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

From: Michal Hocko <mhocko@kernel.org>
To: riel@redhat.com
Cc: linux-kernel@vger.kernel.org, mike.kravetz@oracle.com,
	linux-mm@kvack.org, fweimer@redhat.com, colm@allcosts.net,
	akpm@linux-foundation.org, keescook@chromium.org,
	luto@amacapital.net, wad@chromium.org, mingo@kernel.org,
	kirill@shutemov.name, dave.hansen@intel.com,
	linux-api@vger.kernel.org
Subject: Re: [PATCH v2 0/2] mm,fork,security: introduce MADV_WIPEONFORK
Date: Mon, 7 Aug 2017 15:22:57 +0200	[thread overview]
Message-ID: <20170807132257.GH32434@dhcp22.suse.cz> (raw)
In-Reply-To: <20170806140425.20937-1-riel@redhat.com>

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