linux-kernel - Re: [RFC] c/r: prctl: Add ability to set new mm_struct::exe

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Message-ID: <20120301200026.GG9930@moon>
Date:	Fri, 2 Mar 2012 00:00:26 +0400
From:	Cyrill Gorcunov <gorcunov@...nvz.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Kees Cook <keescook@...omium.org>, Tejun Heo <tj@...nel.org>
Subject: Re: [RFC] c/r: prctl: Add ability to set new mm_struct::exe_file

On Thu, Mar 01, 2012 at 08:41:20PM +0100, Oleg Nesterov wrote:
...
> 
> Still can't understand. I think you need:
> 
> 	file = fget(fd);
> 	if (!file)
> 		return -EBADF;
> 
> 	down_write(&mm->mmap_sem);
> 	if (mm->num_exe_file_vmas) {
> 		fput(mm->exe_file);
> 		mm->exe_file = file;
> 		file = NULL;
> 	}
> 	up_write(&mm->mmap_sem);
> 
> 	if (!file)
> 		return 0;
> 
> 	fput(file);
> 	return -ESOMETHING;
> 
> and that is all.

This breaks overall logic of num_exe_file_vmas.
What the point to have it at all then? I mean,
if there several executable sections in elf file,
once loader finish its work we will have
num_exe_file_vmas more than 1.

Then the process calls for prctl and replaces
own exe_file (I'm talking about possible scenario
since for our own tool we know that there will be
only one .text section and we're more-less safe
in replacing own exe_file, but this interface
will be available for everyone who has c/r config
entry turned on, so I'm trying to find which
negative impact this feature might have, call me
paranoid), so once process has replaced own exe_file
to something else the code which depends on
num_exe_file_vmas become broken.

May not we have a scenario when removed_exe_file_vma
is be called somewhere else later, once this prctl
finished its work? That's what I fear of.

	Cyrill
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