linux-kernel - Re: [CRIU] [PATCH 1/3] prctl: reduce permissions to change boundaries of data, brk and stack

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Date:	Fri, 14 Feb 2014 11:16:41 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	Andrew Vagin <avagin@...il.com>,
	Aditya Kali <adityakali@...gle.com>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	Pavel Emelyanov <xemul@...allels.com>,
	Oleg Nesterov <oleg@...hat.com>, linux-kernel@...r.kernel.org,
	criu@...nvz.org, Al Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>
Subject: Re: [CRIU] [PATCH 1/3] prctl: reduce permissions to change boundaries of data, brk and stack

Cyrill Gorcunov <gorcunov@...il.com> writes:

> On Fri, Feb 14, 2014 at 09:43:14PM +0400, Andrew Vagin wrote:
>> > My brain hurts just looking at this patch and how you are justifying it.
>> > 
>> > For the resources you are mucking with below all you have to do is to
>> > verify that you are below the appropriate rlimit at all times and no
>> > CAP_SYS_RESOURCE check is needed.  You only need CAP_SYS_RESOURCE
>> > to exceed your per process limits.
>> > 
>> > All you have to do is to fix the current code to properly enforce the
>> > limits.
>> 
>> I'm afraid what you are suggesting doesn't work.
>> 
>> The first reason is that we can not change both boundaries in one call.
>> But when we are restoring these attributes, we may need to move their
>> too far.
>
> When this code was introduced, there were no user-namespace implementation,
> if I remember correctly, so CAP_SYS_RESOURCE was enough barrier point
> to prevent modifying this values by anyone. Now user-ns brings a limit --
> we need somehow to provide a way to modify these mm fields having no
> CAP_SYS_RESOURCE set. "Verifying rlimit" not an option here because
> we're modifying members one by one (looking back I think this was not
> a good idea to modify the fields in this manner).
>
> Maybe we could improve this api and provide argument as a pointer
> to a structure, which would have all the fields we're going to
> modify, which in turn would allow us to verify that all new values
> are sane and fit rlimits, then we could (probably) deprecate old
> api if noone except c/r camp is using it (I actually can't imagine
> who else might need this api). Then CAP_SYS_RESOURCE requirement
> could be ripped off. Hm? (sure touching api is always "no-no"
> case, but maybe...)

Hmm.  Let me rewind this a little bit.

I want to be very stupid and ask the following.

Why can't you have the process of interest do:
	ptrace(PTRACE_ATTACHME);
	execve(executable, args, ...);
        
        /* Have the ptracer inject the recovery/fixup code */
	/* Fix up the mostly correct process to look like it has been
         * executing for a while.
         */

That should work, set all of the interesting fields, and works as
non-root today.  My gut feel says do that and we can just
deprecate/remove prctl_set_mm.

I am hoping we can move this conversation what makes sense from oh ick
checkpoint/restort does not work with user namespaces.

Eric
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