linux-kernel - Re: [PATCH 3.5] c/r: prctl: less paranoid prctl_set_mm_exe

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Message-ID: <20120616091712.GA2021@moon>
Date:	Sat, 16 Jun 2012 13:17:12 +0400
From:	Cyrill Gorcunov <gorcunov@...nvz.org>
To:	Konstantin Khlebnikov <khlebnikov@...nvz.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	linux-kernel@...r.kernel.org, Oleg Nesterov <oleg@...hat.com>,
	linux-mm@...ck.org, Matt Helsley <matthltc@...ibm.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH 3.5] c/r: prctl: less paranoid prctl_set_mm_exe_file()

On Sat, Jun 16, 2012 at 01:06:46PM +0400, Cyrill Gorcunov wrote:
> On Sat, Jun 16, 2012 at 12:51:04PM +0400, Konstantin Khlebnikov wrote:
> > "no other files mapped" requirement from my previous patch
> > (c/r: prctl: update prctl_set_mm_exe_file() after mm->num_exe_file_vmas removal)
> > is too paranoid, it forbids operation even if there mapped one shared-anon vma.
> > 
> > Let's check that current mm->exe_file already unmapped, in this case exe_file
> > symlink already outdated and its changing is reasonable.
> > 
> > Plus, this patch fixes exit code in case operation success.
> > 
> > Signed-off-by: Konstantin Khlebnikov <khlebnikov@...nvz.org>
> > Reported-by: Cyrill Gorcunov <gorcunov@...nvz.org>
> > Cc: Oleg Nesterov <oleg@...hat.com>
> > Cc: Matt Helsley <matthltc@...ibm.com>
> > Cc: Kees Cook <keescook@...omium.org>
> > Cc: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
> > Cc: Tejun Heo <tj@...nel.org>
> > Cc: Pavel Emelyanov <xemul@...allels.com>
> > ---
> 
> Ack! Thanks again, Konstantin!

Side note: there is a little nit with this patch actually,
because while when we do c/r we do "right things" and unmap
all vm-executable mappings before we set up new exe_file. But
we can't guarantee that some brave soul would not setup
new exe-file just for it's own, then what we migh have

 - mm::exe_file set up and points to some file, moreover num_exe_file_vmas might be > 1
 - application calls for prctl_set_mm_exe_file
 - set_mm_exe_file(mm, exe_file) called, and it drops num_exe_file_vmas to 0
 - finally application might call for removed_exe_file_vma

void removed_exe_file_vma(struct mm_struct *mm)
{
	mm->num_exe_file_vmas--;
	if ((mm->num_exe_file_vmas == 0) && mm->exe_file) {
		fput(mm->exe_file);
		mm->exe_file = NULL;
	}

}

and it does _not_ test for num_exe_file_vmas being 0 before doing decrement,
thus we get inconsistency in counter.

	Cyrill
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