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    Subject[ 11/91] s390/pfault: fix task state race
    3.4-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Heiko Carstens <heiko.carstens@de.ibm.com>

    commit d5e50a51ccbda36b379aba9d1131a852eb908dda upstream.

    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 <heiko.carstens@de.ibm.com>
    Reviewed-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
    Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    arch/s390/mm/fault.c | 14 ++++++++++++--
    1 file changed, 12 insertions(+), 2 deletions(-)

    --- a/arch/s390/mm/fault.c
    +++ b/arch/s390/mm/fault.c
    @@ -574,6 +574,7 @@ static void pfault_interrupt(struct ext_
    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_
    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(s
    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;



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