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

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Message-ID: <20140214200622.GN13358@moon>
Date:	Sat, 15 Feb 2014 00:06:22 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Pavel Emelyanov <xemul@...allels.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.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

On Fri, Feb 14, 2014 at 11:47:13PM +0400, Pavel Emelyanov wrote:
> >> 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.
> >          */

Erik, it seems I don't understand how it will help us to restore
the mm fields mentioned above?

> 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.

I fear you've got a wrong impression that we're "ick'ing" about user-ns ;)
Actually it's "must have" feature for containers thus we would _really_
love to be able to c/r them.
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