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