From f67f6eb717ae6277ddd6c1685b0e85ef17420096 Mon Sep 17 00:00:00 2001 From: Michael Pratt Date: Mon, 1 Nov 2021 17:06:15 -0400 Subject: [PATCH] posix-cpu-timers: Clear task::posix_cputimers_work in copy_process() commit ca7752caeaa70bd31d1714af566c9809688544af upstream. copy_process currently copies task_struct.posix_cputimers_work as-is. If a timer interrupt arrives while handling clone and before dup_task_struct completes then the child task will have: 1. posix_cputimers_work.scheduled = true 2. posix_cputimers_work.work queued. copy_process clears task_struct.task_works, so (2) will have no effect and posix_cpu_timers_work will never run (not to mention it doesn't make sense for two tasks to share a common linked list). Since posix_cpu_timers_work never runs, posix_cputimers_work.scheduled is never cleared. Since scheduled is set, future timer interrupts will skip scheduling work, with the ultimate result that the task will never receive timer expirations. Together, the complete flow is: 1. Task 1 calls clone(), enters kernel. 2. Timer interrupt fires, schedules task work on Task 1. 2a. task_struct.posix_cputimers_work.scheduled = true 2b. task_struct.posix_cputimers_work.work added to task_struct.task_works. 3. dup_task_struct() copies Task 1 to Task 2. 4. copy_process() clears task_struct.task_works for Task 2. 5. Future timer interrupts on Task 2 see task_struct.posix_cputimers_work.scheduled = true and skip scheduling work. Fix this by explicitly clearing contents of task_struct.posix_cputimers_work in copy_process(). This was never meant to be shared or inherited across tasks in the first place. Fixes: 1fb497dd0030 ("posix-cpu-timers: Provide mechanisms to defer timer handling to task_work") Reported-by: Rhys Hiltner Signed-off-by: Michael Pratt Signed-off-by: Thomas Gleixner Cc: Link: https://lore.kernel.org/r/20211101210615.716522-1-mpratt@google.com Signed-off-by: Greg Kroah-Hartman --- include/linux/posix-timers.h | 2 ++ kernel/fork.c | 1 + kernel/time/posix-cpu-timers.c | 19 +++++++++++++++++-- 3 files changed, 20 insertions(+), 2 deletions(-) diff --git a/include/linux/posix-timers.h b/include/linux/posix-timers.h index 896c16d2c5fb..913aa60228b1 100644 --- a/include/linux/posix-timers.h +++ b/include/linux/posix-timers.h @@ -177,8 +177,10 @@ static inline void posix_cputimers_group_init(struct posix_cputimers *pct, #endif #ifdef CONFIG_POSIX_CPU_TIMERS_TASK_WORK +void clear_posix_cputimers_work(struct task_struct *p); void posix_cputimers_init_work(void); #else +static inline void clear_posix_cputimers_work(struct task_struct *p) { } static inline void posix_cputimers_init_work(void) { } #endif diff --git a/kernel/fork.c b/kernel/fork.c index 773b44be81f9..e465903abed9 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -2191,6 +2191,7 @@ static __latent_entropy struct task_struct *copy_process( p->pdeath_signal = 0; INIT_LIST_HEAD(&p->thread_group); p->task_works = NULL; + clear_posix_cputimers_work(p); /* * Ensure that the cgroup subsystem policies allow the new process to be diff --git a/kernel/time/posix-cpu-timers.c b/kernel/time/posix-cpu-timers.c index 08c033b80256..5d76edd0ad9c 100644 --- a/kernel/time/posix-cpu-timers.c +++ b/kernel/time/posix-cpu-timers.c @@ -1100,14 +1100,29 @@ static void posix_cpu_timers_work(struct callback_head *work) handle_posix_cpu_timers(current); } +/* + * Clear existing posix CPU timers task work. + */ +void clear_posix_cputimers_work(struct task_struct *p) +{ + /* + * A copied work entry from the old task is not meaningful, clear it. + * N.B. init_task_work will not do this. + */ + memset(&p->posix_cputimers_work.work, 0, + sizeof(p->posix_cputimers_work.work)); + init_task_work(&p->posix_cputimers_work.work, + posix_cpu_timers_work); + p->posix_cputimers_work.scheduled = false; +} + /* * Initialize posix CPU timers task work in init task. Out of line to * keep the callback static and to avoid header recursion hell. */ void __init posix_cputimers_init_work(void) { - init_task_work(¤t->posix_cputimers_work.work, - posix_cpu_timers_work); + clear_posix_cputimers_work(current); } /*