Re: [PATCH v2] ptrace: fix ptrace vs tasklist_lock race on PREEMPT_RT.

From: Eric W. Biederman
Date: Thu Apr 07 2022 - 13:52:40 EST

Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes:

> On Tue, Apr 05, 2022 at 01:28:16PM +0200, Peter Zijlstra wrote:
>> On Tue, Apr 05, 2022 at 12:29:03PM +0200, Oleg Nesterov wrote:
>> > On 04/05, Peter Zijlstra wrote:
>> > >
>> > > As is, I think we can write task_is_stopped() like:
>> > >
>> > > #define task_is_stopped(task) ((task)->jobctl & JOBCTL_STOP_PENDING)
>> > >
>> > > Because jobctl is in fact the canonical state.
>> >
>> > Not really. JOBCTL_STOP_PENDING means that the task should stop.
>> >
>> > And this flag is cleared right before set_special_state(TASK_STOPPED)
>> > in do_signal_stop(), see task_participate_group_stop().
>>
>> Argh! More work then :-( Still, I really want to not have p->__state be
>> actual unique state.
>
> How insane is this?
>
> In addition to solving the whole saved_state issue, it also allows my
> freezer rewrite to reconstruct __state. If you don't find the below
> fundamentally broken I can respin that freezer rewrite on top of it.

I am confused about what everyone is trying to fix here.
So I can't comment right on the fundamentals except to see
it looks like more code and more complexity which could be a problem.

If this actually solves the fundamental issue we need to coordinate
because I am just about to allocate jobctl bit 31 for JOBCTL_WILL_EXIT
to implement fatal_signal_pending. Among other things that allows
to not have two meanings for SIGKILL in task->pending.

There are also some interesting interactions between some of these
states and processes that have fatal_signal_pending that I should
look at as well.

I can say that I recently looked and signal_wake_up_state is
consistently called with siglock held.

Eric


>
> ---
> include/linux/sched.h | 8 +++-----
> include/linux/sched/jobctl.h | 8 ++++++++
> include/linux/sched/signal.h | 15 ++++++++++++++-
> kernel/ptrace.c | 18 ++++++++++++++----
> kernel/signal.c | 9 ++++++---
> 5 files changed, 45 insertions(+), 13 deletions(-)
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 67f06f72c50e..6698b97b8e3b 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -118,11 +118,9 @@ struct task_group;
>
> #define task_is_running(task) (READ_ONCE((task)->__state) == TASK_RUNNING)
>
> -#define task_is_traced(task) ((READ_ONCE(task->__state) & __TASK_TRACED) != 0)
> -
> -#define task_is_stopped(task) ((READ_ONCE(task->__state) & __TASK_STOPPED) != 0)
> -
> -#define task_is_stopped_or_traced(task) ((READ_ONCE(task->__state) & (__TASK_STOPPED | __TASK_TRACED)) != 0)
> +#define task_is_traced(task) ((READ_ONCE(task->jobctl) & JOBCTL_TRACED) != 0)
> +#define task_is_stopped(task) ((READ_ONCE(task->jobctl) & JOBCTL_STOPPED) != 0)
> +#define task_is_stopped_or_traced(task) ((READ_ONCE(task->jobctl) & (JOBCTL_STOPPED | JOBCTL_TRACED)) != 0)
>
> /*
> * Special states are those that do not use the normal wait-loop pattern. See
> diff --git a/include/linux/sched/jobctl.h b/include/linux/sched/jobctl.h
> index fa067de9f1a9..ec8b312f7506 100644
> --- a/include/linux/sched/jobctl.h
> +++ b/include/linux/sched/jobctl.h
> @@ -20,6 +20,10 @@ struct task_struct;
> #define JOBCTL_LISTENING_BIT 22 /* ptracer is listening for events */
> #define JOBCTL_TRAP_FREEZE_BIT 23 /* trap for cgroup freezer */
>
> +#define JOBCTL_STOPPED_BIT 24
> +#define JOBCTL_TRACED_BIT 25
> +#define JOBCTL_TRACED_FROZEN_BIT 26
> +
> #define JOBCTL_STOP_DEQUEUED (1UL << JOBCTL_STOP_DEQUEUED_BIT)
> #define JOBCTL_STOP_PENDING (1UL << JOBCTL_STOP_PENDING_BIT)
> #define JOBCTL_STOP_CONSUME (1UL << JOBCTL_STOP_CONSUME_BIT)
> @@ -29,6 +33,10 @@ struct task_struct;
> #define JOBCTL_LISTENING (1UL << JOBCTL_LISTENING_BIT)
> #define JOBCTL_TRAP_FREEZE (1UL << JOBCTL_TRAP_FREEZE_BIT)
>
> +#define JOBCTL_STOPPED (1UL << JOBCTL_STOPPED_BIT)
> +#define JOBCTL_TRACED (1UL << JOBCTL_TRACED_BIT)
> +#define JOBCTL_TRACED_FROZEN (1UL << JOBCTL_TRACED_FROZEN_BIT)
> +
> #define JOBCTL_TRAP_MASK (JOBCTL_TRAP_STOP | JOBCTL_TRAP_NOTIFY)
> #define JOBCTL_PENDING_MASK (JOBCTL_STOP_PENDING | JOBCTL_TRAP_MASK)
>
> diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
> index 3c8b34876744..3e4a14ed402e 100644
> --- a/include/linux/sched/signal.h
> +++ b/include/linux/sched/signal.h
> @@ -294,8 +294,10 @@ static inline int kernel_dequeue_signal(void)
> static inline void kernel_signal_stop(void)
> {
> spin_lock_irq(&current->sighand->siglock);
> - if (current->jobctl & JOBCTL_STOP_DEQUEUED)
> + if (current->jobctl & JOBCTL_STOP_DEQUEUED) {
> + current->jobctl |= JOBCTL_STOPPED;
> set_special_state(TASK_STOPPED);
> + }
> spin_unlock_irq(&current->sighand->siglock);
>
> schedule();
> @@ -437,10 +439,21 @@ extern void signal_wake_up_state(struct task_struct *t, unsigned int state);
>
> static inline void signal_wake_up(struct task_struct *t, bool resume)
> {
> + lockdep_assert_held(t->sighand->siglock);
> +
> + if (resume && !(t->jobctl & JOBCTL_TRACED_FROZEN))
> + t->jobctl &= ~(JOBCTL_STOPPED | JOBCTL_TRACED);
> +
> signal_wake_up_state(t, resume ? TASK_WAKEKILL : 0);
> }
> +
> static inline void ptrace_signal_wake_up(struct task_struct *t, bool resume)
> {
> + lockdep_assert_held(t->sighand->siglock);
> +
> + if (resume)
> + t->jobctl &= ~JOBCTL_TRACED;
> +
> signal_wake_up_state(t, resume ? __TASK_TRACED : 0);
> }
>
> diff --git a/kernel/ptrace.c b/kernel/ptrace.c
> index ccc4b465775b..626f96d275c7 100644
> --- a/kernel/ptrace.c
> +++ b/kernel/ptrace.c
> @@ -185,7 +185,12 @@ static bool looks_like_a_spurious_pid(struct task_struct *task)
> return true;
> }
>
> -/* Ensure that nothing can wake it up, even SIGKILL */
> +/*
> + * Ensure that nothing can wake it up, even SIGKILL
> + *
> + * A task is switched to this state while a ptrace operation is in progress;
> + * such that the ptrace operation is uninterruptible.
> + */
> static bool ptrace_freeze_traced(struct task_struct *task)
> {
> bool ret = false;
> @@ -197,6 +202,7 @@ static bool ptrace_freeze_traced(struct task_struct *task)
> spin_lock_irq(&task->sighand->siglock);
> if (task_is_traced(task) && !looks_like_a_spurious_pid(task) &&
> !__fatal_signal_pending(task)) {
> + task->jobctl |= JOBCTL_TRACED_FROZEN;
> WRITE_ONCE(task->__state, __TASK_TRACED);
> ret = true;
> }
> @@ -218,9 +224,11 @@ static void ptrace_unfreeze_traced(struct task_struct *task)
> */
> spin_lock_irq(&task->sighand->siglock);
> if (READ_ONCE(task->__state) == __TASK_TRACED) {
> - if (__fatal_signal_pending(task))
> + task->jobctl &= ~JOBCTL_TRACED_FROZEN;
> + if (__fatal_signal_pending(task)) {
> + task->jobctl &= ~JOBCTL_TRACED;
> wake_up_state(task, __TASK_TRACED);
> - else
> + } else
> WRITE_ONCE(task->__state, TASK_TRACED);
> }
> spin_unlock_irq(&task->sighand->siglock);
> @@ -475,8 +483,10 @@ static int ptrace_attach(struct task_struct *task, long request,
> * in and out of STOPPED are protected by siglock.
> */
> if (task_is_stopped(task) &&
> - task_set_jobctl_pending(task, JOBCTL_TRAP_STOP | JOBCTL_TRAPPING))
> + task_set_jobctl_pending(task, JOBCTL_TRAP_STOP | JOBCTL_TRAPPING)) {
> + task->jobctl &= ~JOBCTL_STOPPED;
> signal_wake_up_state(task, __TASK_STOPPED);
> + }
>
> spin_unlock(&task->sighand->siglock);
>
> diff --git a/kernel/signal.c b/kernel/signal.c
> index 30cd1ca43bcd..0aea3f0a8002 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -884,7 +884,7 @@ static int check_kill_permission(int sig, struct kernel_siginfo *info,
> static void ptrace_trap_notify(struct task_struct *t)
> {
> WARN_ON_ONCE(!(t->ptrace & PT_SEIZED));
> - assert_spin_locked(&t->sighand->siglock);
> + lockdep_assert_held(&t->sighand->siglock);
>
> task_set_jobctl_pending(t, JOBCTL_TRAP_NOTIFY);
> ptrace_signal_wake_up(t, t->jobctl & JOBCTL_LISTENING);
> @@ -930,9 +930,10 @@ static bool prepare_signal(int sig, struct task_struct *p, bool force)
> for_each_thread(p, t) {
> flush_sigqueue_mask(&flush, &t->pending);
> task_clear_jobctl_pending(t, JOBCTL_STOP_PENDING);
> - if (likely(!(t->ptrace & PT_SEIZED)))
> + if (likely(!(t->ptrace & PT_SEIZED))) {
> + t->jobctl &= ~JOBCTL_STOPPED;
> wake_up_state(t, __TASK_STOPPED);
> - else
> + } else
> ptrace_trap_notify(t);
> }
>
> @@ -2219,6 +2220,7 @@ static int ptrace_stop(int exit_code, int why, int clear_code,
> * schedule() will not sleep if there is a pending signal that
> * can awaken the task.
> */
> + current->jobctl |= JOBCTL_TRACED;
> set_special_state(TASK_TRACED);
>
> /*
> @@ -2460,6 +2462,7 @@ static bool do_signal_stop(int signr)
> if (task_participate_group_stop(current))
> notify = CLD_STOPPED;
>
> + current->jobctl |= JOBCTL_STOPPED;
> set_special_state(TASK_STOPPED);
> spin_unlock_irq(&current->sighand->siglock);
>