Messages in this thread | | | Date | Mon, 14 Mar 2022 10:27:37 +0100 | From | Sebastian Andrzej Siewior <> | Subject | Re: [PATCH] ptrace: fix ptrace vs tasklist_lock race on PREEMPT_RT. |
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On 2022-03-02 22:04:25 [+0100], To linux-kernel@vger.kernel.org wrote: > As explained by Alexander Fyodorov <halcy@yandex.ru>: > > |read_lock(&tasklist_lock) in ptrace_stop() is converted to sleeping > |lock on a PREEMPT_RT kernel, and it can remove __TASK_TRACED from > |task->state (by moving it to task->saved_state). If parent does > |wait() on child followed by a sys_ptrace call, the following race can > |happen: > | > |- child sets __TASK_TRACED in ptrace_stop() > |- parent does wait() which eventually calls wait_task_stopped() and returns > | child's pid > |- child blocks on read_lock(&tasklist_lock) in ptrace_stop() and moves > | __TASK_TRACED flag to saved_state > |- parent calls sys_ptrace, which calls ptrace_check_attach() and > | wait_task_inactive() > > The patch is based on his initial patch where an additional check is > added in case the __TASK_TRACED moved to ->saved_state. The pi_lock is > taken in case the caller is interrupted between looking into ->state and > ->saved_state. > > [ Fix for ptrace_unfreeze_traced() by Oleg Nesterov ] > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
ping.
> --- > > I redid the state matching part compared to what I had in my queue so it > hopefully looks better. > > include/linux/sched.h | 127 ++++++++++++++++++++++++++++++++++++++++-- > kernel/ptrace.c | 25 +++++---- > kernel/sched/core.c | 5 +- > 3 files changed, 140 insertions(+), 17 deletions(-) > > diff --git a/include/linux/sched.h b/include/linux/sched.h > index 75ba8aa60248b..73109ce7ce789 100644 > --- a/include/linux/sched.h > +++ b/include/linux/sched.h > @@ -118,12 +118,8 @@ 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) > - > /* > * Special states are those that do not use the normal wait-loop pattern. See > * the comment with set_special_state(). > @@ -2006,6 +2002,129 @@ static inline int test_tsk_need_resched(struct task_struct *tsk) > return unlikely(test_tsk_thread_flag(tsk,TIF_NEED_RESCHED)); > } > > +#ifdef CONFIG_PREEMPT_RT > + > +static inline bool task_state_match_and(struct task_struct *tsk, long state) > +{ > + unsigned long flags; > + bool match = false; > + > + raw_spin_lock_irqsave(&tsk->pi_lock, flags); > + if (READ_ONCE(tsk->__state) & state) > + match = true; > + else if (tsk->saved_state & state) > + match = true; > + raw_spin_unlock_irqrestore(&tsk->pi_lock, flags); > + return match; > +} > + > +static inline bool __task_state_match_eq(struct task_struct *tsk, long state) > +{ > + bool match = false; > + > + if (READ_ONCE(tsk->__state) == state) > + match = true; > + else if (tsk->saved_state == state) > + match = true; > + return match; > +} > + > +static inline bool task_state_match_eq(struct task_struct *tsk, long state) > +{ > + unsigned long flags; > + bool match; > + > + raw_spin_lock_irqsave(&tsk->pi_lock, flags); > + match = __task_state_match_eq(tsk, state); > + raw_spin_unlock_irqrestore(&tsk->pi_lock, flags); > + return match; > +} > + > +static inline bool task_state_match_and_set(struct task_struct *tsk, long state, > + long new_state) > +{ > + unsigned long flags; > + bool match = false; > + > + raw_spin_lock_irqsave(&tsk->pi_lock, flags); > + if (READ_ONCE(tsk->__state) & state) { > + WRITE_ONCE(tsk->__state, new_state); > + match = true; > + } else if (tsk->saved_state & state) { > + tsk->__state = new_state; > + match = true; > + } > + raw_spin_unlock_irqrestore(&tsk->pi_lock, flags); > + return match; > +} > + > +static inline bool task_state_match_eq_set(struct task_struct *tsk, long state, > + long new_state) > +{ > + unsigned long flags; > + bool match = false; > + > + raw_spin_lock_irqsave(&tsk->pi_lock, flags); > + if (READ_ONCE(tsk->__state) == state) { > + WRITE_ONCE(tsk->__state, new_state); > + match = true; > + } else if (tsk->saved_state == state) { > + tsk->saved_state = new_state; > + match = true; > + } > + raw_spin_unlock_irqrestore(&tsk->pi_lock, flags); > + return match; > +} > + > +#else > + > +static inline bool task_state_match_and(struct task_struct *tsk, long state) > +{ > + return READ_ONCE(tsk->__state) & state; > +} > + > +static inline bool __task_state_match_eq(struct task_struct *tsk, long state) > +{ > + return READ_ONCE(tsk->__state) == state; > +} > + > +static inline bool task_state_match_eq(struct task_struct *tsk, long state) > +{ > + return __task_state_match_eq(tsk, state); > +} > + > +static inline bool task_state_match_and_set(struct task_struct *tsk, long state, > + long new_state) > +{ > + if (READ_ONCE(tsk->__state) & state) { > + WRITE_ONCE(tsk->__state, new_state); > + return true; > + } > + return false; > +} > + > +static inline bool task_state_match_eq_set(struct task_struct *tsk, long state, > + long new_state) > +{ > + if (READ_ONCE(tsk->__state) == state) { > + WRITE_ONCE(tsk->__state, new_state); > + return true; > + } > + return false; > +} > + > +#endif > + > +static inline bool task_is_traced(struct task_struct *tsk) > +{ > + return task_state_match_and(tsk, __TASK_TRACED); > +} > + > +static inline bool task_is_stopped_or_traced(struct task_struct *tsk) > +{ > + return task_state_match_and(tsk, __TASK_STOPPED | __TASK_TRACED); > +} > + > /* > * cond_resched() and cond_resched_lock(): latency reduction via > * explicit rescheduling in places that are safe. The return > diff --git a/kernel/ptrace.c b/kernel/ptrace.c > index eea265082e975..5ce0948c0c0a7 100644 > --- a/kernel/ptrace.c > +++ b/kernel/ptrace.c > @@ -195,10 +195,10 @@ static bool ptrace_freeze_traced(struct task_struct *task) > return ret; > > spin_lock_irq(&task->sighand->siglock); > - if (task_is_traced(task) && !looks_like_a_spurious_pid(task) && > - !__fatal_signal_pending(task)) { > - WRITE_ONCE(task->__state, __TASK_TRACED); > - ret = true; > + if (!looks_like_a_spurious_pid(task) && !__fatal_signal_pending(task)) { > + > + ret = task_state_match_and_set(task, __TASK_TRACED, > + __TASK_TRACED); > } > spin_unlock_irq(&task->sighand->siglock); > > @@ -207,7 +207,10 @@ static bool ptrace_freeze_traced(struct task_struct *task) > > static void ptrace_unfreeze_traced(struct task_struct *task) > { > - if (READ_ONCE(task->__state) != __TASK_TRACED) > + bool frozen; > + > + if (!IS_ENABLED(CONFIG_PREEMPT_RT) && > + READ_ONCE(task->__state) != __TASK_TRACED) > return; > > WARN_ON(!task->ptrace || task->parent != current); > @@ -217,12 +220,12 @@ static void ptrace_unfreeze_traced(struct task_struct *task) > * Recheck state under the lock to close this race. > */ > spin_lock_irq(&task->sighand->siglock); > - if (READ_ONCE(task->__state) == __TASK_TRACED) { > - if (__fatal_signal_pending(task)) > - wake_up_state(task, __TASK_TRACED); > - else > - WRITE_ONCE(task->__state, TASK_TRACED); > - } > + > + frozen = task_state_match_eq_set(task, __TASK_TRACED, TASK_TRACED); > + > + if (frozen && __fatal_signal_pending(task)) > + wake_up_state(task, __TASK_TRACED); > + > spin_unlock_irq(&task->sighand->siglock); > } > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index 9745613d531ce..a44414946de3d 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -3239,7 +3239,8 @@ unsigned long wait_task_inactive(struct task_struct *p, unsigned int match_state > * is actually now running somewhere else! > */ > while (task_running(rq, p)) { > - if (match_state && unlikely(READ_ONCE(p->__state) != match_state)) > + if (match_state && > + unlikely(!task_state_match_eq(p, match_state))) > return 0; > cpu_relax(); > } > @@ -3254,7 +3255,7 @@ unsigned long wait_task_inactive(struct task_struct *p, unsigned int match_state > running = task_running(rq, p); > queued = task_on_rq_queued(p); > ncsw = 0; > - if (!match_state || READ_ONCE(p->__state) == match_state) > + if (!match_state || __task_state_match_eq(p, match_state)) > ncsw = p->nvcsw | LONG_MIN; /* sets MSB */ > task_rq_unlock(rq, p, &rf); > > -- > 2.35.1 >
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