can't find corresponding sid\n");
-			rc = -EACCES;
+			unsigned int allowed = 0;
+
+			/*
+			 * No ACE names the owner, who may still read and
+			 * change the DACL unless OWNER RIGHTS applies
+			 * (MS-DTYP 2.5.3.2).
+			 */
+			if (is_owner && !or_ace)
+				allowed = READ_CONTROL | WRITE_DAC;
+			if (*pdaccess & FILE_MAXIMAL_ACCESS_LE)
+				requested |= allowed;
+			if (!allowed || requested & ~allowed) {
+				ksmbd_debug(SMB, "Can't find corresponding sid\n");
+				rc = -EACCES;
+				goto err_out;
+			}
+			*pdaccess = cpu_to_le32(requested);
 			goto err_out;
 		}
 	}
-- 
2.47.3