linux-kernel - Re: [PATCH v3 0/4] sysctl: fix incorrect write position handling

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Date:	Mon, 5 May 2014 18:28:35 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	LKML <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" <linux-doc@...r.kernel.org>
Subject: Re: [PATCH v3 0/4] sysctl: fix incorrect write position handling

On Mon, May 5, 2014 at 3:00 PM, Andrew Morton <akpm@...ux-foundation.org> wrote:
> 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.

Great, thanks!

> 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.

I was worried we'd miss different processed tripping it later. On the
other hand, I didn't like the idea of being able to spam dmesg. Do you
want me to send a patch to replace that with pr_warn_once()?

-Kees

-- 
Kees Cook
Chrome OS Security
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