seccomp: Only dump core when single-threaded

The SECCOMP_RET_KILL filter return code has always killed the current
thread, not the entire process. Changing this as a side-effect of dumping
core isn't a safe thing to do (a few test suites have already flagged this
behavioral change). Instead, restore the RET_KILL semantics, but still
dump core when a RET_KILL delivers SIGSYS to a single-threaded process.

Fixes: b25e6716 ("seccomp: dump core when using SECCOMP_RET_KILL")
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>

kernel/seccomp.c

@@ -643,11 +643,14 @@ static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
 	default: {
 		siginfo_t info;
 		audit_seccomp(this_syscall, SIGSYS, action);
+		/* Dump core only if this is the last remaining thread. */
+		if (get_nr_threads(current) == 1) {
 			/* Show the original registers in the dump. */
 			syscall_rollback(current, task_pt_regs(current));
 			/* Trigger a manual coredump since do_exit skips it. */
 			seccomp_init_siginfo(&info, this_syscall, data);
 			do_coredump(&info);
+		}
 		do_exit(SIGSYS);
 	}
 	}