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

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Message-ID: <87txc1pibc.fsf@xmission.com>
Date:	Fri, 14 Feb 2014 12:09:43 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Pavel Emelyanov <xemul@...allels.com>
Cc:	Cyrill Gorcunov <gorcunov@...il.com>,
	Andrew Vagin <avagin@...il.com>,
	Aditya Kali <adityakali@...gle.com>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	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

Pavel Emelyanov <xemul@...allels.com> writes:

> On 02/14/2014 11:16 PM, Eric W. Biederman wrote:
>> 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.
>>          */
>
> Let's imagine we do that.
>
> This means, that the whole memory contents should be restored _after_
> the execve() call, since the execve() flushes old mappings. In
> that case we lose the ability to preserve any shared memory regions
> between any two processes. This "shared" can be either regular
> MAP_SHARED mappings or MAP_ANONYMOUS but still not COW-ed ones.

If we have MAP_ANONYMOUS but not COW-ed mappings we have the correct
executable, which implies we have everything else correct except for the
brk and the stack addresses, because the process was started with fork.

So while that sounds like an interesting case to handle it does not seem
to invalidate the idea of using exec to set all of the other fields when
we need to set them.

Eric

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