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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Mon, 11 Jan 2021 09:09:13 -0800
Message-ID: <CAJuCfpHw8RKPPJFXt6sCmxDHKFvgKTJd_gJ4HV68BTWdTsvkeg@mail.gmail.com>
Subject: Re: [PATCH 1/1] mm/madvise: replace ptrace attach requirement for process_madvise
To:     Florian Weimer <fweimer@redhat.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Jann Horn <jannh@google.com>,
        Kees Cook <keescook@chromium.org>,
        Jeffrey Vander Stoep <jeffv@google.com>,
        Minchan Kim <minchan@kernel.org>,
        Michal Hocko <mhocko@suse.com>,
        Shakeel Butt <shakeelb@google.com>,
        David Rientjes <rientjes@google.com>,
        =?UTF-8?Q?Edgar_Arriaga_Garc=C3=ADa?= <edgararriaga@google.com>,
        Tim Murray <timmurray@google.com>,
        linux-mm <linux-mm@kvack.org>, selinux@vger.kernel.org,
        Linux API <linux-api@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        kernel-team <kernel-team@android.com>
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On Mon, Jan 11, 2021 at 9:05 AM Suren Baghdasaryan <surenb@google.com> wrote:
>
> 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.
>

V2 with Minchan's fix is posted at:
https://lore.kernel.org/lkml/20210111170622.2613577-1-surenb@google.com/T/#u

> >
> > Thanks,
> > Florian
> > --
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