Re: [PATCH -mm 1/1] ptrace: PTRACE_GETFDPIC: fix the unsafe usage

[Oleg Nesterov <oleg@redhat.com>]

On 05/24, David Howells wrote:
>
> Oleg Nesterov <oleg@redhat.com> wrote:
>
> > Now that Mike Frysinger unified the FDPIC ptrace code, we can fix
> > the unsafe usage of child->mm in ptrace_request(PTRACE_GETFDPIC).
> >
> > We have the reference to task_struct, and ptrace_check_attach()
> > verified the tracee is stopped. But nothing can protect from
> > SIGKILL after that, we must not assume child->mm != NULL.
> >
> > Signed-off-by: Oleg Nesterov <oleg@redhat.com>
>
> Acked-by: David Howells <dhowells@redhat.com>
>
> Does it make sense to move the call to get_task_mm() up to sys_ptrace() since
> several ptrace functions use it?  The mm pointer could then be handed down the
> ptrace hierarchy.

You mean, pass it to arch_ptrace() ?

grep, grep, grep. I guess I understand you. We have more unsafe code
like this in arch/*/kernel/ptrace.c. Of course, it can be fixed without
doing get_task_mm() in sys_ptrace(), but perhaps it would be more clean
to do what you suggest.

Roland, what do you think?

Oleg.