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

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Date:	Tue, 13 Mar 2012 20:00:44 +0400
From:	Cyrill Gorcunov <gorcunov@...nvz.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Matt Helsley <matthltc@...ibm.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Kees Cook <keescook@...omium.org>, Tejun Heo <tj@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] c/r: prctl: Add ability to set new mm_struct::exe_file v3

On Tue, Mar 13, 2012 at 04:43:37PM +0100, Oleg Nesterov wrote:
> > +
> > +	/*
> > +	 * For security reason changing mm->exe_file
> > +	 * is one-shot action.
> > +	 */
> > +	down_write(&mm->mmap_sem);
> > +	if (likely(!mm->exe_file))
> 
> This means that the num_exe_file_vmas check at the start is not needed.
> If you want it as a "fast-path" check, please fix the comment. Or just
> remove it. Otherwise the code looks as if we have to check them both.

Yes, I wanted a fast test first, while the second test will give
one-shot condition and the second attempt to setup new exe_file
will fail. OK, I'll update the comment block.

> 
> Matt, is it really possible to hit mm->exe_file = NULL in
> removed_exe_file_vma ? Unless I missed something, this check just
> hides the potentional problem, no?
> 
> IOW, shouldn't it do
> 
> 	void removed_exe_file_vma(struct mm_struct *mm)
> 	{
> 		WARN_ON(!mm->exe_file);
> 		WARN_ON(mm->num_exe_file_vmas <= 0);
> 

I guess if num_exe_file_vmas < 1 here we've a bug somewhere
and should not decrement the counter at all.

	Cyrill
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