linux-kernel - Re: [PATCH v2 0/2] mm,fork,security: introduce MADV

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Message-ID: <20170810153639.GB23863@dhcp22.suse.cz>
Date:   Thu, 10 Aug 2017 17:36:39 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Colm MacCárthaigh <colm@...costs.net>
Cc:     Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
        Mike Kravetz <mike.kravetz@...cle.com>, linux-mm@...ck.org,
        Florian Weimer <fweimer@...hat.com>, akpm@...ux-foundation.org,
        Kees Cook <keescook@...omium.org>, luto@...capital.net,
        Will Drewry <wad@...omium.org>, mingo@...nel.org,
        kirill@...temov.name, dave.hansen@...el.com,
        linux-api@...r.kernel.org
Subject: Re: [PATCH v2 0/2] mm,fork,security: introduce MADV_WIPEONFORK

On Thu 10-08-17 15:23:05, Colm MacCárthaigh wrote:
> On Thu, Aug 10, 2017 at 3:05 PM, Michal Hocko <mhocko@...nel.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?

-- 
Michal Hocko
SUSE Labs