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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Tue, 12 Jan 2021 09:36:18 -0800
Message-ID: <CAJuCfpFdSNTp+PhJ1ztyfbzdVWVHy=qJJPVizXPGizgBQqTKYw@mail.gmail.com>
Subject: Re: [PATCH v2 1/1] mm/madvise: replace ptrace attach requirement for process_madvise
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     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 5:22 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Mon, 11 Jan 2021 09:06:22 -0800 Suren Baghdasaryan <surenb@google.com> wrote:
>
> > process_madvise currently requires ptrace attach capability.
> > PTRACE_MODE_ATTACH gives one process complete control over another
> > process. It effectively removes the security boundary between the
> > two processes (in one direction). Granting ptrace attach capability
> > even to a system process is considered dangerous since it creates an
> > attack surface. This severely limits the usage of this API.
> > The operations process_madvise can perform do not affect the correctness
> > of the operation of the target process; they only affect where the data
> > is physically located (and therefore, how fast it can be accessed).
> > What we want is the ability for one process to influence another process
> > in order to optimize performance across the entire system while leaving
> > the security boundary intact.
> > Replace PTRACE_MODE_ATTACH with a combination of PTRACE_MODE_READ
> > and CAP_SYS_NICE. PTRACE_MODE_READ to prevent leaking ASLR metadata
> > and CAP_SYS_NICE for influencing process performance.
>
> It would be useful to see the proposed manpage update.
>
> process_madvise() was released in 5.10, so this is a
> non-backward-compatible change to a released kernel.
>
> I think it would be OK at this stage to feed this into 5.10.x with a
> cc:stable and suitable words in the changelog explaining why we're
> doing this.

Sure, I will post another patchset that will include manpage update
and will CC:stable. That's of course after Michal's concerns are
addressed.
Thanks!

>
> Alternatively we could retain PTRACE_MODE_ATTACH's behaviour and add
> PTRACE_MODE_READ&CAP_SYS_NICE alongside that.