[RFC][PATCH] signal: Store pending signal exit in tsk.jobctl not in tsk.pending

[Eric W. Biederman]

Recently syzkaller was able to create unkillablle processes by
creating a timer that is delivered as a thread local signal on SIGHUP,
and receiving SIGHUP SA_NODEFERER.  Ultimately causing a loop
failing to deliver SIGHUP but always trying.

Upon examination it turns out part of the problem is actually most of
the solution.  Since 2.5 complete_signal has found all fatal signals
and queued SIGKILL in every threads thread queue relying on
signal->group_exit_code to preserve the information of which was the
actual fatal signal.

The conversion of all fatal signals to SIGKILL results in the
synchronous signal heuristic in next_signal kicking in and preferring
SIGHUP to SIGKILL.  Which is especially problematic as all
fatal signals have already been transformed into SIGKILL.

Now that we have task->jobctl we can do better and set a bit in
task->jobctl instead of reusing tsk->pending[SIGKILL].  This allows
giving already detected process exits a higher priority than any
pending signal.

Cc: sta...@vger.kernel.org
Reported-by: Dmitry Vyukov <dvyu...@google.com>
Ref: ebf5ebe31d2c ("[PATCH] signal-fixes-2.5.59-A4")
History Tree: https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git
Signed-off-by: "Eric W. Biederman" <ebied...@xmission.com>
---
Oleg, can you give this a quick review and see if I am missing anything?
Dmitry, can you verify this runs cleanly in your test setups?

 fs/coredump.c                |  2 +-
 include/linux/sched/jobctl.h |  1 +
 include/linux/sched/signal.h |  2 +-
 kernel/signal.c              | 10 ++++++++--
 4 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/fs/coredump.c b/fs/coredump.c
index e42e17e55bfd..487995293ef0 100644
--- a/fs/coredump.c
+++ b/fs/coredump.c
@@ -322,7 +322,7 @@ static int zap_process(struct task_struct *start, int 
exit_code, int flags)
        for_each_thread(start, t) {
                task_clear_jobctl_pending(t, JOBCTL_PENDING_MASK);
                if (t != current && t->mm) {
-                       sigaddset(&t->pending.signal, SIGKILL);
+                       t->jobctl |= JOBCTL_TASK_EXIT;
                        signal_wake_up(t, 1);
                        nr++;
                }
diff --git a/include/linux/sched/jobctl.h b/include/linux/sched/jobctl.h
index 98228bd48aee..ff7b3ea43f4c 100644
--- a/include/linux/sched/jobctl.h
+++ b/include/linux/sched/jobctl.h
@@ -18,6 +18,7 @@ struct task_struct;
 #define JOBCTL_TRAP_NOTIFY_BIT 20      /* trap for NOTIFY */
 #define JOBCTL_TRAPPING_BIT    21      /* switching to TRACED */
 #define JOBCTL_LISTENING_BIT   22      /* ptracer is listening for events */
+#define JOBCTL_TASK_EXIT       23      /* task is exiting */
 
 #define JOBCTL_STOP_DEQUEUED   (1UL << JOBCTL_STOP_DEQUEUED_BIT)
 #define JOBCTL_STOP_PENDING    (1UL << JOBCTL_STOP_PENDING_BIT)
diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
index 13789d10a50e..3f3edadf1ae1 100644
--- a/include/linux/sched/signal.h
+++ b/include/linux/sched/signal.h
@@ -354,7 +354,7 @@ static inline int signal_pending(struct task_struct *p)
 
 static inline int __fatal_signal_pending(struct task_struct *p)
 {
-       return unlikely(sigismember(&p->pending.signal, SIGKILL));
+       return unlikely(p->jobctl & JOBCTL_TASK_EXIT);
 }
 
 static inline int fatal_signal_pending(struct task_struct *p)
diff --git a/kernel/signal.c b/kernel/signal.c
index 9ca8e5278c8e..0577e37fdf43 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -989,7 +989,7 @@ static void complete_signal(int sig, struct task_struct *p, 
enum pid_type type)
                        t = p;
                        do {
                                task_clear_jobctl_pending(t, 
JOBCTL_PENDING_MASK);
-                               sigaddset(&t->pending.signal, SIGKILL);
+                               t->jobctl |= JOBCTL_TASK_EXIT;
                                signal_wake_up(t, 1);
                        } while_each_thread(p, t);
                        return;
@@ -1273,7 +1273,7 @@ int zap_other_threads(struct task_struct *p)
                /* Don't bother with already dead threads */
                if (t->exit_state)
                        continue;
-               sigaddset(&t->pending.signal, SIGKILL);
+               t->jobctl |= JOBCTL_TASK_EXIT;
                signal_wake_up(t, 1);
        }
 
@@ -2393,6 +2393,11 @@ bool get_signal(struct ksignal *ksig)
                goto relock;
        }
 
+       /* Has this task already been flagged for death? */
+       ksig->info.si_signo = signr = SIGKILL;
+       if (current->jobctl & JOBCTL_TASK_EXIT)
+               goto fatal;
+
        for (;;) {
                struct k_sigaction *ka;
 
@@ -2488,6 +2493,7 @@ bool get_signal(struct ksignal *ksig)
                        continue;
                }
 
+       fatal:
                spin_unlock_irq(&sighand->siglock);
 
                /*
-- 
2.17.1