This is a note to let you know that I've just added the patch titled

kernel/signal.c: remove the no longer needed SIGNAL_UNKILLABLE check in complete_signal()

to the 4.9-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git%3Ba=su...

The filename of the patch is: kernel-signal.c-remove-the-no-longer-needed-signal_unkillable-check-in-complete_signal.patch and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let stable@vger.kernel.org know about it.

From 426915796ccaf9c2bd9bb06dc5702225957bc2e5 Mon Sep 17 00:00:00 2001

From: Oleg Nesterov oleg@redhat.com Date: Fri, 17 Nov 2017 15:30:08 -0800 Subject: kernel/signal.c: remove the no longer needed SIGNAL_UNKILLABLE check in complete_signal()

From: Oleg Nesterov oleg@redhat.com

commit 426915796ccaf9c2bd9bb06dc5702225957bc2e5 upstream.

complete_signal() checks SIGNAL_UNKILLABLE before it starts to destroy the thread group, today this is wrong in many ways.

If nothing else, fatal_signal_pending() should always imply that the whole thread group (except ->group_exit_task if it is not NULL) is killed, this check breaks the rule.

After the previous changes we can rely on sig_task_ignored(); sig_fatal(sig) && SIGNAL_UNKILLABLE can only be true if we actually want to kill this task and sig == SIGKILL OR it is traced and debugger can intercept the signal.

This should hopefully fix the problem reported by Dmitry. This test-case

static int init(void *arg) { for (;;) pause(); }

int main(void) { char stack[16 * 1024];

for (;;) { int pid = clone(init, stack + sizeof(stack)/2, CLONE_NEWPID | SIGCHLD, NULL); assert(pid > 0);

assert(ptrace(PTRACE_ATTACH, pid, 0, 0) == 0); assert(waitpid(-1, NULL, WSTOPPED) == pid);

assert(ptrace(PTRACE_DETACH, pid, 0, SIGSTOP) == 0); assert(syscall(__NR_tkill, pid, SIGKILL) == 0); assert(pid == wait(NULL)); } }

triggers the WARN_ON_ONCE(!(task->jobctl & JOBCTL_STOP_PENDING)) in task_participate_group_stop(). do_signal_stop()->signal_group_exit() checks SIGNAL_GROUP_EXIT and return false, but task_set_jobctl_pending() checks fatal_signal_pending() and does not set JOBCTL_STOP_PENDING.

And his should fix the minor security problem reported by Kyle, SECCOMP_RET_TRACE can miss fatal_signal_pending() the same way if the task is the root of a pid namespace.

Link: http://lkml.kernel.org/r/20171103184246.GD21036@redhat.com Signed-off-by: Oleg Nesterov oleg@redhat.com Reported-by: Dmitry Vyukov dvyukov@google.com Reported-by: Kyle Huey me@kylehuey.com Reviewed-by: Kees Cook keescook@chromium.org Tested-by: Kyle Huey me@kylehuey.com Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org

--- kernel/signal.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/kernel/signal.c +++ b/kernel/signal.c @@ -919,9 +919,9 @@ static void complete_signal(int sig, str * then start taking the whole group down immediately. */ if (sig_fatal(p, sig) && - !(signal->flags & (SIGNAL_UNKILLABLE | SIGNAL_GROUP_EXIT)) && + !(signal->flags & SIGNAL_GROUP_EXIT) && !sigismember(&t->real_blocked, sig) && - (sig == SIGKILL || !t->ptrace)) { + (sig == SIGKILL || !p->ptrace)) { /* * This signal will be fatal to the whole group. */

Patches currently in stable-queue which might be from oleg@redhat.com are

queue-4.9/kernel-signal.c-remove-the-no-longer-needed-signal_unkillable-check-in-complete_signal.patch queue-4.9/kernel-signal.c-protect-the-signal_unkillable-tasks-from-sig_kernel_only-signals.patch queue-4.9/kernel-acct.c-fix-the-acct-needcheck-check-in-check_free_space.patch queue-4.9/kernel-signal.c-protect-the-traced-signal_unkillable-tasks-from-sigkill.patch