From: Heiko Carstens Date: Wed, 9 May 2012 07:37:30 +0000 (+0200) Subject: s390/pfault: fix task state race X-Git-Url: https://git.karo-electronics.de/?a=commitdiff_plain;h=d5e50a51ccbda36b379aba9d1131a852eb908dda;p=linux-beck.git s390/pfault: fix task state race When setting the current task state to TASK_UNINTERRUPTIBLE this can race with a different cpu. The other cpu could set the task state after it inspected it (while it was still TASK_RUNNING) to TASK_RUNNING which would change the state from TASK_UNINTERRUPTIBLE to TASK_RUNNING again. This race was always present in the pfault interrupt code but didn't cause anything harmful before commit f2db2e6c "[S390] pfault: cpu hotplug vs missing completion interrupts" which relied on the fact that after setting the task state to TASK_UNINTERRUPTIBLE the task would really sleep. Since this is not necessarily the case the result may be a list corruption of the pfault_list or, as observed, a use-after-free bug while trying to access the task_struct of a task which terminated itself already. To fix this, we need to get a reference of the affected task when receiving the initial pfault interrupt and add special handling if we receive yet another initial pfault interrupt when the task is already enqueued in the pfault list. Signed-off-by: Heiko Carstens Reviewed-by: Martin Schwidefsky Cc: # needed for v3.0 and newer Signed-off-by: Martin Schwidefsky --- diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c index b9aeaca26d3a..67e2d4d14ae6 100644 --- a/arch/s390/mm/fault.c +++ b/arch/s390/mm/fault.c @@ -574,6 +574,7 @@ static void pfault_interrupt(struct ext_code ext_code, tsk->thread.pfault_wait = 0; list_del(&tsk->thread.list); wake_up_process(tsk); + put_task_struct(tsk); } else { /* Completion interrupt was faster than initial * interrupt. Set pfault_wait to -1 so the initial @@ -588,14 +589,22 @@ static void pfault_interrupt(struct ext_code ext_code, put_task_struct(tsk); } else { /* signal bit not set -> a real page is missing. */ - if (tsk->thread.pfault_wait == -1) { + if (tsk->thread.pfault_wait == 1) { + /* Already on the list with a reference: put to sleep */ + set_task_state(tsk, TASK_UNINTERRUPTIBLE); + set_tsk_need_resched(tsk); + } else if (tsk->thread.pfault_wait == -1) { /* Completion interrupt was faster than the initial * interrupt (pfault_wait == -1). Set pfault_wait * back to zero and exit. */ tsk->thread.pfault_wait = 0; } else { /* Initial interrupt arrived before completion - * interrupt. Let the task sleep. */ + * interrupt. Let the task sleep. + * An extra task reference is needed since a different + * cpu may set the task state to TASK_RUNNING again + * before the scheduler is reached. */ + get_task_struct(tsk); tsk->thread.pfault_wait = 1; list_add(&tsk->thread.list, &pfault_list); set_task_state(tsk, TASK_UNINTERRUPTIBLE); @@ -620,6 +629,7 @@ static int __cpuinit pfault_cpu_notify(struct notifier_block *self, list_del(&thread->list); tsk = container_of(thread, struct task_struct, thread); wake_up_process(tsk); + put_task_struct(tsk); } spin_unlock_irq(&pfault_lock); break;