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    Subject[tip:core/urgent] futex: Fix handling of bad requeue syscall pairing
    Commit-ID:  392741e0a4e17c82e3978b7fcbf04291294dc0a1
    Gitweb: http://git.kernel.org/tip/392741e0a4e17c82e3978b7fcbf04291294dc0a1
    Author: Darren Hart <dvhltc@us.ibm.com>
    AuthorDate: Fri, 7 Aug 2009 15:20:48 -0700
    Committer: Ingo Molnar <mingo@elte.hu>
    CommitDate: Mon, 10 Aug 2009 20:38:11 +0200

    futex: Fix handling of bad requeue syscall pairing

    If futex_requeue(requeue_pi=1) finds a futex_q that was created by a call
    other the futex_wait_requeue_pi(), the q.rt_waiter may 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 mispairing in futex_requeue(), bail out, seding -EINVAL to
    user-space.

    V2: Fix parenthesis warnings.

    Signed-off-by: Darren Hart <dvhltc@us.ibm.com>
    Acked-by: Peter Zijlstra <peterz@infradead.org>
    Cc: Steven Rostedt <rostedt@goodmis.org>
    Cc: John Kacur <jkacur@redhat.com>
    Cc: Eric Dumazet <eric.dumazet@gmail.com>
    Cc: Dinakar Guniguntala <dino@in.ibm.com>
    Cc: John Stultz <johnstul@linux.vnet.ibm.com>
    LKML-Reference: <4A7CA8C0.7010809@us.ibm.com>
    Signed-off-by: Ingo Molnar <mingo@elte.hu>


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

    diff --git a/kernel/futex.c b/kernel/futex.c
    index 8cc3ee1..e18cfbd 100644
    --- a/kernel/futex.c
    +++ b/kernel/futex.c
    @@ -1256,8 +1256,15 @@ 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

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