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

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Message-ID: <20140215062922.GA22779@moon>
Date:	Sat, 15 Feb 2014 10:29:22 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Pavel Emelyanov <xemul@...allels.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 12:18:46PM -0800, Eric W. Biederman wrote:
> >> > 
> >> > 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?
> 
> Because exec is how those mm fields are set when you don't use
> prctl_set_mm.  So execpt for the stack and the brk limits that
> will simply result in the values being set to what the usually
> would be set to.

Yes, all these fields are set up by kernel's elf loader but this
routine is a way more time consuming than a clone call. But gimme
some time to examine all possible problems we might have with such
approach and if there a way to solve them.

> >> 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.
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