From: Michael Pratt mpratt@google.com
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 rhys@justin.tv Signed-off-by: Michael Pratt mpratt@google.com Signed-off-by: Thomas Gleixner tglx@linutronix.de Cc: stable@vger.kernel.org Link: https://lore.kernel.org/r/20211101210615.716522-1-mpratt@google.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- include/linux/posix-timers.h | 2 ++ kernel/fork.c | 1 + kernel/time/posix-cpu-timers.c | 19 +++++++++++++++++-- 3 files changed, 20 insertions(+), 2 deletions(-)
--- a/include/linux/posix-timers.h +++ b/include/linux/posix-timers.h @@ -184,8 +184,10 @@ static inline void posix_cputimers_group #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
--- a/kernel/fork.c +++ b/kernel/fork.c @@ -2280,6 +2280,7 @@ static __latent_entropy struct task_stru p->pdeath_signal = 0; INIT_LIST_HEAD(&p->thread_group); p->task_works = NULL; + clear_posix_cputimers_work(p);
#ifdef CONFIG_KRETPROBES p->kretprobe_instances.first = NULL; --- a/kernel/time/posix-cpu-timers.c +++ b/kernel/time/posix-cpu-timers.c @@ -1159,13 +1159,28 @@ static void posix_cpu_timers_work(struct }
/* + * 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); }
/*