linux-kernel - Re: [PATCH] prctl: remove one-shot limitation for changing exe link

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Date:	Mon, 25 Jul 2016 14:56:43 -0500
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	Stanislav Kinsburskiy <skinsbursky@...tuozzo.com>,
	peterz@...radead.org, mingo@...hat.com, mhocko@...e.com,
	keescook@...omium.org, linux-kernel@...r.kernel.org,
	mguzik@...hat.com, bsegall@...gle.com, john.stultz@...aro.org,
	oleg@...hat.com, matthltc@...ibm.com, akpm@...ux-foundation.org,
	luto@...capital.net, vbabka@...e.cz, xemul@...tuozzo.com
Subject: Re: [PATCH] prctl: remove one-shot limitation for changing exe link

Cyrill Gorcunov <gorcunov@...il.com> writes:

> On Mon, Jul 25, 2016 at 01:21:51PM -0500, Eric W. Biederman wrote:
>> Stanislav Kinsburskiy <skinsbursky@...tuozzo.com> writes:
>> 
>> > Gentlemen,
>> >
>> > Looks like there are no objections to this patch.
>> 
>> There has been objection.
>> 
>> The only justification for the change that has been put forward is
>> someone doing a restore lazily.  I don't see a reason why you can't call
>> prctl_set_mm_exe_file until you have the file in place instead of a
>> place holder that sounds like a trivial solution to any restore issues.
>> 
>> The truth is an unlimited settable exe link is essentially meaningless,
>> as you can't depend on it for anything.  One shot seems the best
>> compromise I have seen put forward between the definite
>> checkpoint/restart requirement to set the this value and the general
>> need to have something that makes sense and people can depend on for
>> system management.
>>
>> Also there is a big fat bug in prctl_set_mm_exe_file.  It doesn't
>> validate that the new file is a actually mmaped executable.  We would
>> definitely need that to be fixed before even considering removing the
>> limit.
>
> Could you please elaborate? We check for inode being executable,
> what else needed?

That the inode is mmaped into the process with executable mappings.

Effectively what we check the old mapping for and refuse to remove the old
mm_exe_file if it exists.

I think a reasonable argument can be made that if the file is
executable, and it is mmaped with executable pages that exe_file is not
a complete lie.

Which is the important part.  At the end of the day how much can
userspace trust /proc/pid/exe?  If we are too lax it is just a random
file descriptor we can not trust at all.  At which point there is
exactly no point in preserving it in checkpoint/restart, because nothing
will trust or look at it.

If the only user is checkpoint/restart perhaps it should be only ptrace
that can set this and not the process itself with a prctl.  I don't
know.  All I know is that we should work on making it a very trustable
value even though in some specific instances we can set it.

Eric