openat2: reject RESOLVE_BENEATH|RESOLVE_IN_ROOT

This was an oversight in the original implementation, as it makes no
sense to specify both scoping flags to the same openat2(2) invocation
(before this patch, the result of such an invocation was equivalent to
RESOLVE_IN_ROOT being ignored).

This is a userspace-visible ABI change, but the only user of openat2(2)
at the moment is LXC which doesn't specify both flags and so no
userspace programs will break as a result.

Fixes: fddb5d4 ("open: introduce openat2(2) syscall")
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Cc: <stable@vger.kernel.org> # v5.6+
Link: https://lore.kernel.org/r/20201027235044.5240-2-cyphar@cyphar.com
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

fs/open.c

@@ -1010,6 +1010,10 @@ inline int build_open_flags(const struct open_how *how, struct open_flags *op)
 	if (how->resolve & ~VALID_RESOLVE_FLAGS)
 		return -EINVAL;
 
+	/* Scoping flags are mutually exclusive. */
+	if ((how->resolve & RESOLVE_BENEATH) && (how->resolve & RESOLVE_IN_ROOT))
+		return -EINVAL;
+
 	/* Deal with the mode. */
 	if (WILL_CREATE(flags)) {
 		if (how->mode & ~S_IALLUGO)