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    SubjectRe: Race in ptrace.
    On 02/09, Oleg Nesterov wrote:
    >
    > Salman Qazi wrote:
    > >
    > > A race in ptrace was pointed to us by a fellow Google engineer, Tavis
    > > Ormandy. The race involves interaction between a tracer, a tracee and
    > > an antagonist. The tracer is tracing the tracee with PTRACE_SYSCALL and
    > > waits on the tracee. In the mean time, an antagonist blasts the tracee
    > > with SIGCONTs.
    >
    > Could you please explain how did observe this race? Do you have a
    > test-case, or could you explain how we can reproduce it?
    >
    > Because,
    >
    > > It turns out that a SIGCONT wakes up the
    > > tracee in kernel mode,
    >
    > SIGCONT must not wake up a TASK_TRACED task.

    In case I wasn't clear...

    --- a/kernel/signal.c
    +++ b/kernel/signal.c
    @@ -697,6 +697,10 @@ static int prepare_signal(int sig, struct task_struct *p, int from_ancestor_ns)
    * and wake all threads.
    */
    rm_from_queue(SIG_KERNEL_STOP_MASK, &signal->shared_pending);
    + if (p->ptrace & PT_PTRACED) {
    + p->ptrace |= PT_WAKING;
    + mb();
    + }

    Please note that we are going to do wake_up_state(state), and
    this state can never have __TASK_TRACED bit set.

    And we can't change ->ptrace here, we can race with the tracer.

    There are other problems with this patch, but the main problem
    is that I can't understand what this patch tries to fix.
    IOW, please provide more info ;)

    Oleg.



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