fs/posix_acl: apply umask if superblock disables ACL support

The function posix_acl_create() applies the umask only if the inode has no ACL (= NULL) or if ACLs are not supported by the filesystem driver (= -EOPNOTSUPP).

However, this happens only after after the IS_POSIXACL() check succeeded. If the superblock doesn't enable ACL support, umask will never be applied. A filesystem which has no ACL support will of course not enable SB_POSIXACL, rendering the umask-applying code path unreachable.

This fixes a bug which causes the umask to be ignored with O_TMPFILE on tmpfs:

https://github.com/MusicPlayerDaemon/MPD/issues/558
https://bugs.gentoo.org/show_bug.cgi?id=686142#c3
https://bugzilla.kernel.org/show_bug.cgi?id=203625

---
 fs/posix_acl.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

index a05fe94970ce..79831269dd2f 100644
@@ -642,9 +642,14 @@ posix_acl_create(struct inode *dir, umode_t *mode,
 	*acl = NULL;
 	*default_acl = NULL;
 
-	if (S_ISLNK(*mode) || !IS_POSIXACL(dir))
+	if (S_ISLNK(*mode))
 		return 0;
 
+	if (!IS_POSIXACL(dir)) {
+		*mode &= ~current_umask();
+		return 0;
+	}
+
 	p = get_inode_acl(dir, ACL_TYPE_DEFAULT);
 	if (!p || p == ERR_PTR(-EOPNOTSUPP)) {
 		*mode &= ~current_umask();

2.39.2

The fix is in the wrong place if !IS_POSIXACL() umask stripping happens in the VFS. So if at all we need to fix stripping umask for O_TMPFILE in the vfs.

Have you verified that commit ac6800e279a2 ("fs: Add missing umask strip in vfs_tmpfile") doesn't already fix this?

No, I havn't - I submitted this patch already several years ago, but it was never merged, and since then, I've been carrying this patch around in all kernels I ever used. While doing some other kernel work this week, I decided to resubmit it, because I thought it's a security vulnerability to ignore the umask. But thanks, it's a good hint, I'll check that 2022 commit.