Messages in this thread Patch in this message | | | Date | Thu, 31 Mar 2022 16:25:42 +0200 | From | Sebastian Andrzej Siewior <> | Subject | [PATCH v2] ptrace: fix ptrace vs tasklist_lock race on PREEMPT_RT. |
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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 acquired to have stable view on ->__state and ->saved_state.
wait_task_inactive() needs to check both task states while waiting for the expected task state. Should the expected task state be in ->saved_state then the task is blocked on a sleeping lock. In this case wait_task_inactive() needs to wait until the lock situtation has been resolved (the expected state is in ->__state). This ensures that the task is idle and does not wakeup as part of lock resolving and races for instance with __switch_to_xtra() while the debugger clears TIF_BLOCKSTEP() (noted by Oleg Nesterov).
[ Fix for ptrace_unfreeze_traced() by Oleg Nesterov ]
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> --- v1…v2: - Use also ->saved_state in task_state_match_and_set(). - Wait in wait_task_inactive() until the desired task state is in ->__state so that the task won't wake up a as part of lock resolving. Pointed out by Oleg Nesterov.
include/linux/sched.h | 128 ++++++++++++++++++++++++++++++++++++++++++++++++-- kernel/ptrace.c | 25 +++++---- kernel/sched/core.c | 11 +++- 3 files changed, 146 insertions(+), 18 deletions(-)
--- 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(). @@ -2009,6 +2005,130 @@ static inline int test_tsk_need_resched( 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 int __task_state_match_eq(struct task_struct *tsk, long state) +{ + int match = 0; + + if (READ_ONCE(tsk->__state) == state) + match = 1; + else if (tsk->saved_state == state) + match = -1; + + return match; +} + +static inline int task_state_match_eq(struct task_struct *tsk, long state) +{ + unsigned long flags; + int 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->saved_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 int __task_state_match_eq(struct task_struct *tsk, long state) +{ + return READ_ONCE(tsk->__state) == state; +} + +static inline int 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 --- a/kernel/ptrace.c +++ b/kernel/ptrace.c @@ -195,10 +195,10 @@ static bool ptrace_freeze_traced(struct 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 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(struc * 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); } --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -3219,6 +3219,8 @@ unsigned long wait_task_inactive(struct struct rq *rq; for (;;) { + int match_type = 0; + /* * We do the initial early heuristics without holding * any task-queue locks at all. We'll only try to get @@ -3239,7 +3241,8 @@ unsigned long wait_task_inactive(struct * 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 +3257,9 @@ unsigned long wait_task_inactive(struct 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) + match_type = __task_state_match_eq(p, match_state); + if (!match_state || match_type) ncsw = p->nvcsw | LONG_MIN; /* sets MSB */ task_rq_unlock(rq, p, &rf); @@ -3284,7 +3289,7 @@ unsigned long wait_task_inactive(struct * running right now), it's preempted, and we should * yield - it could be a while. */ - if (unlikely(queued)) { + if (unlikely(queued || match_type < 0)) { ktime_t to = NSEC_PER_SEC / HZ; set_current_state(TASK_UNINTERRUPTIBLE);
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