linux-kernel - Re: [patch 1/2] c/r: prctl: Add ability to set new mm_struct::exe

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Message-ID: <CAFLxGvxEDs=RG7tX+j6XEUx2+wEvuCGipUzh2vSp3rj15Rq6zA@mail.gmail.com>
Date:	Mon, 19 Mar 2012 23:41:36 +0100
From:	richard -rw- weinberger <richard.weinberger@...il.com>
To:	Cyrill Gorcunov <gorcunov@...nvz.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Kees Cook <keescook@...omium.org>, Tejun Heo <tj@...nel.org>,
	Matt Helsley <matthltc@...ibm.com>
Subject: Re: [patch 1/2] c/r: prctl: Add ability to set new mm_struct::exe_file

On Mon, Mar 19, 2012 at 11:39 PM, Cyrill Gorcunov <gorcunov@...nvz.org> wrote:
> On Mon, Mar 19, 2012 at 03:15:07PM -0700, Andrew Morton wrote:
> ...
>> >
>> > Also this action is one-shot only. For security reason
>> > we don't allow to change the symlink several times.
>>
>> What is this mysterious "security reason"?
>>
>
> Oh, sorry I should have included Matt's comment here
>
>        | Before this patch that state was rather ephemeral and almost entirely
>        | under the control of the kernel. The only way userspace could change it
>        | was by unmapping the region(s) mapped during exec*(). At that point it
>        | could not "lie" and insert some other symlink there and the admin would
>        | be better able to determine what had happened.
>        |
>        | With this patch -- especially the multi-shot form -- the symlink will
>        | be entirely under the control of (potentially untrusted) userspace code
>        | and the admin is totally at the mercy of the userspace code. In
>        | single-shot form programs could use the prctl() to ensure the symlink
>        | could not be changed later -- the restart tool would be the only program
>        | that would need to ensure that prctl() had not been used since the last
>        | exec*().
> ...
>>
>> It should be explained here also.  The comment is pretty useless - if
>> we don't tell people what this "security reason" is, how can future
>> developers be sure that they aren't violating it?
>>
>
> Actually I liked multi-shot version more but Matt arguments convinced
> me that one-short fashion is more "secure" in terms of overall kernel
> state and potential transitions/changes of this /proc/pid/exe symlink.
>
> At least with one-shot version the admin may be sure that the symlink
> is never changed more than once, ever.
>

And changing it once does not harm security?
I'm sure that rootkit writers will like this feature...

-- 
Thanks,
//richard
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