Re: [PATCH 1/1] mm/madvise: replace ptrace attach requirement for process_madvise

[Suren Baghdasaryan <surenb@google.com>]

On Mon, Jan 11, 2021 at 2:20 AM Florian Weimer <fweimer@redhat.com> wrote:
>
> * Suren Baghdasaryan:
>
> > diff --git a/mm/madvise.c b/mm/madvise.c
> > index 6a660858784b..c2d600386902 100644
> > --- a/mm/madvise.c
> > +++ b/mm/madvise.c
> > @@ -1197,12 +1197,22 @@ SYSCALL_DEFINE5(process_madvise, int, pidfd, const struct iovec __user *, vec,
> >               goto release_task;
> >       }
> >
> > -     mm = mm_access(task, PTRACE_MODE_ATTACH_FSCREDS);
> > +     /* Require PTRACE_MODE_READ to avoid leaking ASLR metadata. */
> > +     mm = mm_access(task, PTRACE_MODE_READ_FSCREDS);
> >       if (IS_ERR_OR_NULL(mm)) {
> >               ret = IS_ERR(mm) ? PTR_ERR(mm) : -ESRCH;
> >               goto release_task;
> >       }
>
> Shouldn't this depend on the requested behavior?  Several operations
> directly result in observable changes, and go beyond performance tuning.

Thanks for the comment Florian.
process_madvise supports only MADV_COLD and MADV_PAGEOUT hints which
are both non-destructive (see process_madvise_behavior_valid()
function). Maybe you meant something else by "observable changes", if
so please clarify.
Thanks,
Suren.

>
> Thanks,
> Florian
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