On Fri, Nov 3, 2017 at 11:42 AM, Oleg Nesterov <[email protected]> wrote: > 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. > > Reported-by: Dmitry Vyukov <[email protected]> > Reported-by: Kyle Huey <[email protected]> > Signed-off-by: Oleg Nesterov <[email protected]>
Reviewed-by: Kees Cook <[email protected]> Thanks for digging through this! Two birds, one stone, etc. :) -Kees > --- > kernel/signal.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/kernel/signal.c b/kernel/signal.c > index 8fc0182..7e15b56 100644 > --- a/kernel/signal.c > +++ b/kernel/signal.c > @@ -931,9 +931,9 @@ 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. > */ > -- > 2.5.0 > > -- Kees Cook Pixel Security
