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    Subjectrcu_read_lock/unlock protect find_task_by_vpid call in check_clock
    ===================================================
    [ INFO: suspicious rcu_dereference_check() usage. ]
    ---------------------------------------------------
    kernel/pid.c:419 invoked rcu_dereference_check() without protection!

    other info that might help us debug this:


    rcu_scheduler_active = 1, debug_locks = 1
    1 lock held by scrashme/13382:
    #0: (tasklist_lock){.?.?..}, at: [<ffffffff8106ddea>] check_clock+0x46/0x9a

    stack backtrace:
    Pid: 13382, comm: scrashme Not tainted 2.6.37-rc3+ #8
    Call Trace:
    [<ffffffff8107cfe1>] lockdep_rcu_dereference+0x9d/0xa5
    [<ffffffff81069d08>] find_task_by_pid_ns+0x44/0x5d
    [<ffffffff81069d43>] find_task_by_vpid+0x22/0x24
    [<ffffffff8106ddf2>] check_clock+0x4e/0x9a
    [<ffffffff8106deac>] posix_cpu_clock_getres+0x16/0x41
    [<ffffffff8106be74>] sys_clock_getres+0x39/0xa0
    [<ffffffff81009cb2>] system_call_fastpath+0x16/0x1b

    Signed-off-by: Dave Jones <davej@redhat.com>

    diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
    index 6842eeb..4bef9aa 100644
    --- a/kernel/posix-cpu-timers.c
    +++ b/kernel/posix-cpu-timers.c
    @@ -38,11 +38,13 @@ static int check_clock(const clockid_t which_clock)
    return 0;

    read_lock(&tasklist_lock);
    + rcu_read_lock();
    p = find_task_by_vpid(pid);
    if (!p || !(CPUCLOCK_PERTHREAD(which_clock) ?
    same_thread_group(p, current) : thread_group_leader(p))) {
    error = -EINVAL;
    }
    + rcu_read_unlock();
    read_unlock(&tasklist_lock);

    return error;


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