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Date:   Wed, 13 Jan 2021 15:19:32 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Suren Baghdasaryan <surenb@google.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>,
        Shakeel Butt <shakeelb@google.com>,
        David Rientjes <rientjes@google.com>,
        Edgar Arriaga =?iso-8859-1?Q?Garc=EDa?= 
        <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>,
        Oleg Nesterov <oleg@redhat.com>
Subject: Re: [PATCH v2 1/1] mm/madvise: replace ptrace attach requirement for
 process_madvise
Message-ID: <20210113141932.GB22493@dhcp22.suse.cz>
References: <20210111170622.2613577-1-surenb@google.com>
 <20210112074629.GG22493@dhcp22.suse.cz>
 <CAJuCfpHazLXJ1rpJQ+w9=8-O==rzz3yEVuVtSn-sYMS+a3FoXQ@mail.gmail.com>
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On Tue 12-01-21 10:12:03, Suren Baghdasaryan wrote:
> On Mon, Jan 11, 2021 at 11:46 PM Michal Hocko <mhocko@suse.com> wrote:
> >
> > On Mon 11-01-21 09:06:22, Suren Baghdasaryan 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).
> >
> > Yes it doesn't influence the correctness but it is still a very
> > sensitive operation because it can allow a targeted side channel timing
> > attacks so we should be really careful.
> 
> Sorry, I missed this comment in my answer. Possibility of affecting
> the target's performance including side channel attack is why we
> require CAP_SYS_NICE.

OK. It would be really good to document that in the man page. From the
current wording it seems we already rely on this cap for migration on a
remote process which is not the same thing but it roughly falls into the
similar category.
-- 
Michal Hocko
SUSE Labs