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

[Pavel Emelyanov <xemul@parallels.com>]

On 02/14/2014 11:16 PM, Eric W. Biederman wrote:
> Cyrill Gorcunov <gorcunov@gmail.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.

> 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
> .

Thanks,
Pavel