fanotify: avoid/silence premature LSM capability checks

Make sure calling capable()/ns_capable() actually leads to access denied
when false is returned, because these functions emit an audit record
when a Linux Security Module denies the capability, which makes it
difficult to avoid allowing/silencing unnecessary permissions in
security policies (namely with SELinux).

Where the return value just used to set a flag, use the non-auditing
ns_capable_noaudit() instead.

Fixes: 7cea2a3 ("fanotify: support limited functionality for unprivileged users")
Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
Reviewed-by: Paul Moore <paul@paul-moore.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Link: https://patch.msgid.link/20260216150625.793013-2-omosnace@redhat.com
Signed-off-by: Jan Kara <jack@suse.cz>

Author: WOnder93

fs/notify/fanotify/fanotify_user.c

@@ -1615,17 +1615,18 @@ SYSCALL_DEFINE2(fanotify_init, unsigned int, flags, unsigned int, event_f_flags)
 	pr_debug("%s: flags=%x event_f_flags=%x\n",
 		 __func__, flags, event_f_flags);
 
-	if (!capable(CAP_SYS_ADMIN)) {
-		/*
-		 * An unprivileged user can setup an fanotify group with
-		 * limited functionality - an unprivileged group is limited to
-		 * notification events with file handles or mount ids and it
-		 * cannot use unlimited queue/marks.
-		 */
-		if ((flags & FANOTIFY_ADMIN_INIT_FLAGS) ||
-		    !(flags & (FANOTIFY_FID_BITS | FAN_REPORT_MNT)))
-			return -EPERM;
+	/*
+	 * An unprivileged user can setup an fanotify group with limited
+	 * functionality - an unprivileged group is limited to notification
+	 * events with file handles or mount ids and it cannot use unlimited
+	 * queue/marks.
+	 */
+	if (((flags & FANOTIFY_ADMIN_INIT_FLAGS) ||
+	     !(flags & (FANOTIFY_FID_BITS | FAN_REPORT_MNT))) &&
+	    !capable(CAP_SYS_ADMIN))
+		return -EPERM;
 
+	if (!ns_capable_noaudit(&init_user_ns, CAP_SYS_ADMIN)) {
 		/*
 		 * Setting the internal flag FANOTIFY_UNPRIV on the group
 		 * prevents setting mount/filesystem marks on this group and
@@ -1990,8 +1991,8 @@ static int do_fanotify_mark(int fanotify_fd, unsigned int flags, __u64 mask,
 	 * A user is allowed to setup sb/mount/mntns marks only if it is
 	 * capable in the user ns where the group was created.
 	 */
-	if (!ns_capable(group->user_ns, CAP_SYS_ADMIN) &&
-	    mark_type != FAN_MARK_INODE)
+	if (mark_type != FAN_MARK_INODE &&
+	    !ns_capable(group->user_ns, CAP_SYS_ADMIN))
 		return -EPERM;
 
 	/*