From: Oleg Nesterov oleg@redhat.com
This patch has been added to the 4.1 stable tree. If you have any objections, please let us know.
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[ Upstream commit 426915796ccaf9c2bd9bb06dc5702225957bc2e5 ]
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: Sasha Levin alexander.levin@microsoft.com --- kernel/signal.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/kernel/signal.c b/kernel/signal.c index 4e61b75be17a..46a2471173b8 100644 --- a/kernel/signal.c +++ b/kernel/signal.c @@ -962,9 +962,9 @@ static void complete_signal(int sig, struct task_struct *p, int group) * 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. */