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    Subject[PATCH] futex: Fix handling of bad requeue syscall pairing
    From: Darren Hart <dvhltc@us.ibm.com>

    If futex_requeue(requeue_pi=1) finds a futex_q that was created by a call
    other than futex_wait_requeue_pi(), the q.rt_waiter will be null. If so,
    this will result in an oops from the following call graph:

    futex_requeue()
    rt_mutex_start_proxy_lock()
    task_blocks_on_rt_mutex()
    waiter->task dereference
    OOPS

    We currently WARN_ON() if this is detected, clearly this is inadequate.
    If we detect a mis-pairing in futex_requeue(), bail out, sending -EINVAL
    to user-space.

    Signed-off-by: Darren Hart <dvhltc@us.ibm.com>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Steven Rostedt <rostedt@goodmis.org>
    Cc: Ingo Molnar <mingo@elte.hu>
    CC: Eric Dumazet <eric.dumazet@gmail.com>
    CC: John Kacur <jkacur@redhat.com>
    CC: Dinakar Guniguntala <dino@in.ibm.com>
    CC: John Stultz <johnstul@us.ibm.com>
    ---

    kernel/futex.c | 11 +++++++++--
    1 files changed, 9 insertions(+), 2 deletions(-)


    diff --git a/kernel/futex.c b/kernel/futex.c
    index 578c7b7..d806d6c 100644
    --- a/kernel/futex.c
    +++ b/kernel/futex.c
    @@ -1306,8 +1306,15 @@ retry_private:
    if (!match_futex(&this->key, &key1))
    continue;

    - WARN_ON(!requeue_pi && this->rt_waiter);
    - WARN_ON(requeue_pi && !this->rt_waiter);
    + /*
    + * FUTEX_WAIT_REQEUE_PI and FUTEX_CMP_REQUEUE_PI should always
    + * be paired with each other and no other futex ops.
    + */
    + if (requeue_pi && !this->rt_waiter ||
    + !requeue_pi && this->rt_waiter) {
    + ret = -EINVAL;
    + break;
    + }

    /*
    * Wake nr_wake waiters. For requeue_pi, if we acquired the
    --
    Darren Hart
    IBM Linux Technology Center
    Real-Time Linux Team

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