Re: [PATCH v2 0/2] mm,fork,security: introduce MADV_WIPEONFORK

From: Michal Hocko <mhocko@kernel.org>
To: "Colm MacCárthaigh" <colm@allcosts.net>
Cc: Florian Weimer <fweimer@redhat.com>,
	Kees Cook <keescook@chromium.org>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Rik van Riel <riel@redhat.com>, Will Drewry <wad@chromium.org>,
	akpm@linux-foundation.org, dave.hansen@intel.com,
	kirill@shutemov.name, linux-api@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	luto@amacapital.net, mingo@kernel.org
Subject: Re: [PATCH v2 0/2] mm,fork,security: introduce MADV_WIPEONFORK
Date: Thu, 10 Aug 2017 19:01:44 +0200	[thread overview]
Message-ID: <20170810170144.GA987@dhcp22.suse.cz> (raw)
In-Reply-To: <CAAF6GDeno6RpHf1KORVSxUL7M-CQfbWFFdyKK8LAWd_6PcJ55Q@mail.gmail.com>

On Thu 10-08-17 16:17:18, Colm MacCárthaigh wrote:
> On Déar 10 Lún 2017 at 17:36 Michal Hocko <mhocko@kernel.org> wrote:
> 
> > On Thu 10-08-17 15:23:05, Colm MacCįrthaigh wrote:
> > > On Thu, Aug 10, 2017 at 3:05 PM, Michal Hocko <mhocko@kernel.org> wrote:
> > > >> Too late for that. VM_DONTFORK is already implemented
> > > >> through MADV_DONTFORK & MADV_DOFORK, in a way that is
> > > >> very similar to the MADV_WIPEONFORK from these patches.
> > > >
> > > > Yeah, those two seem to be breaking the "madvise as an advise"
> > semantic as
> > > > well but that doesn't mean we should follow that pattern any further.
> > >
> > > I would imagine that many of the crypto applications using
> > > MADV_WIPEONFORK will also be using MADV_DONTDUMP. In cases where it's
> > > for protecting secret keys, I'd like to use both in my code, for
> > > example. Though that doesn't really help decide this.
> > >
> > > There is also at least one case for being able to turn WIPEONFORK
> > > on/off with an existing page; a process that uses privilege separation
> > > often goes through the following flow:
> > >
> > > 1. [ Access privileged keys as a power user and initialize memory ]
> > > 2. [ Fork a child process that actually does the work ]
> > > 3. [ Child drops privileges and uses the memory to do work ]
> > > 4. [ Parent hangs around to re-spawn a child if it crashes ]
> > >
> > > In that mode it would be convenient to be able to mark the memory as
> > > WIPEONFORK in the child, but not the parent.
> >
> > I am not sure I understand. The child will have an own VMA so chaging
> > the attribute will not affect parent. Or did I misunderstand your
> > example?
> >
> 
> Typically with privilege separation the parent has to share some minimal
> state with the child. In this case that's why the page is left alone.
> Though a smart parent could unset and set just immediately around the fork.
> 
> The point then of protecting it in the child is to ensure that a grandchild
> doesn't inherit the secret data.

Does anybody actually do that using the minherit BSD interface?
-- 
Michal Hocko
SUSE Labs