seccomp: mask setns() nstype to 32 bits in RestrictNamespaces=

The nstype argument of setns() is an int, but it is passed to the kernel
in a 64-bit register, and the kernel truncates it to the low 32 bits.
The rule that blocks setns() with nstype == 0 compared the full 64-bit
register with SCMP_CMP_EQ, so setting any bit above bit 31 let the call
through, and the kernel then joined whichever namespace the fd referred
to, even one that RestrictNamespaces= is supposed to deny.

Use SCMP_CMP_MASKED_EQ with a 32-bit mask so we match on the same value
the kernel does.

Add a test that issues setns() via syscall() with an upper bit set.

Follow-up for #44134

Signed-off-by: Ricardo Branco <rbranco@suse.de>
(cherry picked from commit da72537164910f9050793055fef030711c00b954)

src/shared/seccomp-util.c

@@ -1429,13 +1429,14 @@ int seccomp_restrict_namespaces(unsigned long retain) {
                                         0);
                 else
                         /* Otherwise, block only the invocations with the appropriate flags in the loop
-                         * below, but also the special invocation with a zero flags argument, right here. */
+                         * below, but also the special invocation with a zero flags argument, right here. The
+                         * kernel takes nstype as an int and truncates it to 32 bits, hence only compare those. */
                         r = sym_seccomp_rule_add_exact(
                                         seccomp,
                                         SCMP_ACT_ERRNO(EPERM),
                                         SCMP_SYS(setns),
                                         1,
-                                        SCMP_A1(SCMP_CMP_EQ, 0));
+                                        SCMP_A1(SCMP_CMP_MASKED_EQ, UINT32_MAX, 0));
                 if (r < 0) {
                         log_debug_errno(r, "Failed to add setns() rule for architecture %s, skipping: %m",
                                         seccomp_arch_to_string(arch));