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

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Date:   Mon, 7 Aug 2017 15:22:57 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     riel@...hat.com
Cc:     linux-kernel@...r.kernel.org, mike.kravetz@...cle.com,
        linux-mm@...ck.org, fweimer@...hat.com, colm@...costs.net,
        akpm@...ux-foundation.org, keescook@...omium.org,
        luto@...capital.net, 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

This is an user visible API so make sure you CC linux-api (added)

On Sun 06-08-17 10:04:23, Rik van Riel wrote:
> v2: fix MAP_SHARED case and kbuild warnings
> 
> Introduce MADV_WIPEONFORK semantics, which result in a VMA being
> empty in the child process after fork. This differs from MADV_DONTFORK
> in one important way.
> 
> If a child process accesses memory that was MADV_WIPEONFORK, it
> will get zeroes. The address ranges are still valid, they are just empty.
> 
> If a child process accesses memory that was MADV_DONTFORK, it will
> get a segmentation fault, since those address ranges are no longer
> valid in the child after fork.
> 
> Since MADV_DONTFORK also seems to be used to allow very large
> programs to fork in systems with strict memory overcommit restrictions,
> changing the semantics of MADV_DONTFORK might break existing programs.
> 
> The use case is libraries that store or cache information, and
> want to know that they need to regenerate it in the child process
> after fork.
> 
> Examples of this would be:
> - systemd/pulseaudio API checks (fail after fork)
>   (replacing a getpid check, which is too slow without a PID cache)
> - PKCS#11 API reinitialization check (mandated by specification)
> - glibc's upcoming PRNG (reseed after fork)
> - OpenSSL PRNG (reseed after fork)
> 
> The security benefits of a forking server having a re-inialized
> PRNG in every child process are pretty obvious. However, due to
> libraries having all kinds of internal state, and programs getting
> compiled with many different versions of each library, it is
> unreasonable to expect calling programs to re-initialize everything
> manually after fork.
> 
> A further complication is the proliferation of clone flags,
> programs bypassing glibc's functions to call clone directly,
> and programs calling unshare, causing the glibc pthread_atfork
> hook to not get called.
> 
> It would be better to have the kernel take care of this automatically.
> 
> This is similar to the OpenBSD minherit syscall with MAP_INHERIT_ZERO:
> 
>     https://man.openbsd.org/minherit.2
> 
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-- 
Michal Hocko
SUSE Labs