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

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Date:	Sat, 15 Feb 2014 15:05:43 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Andrey Wagin <avagin@...il.com>
Cc:	Cyrill Gorcunov <gorcunov@...il.com>,
	Aditya Kali <adityakali@...gle.com>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	Pavel Emelyanov <xemul@...allels.com>,
	Oleg Nesterov <oleg@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"criu\@openvz.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

Andrey Wagin <avagin@...il.com> writes:

> 2014-02-14 23:16 GMT+04:00 Eric W. Biederman <ebiederm@...ssion.com>:
>>
>> 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.
>
> start_brk and start_stack are randomized each time. I don't understand
> how execve() can restore the origin values of attributes.

As is the location of the vdso and there isn't a way to set that.

So perhaps what we want to do is to change the randomization with
mremap(old_addr, size, size, MREMAP_MAYMOVE | MREMAP_FIXED, new_addr)
and just have the kernel update all of the addresses in bulk when we
move the location.

I don't know what the folks who are worried about losing tampering
evidence will think but as a targeted special case it may not be at all
crazy.

Eric
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