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SubjectRe: [ 020/184] ptrace: ensure arch_ptrace/ptrace_request can never
On 06/05, Luis Henriques wrote:
>
> Willy Tarreau <w@1wt.eu> writes:
>
> > 2.6.32-longterm review patch. If anyone has any objections, please let me know.
> >
> > ------------------
> > race with SIGKILL
> >
> > From: Oleg Nesterov <oleg@redhat.com>
> >
> > ptrace: ensure arch_ptrace/ptrace_request can never race with SIGKILL
> >
>
> This patch actually introduce a regression in the Ubuntu kernel. You
> may want to include the fix below.

Yes, 2.6.32 should also take care of TASK_STOPPED.

> --- a/kernel/ptrace.c
> +++ b/kernel/ptrace.c
> @@ -81,14 +81,18 @@ void __ptrace_unlink(struct task_struct *child)
> }
>
> /* Ensure that nothing can wake it up, even SIGKILL */
> -static bool ptrace_freeze_traced(struct task_struct *task)
> +static bool ptrace_freeze_traced(struct task_struct *task, int kill)
> {
> - bool ret = false;
> + bool ret = true;
>
> spin_lock_irq(&task->sighand->siglock);
> - if (task_is_traced(task) && !__fatal_signal_pending(task)) {
> + if (task_is_stopped(task) && !__fatal_signal_pending(task))
> task->state = __TASK_TRACED;
> - ret = true;
> + else if (!kill) {
> + if (task_is_traced(task) && !__fatal_signal_pending(task))
> + task->state = __TASK_TRACED;
> + else
> + ret = false;
> }
> spin_unlock_irq(&task->sighand->siglock);
>
> @@ -131,7 +135,7 @@ int ptrace_check_attach(struct task_struct *child, int kill)
> * child->sighand can't be NULL, release_task()
> * does ptrace_unlink() before __exit_signal().
> */
> - if (kill || ptrace_freeze_traced(child))
> + if (ptrace_freeze_traced(child, kill))
> ret = 0;

I can't apply this patch, probably I misread it...

But it looks very wrong. It seems that ptrace_freeze_traced(kill => true)
always succeeds? Even if task is TASK_RUNNING/UNINTERRUPTIBLE/etc ?

Note: I can make a _much_ simpler patch for 2.6.32, please let me know
if you need it.

We can rely on sys_ptrace()->lock_kernel() and simply do lock/unlock
if fatal_signal_pending() in ptrace_stop/do_signal_stop. This is not
the same, this doesn't prevent wakeup(), but this should be enough.

Oleg.



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