seccomp: set the seccomp filter to all threads

When using "-seccomp on", the seccomp policy is only applied to the
main thread, the vcpu worker thread and other worker threads created
after seccomp policy is applied; the seccomp policy is not applied to
e.g. the RCU thread because it is created before the seccomp policy is
applied and SECCOMP_FILTER_FLAG_TSYNC isn't used.

This can be verified with
for task in /proc/`pidof qemu`/task/*; do cat $task/status | grep Secc ; done
Seccomp:	2
Seccomp:	0
Seccomp:	0
Seccomp:	2
Seccomp:	2
Seccomp:	2

Starting with libseccomp 2.2.0 and kernel >= 3.17, we can use
seccomp_attr_set(ctx, > SCMP_FLTATR_CTL_TSYNC, 1) to update the policy
on all threads.

libseccomp requirement was bumped to 2.2.0 in previous patch.
libseccomp should fail to set the filter if it can't honour
SCMP_FLTATR_CTL_TSYNC (untested), and thus -sandbox will now fail on
kernel < 3.17.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Acked-by: Eduardo Otubo <otubo@redhat.com>
(cherry picked from commit 70dfabeaa7)
*CVE-2018-15746
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

qemu-seccomp.c

@@ -149,6 +149,11 @@ static int seccomp_start(uint32_t seccomp_opts)
         goto seccomp_return;
     }
 
+    rc = seccomp_attr_set(ctx, SCMP_FLTATR_CTL_TSYNC, 1);
+    if (rc != 0) {
+        goto seccomp_return;
+    }
+
     for (i = 0; i < ARRAY_SIZE(blacklist); i++) {
         if (!(seccomp_opts & blacklist[i].set)) {
             continue;