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Date: Fri, 3 Nov 2017 12:14:25 -0700 From: Kees Cook <keescook@...omium.org> To: Oleg Nesterov <oleg@...hat.com> Cc: Andrew Morton <akpm@...ux-foundation.org>, "Eric W. Biederman" <ebiederm@...ssion.com>, security@...nel.org, "Robert O'Callahan" <robert@...llahan.org>, Andy Lutomirski <luto@...nel.org>, Dmitry Vyukov <dvyukov@...gle.com>, Kyle Huey <me@...ehuey.com>, LKML <linux-kernel@...r.kernel.org> Subject: Re: [PATCH 3/3] remove the no longer needed SIGNAL_UNKILLABLE check in complete_signal() On Fri, Nov 3, 2017 at 11:42 AM, Oleg Nesterov <oleg@...hat.com> 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 <dvyukov@...gle.com> > Reported-by: Kyle Huey <me@...ehuey.com> > Signed-off-by: Oleg Nesterov <oleg@...hat.com> Reviewed-by: Kees Cook <keescook@...omium.org> 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
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