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Date:	Mon, 5 May 2014 15:00:16 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Kees Cook <keescook@...omium.org>
Cc:	linux-kernel@...r.kernel.org, Randy Dunlap <rdunlap@...radead.org>,
	Ingo Molnar <mingo@...nel.org>, Rik van Riel <riel@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Mel Gorman <mgorman@...e.de>,
	Aaron Tomlin <atomlin@...hat.com>,
	Li Zefan <lizefan@...wei.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Ryan Mallon <rmallon@...il.com>,
	Wanpeng Li <liwanp@...ux.vnet.ibm.com>,
	Dario Faggioli <raistlin@...ux.it>, Jens Axboe <axboe@...com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Michael Ellerman <michael@...erman.id.au>,
	linux-doc@...r.kernel.org
Subject: Re: [PATCH v3 0/4] sysctl: fix incorrect write position handling

On Thu,  1 May 2014 14:26:33 -0700 Kees Cook <keescook@...omium.org> wrote:

> When writing to a sysctl string, each write, regardless of VFS position,
> began writing the string from the start. This meant the contents of
> the last write to the sysctl controlled the string contents instead of
> the first.
> 
> This misbehavior was featured in an exploit against Chrome OS. While it's
> not in itself a vulnerability, it's a weirdness that isn't on the mind
> of most auditors: "This filter looks correct, the first line written
> would not be meaningful to sysctl" doesn't apply here, since the size
> of the write and the contents of the final write are what matter when
> writing to sysctls.
> 
> This adds the sysctl kernel.sysctl_writes_strict to control the write
> behavior. The default (0) reports when VFS position is non-0 on a write,
> but retains legacy behavior, -1 disables the warning, and 1 enables the
> position-respecting behavior.
> 

OK, let's try that.  I added this paragraph to the patchset's overall
changelog:

: The long-term plan here is to wait for userspace to be fixed in response
: to the new warning and to then switch the default kernel behavior to the
: new position-respecting behavior.

I'm thinking we should use pr_warn_once() in warn_sysctl_write()?  Otherwise
people will go and shut the thing up permanently and we'll lose the benefits.
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