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

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Date:	Sat, 16 Jun 2012 13:42:23 +0400
From:	Konstantin Khlebnikov <khlebnikov@...nvz.org>
To:	Cyrill Gorcunov <gorcunov@...nvz.org>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>,
	Pavel Emelianov <xemul@...allels.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	"linux-mm@...ck.org" <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()

Cyrill Gorcunov wrote:
> 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 symlink already outdated and its changing is reasonable.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.

No, removed_exe_file_vma() is called only for vma with VM_EXECUTABLE flag,
there no way to get such vma other than sys_execve().
And this brave soul cannot call prctl_set_mm_exe_file() successfully,
just because for vma with VM_EXECUTABLE flag vma->vm_file == mm->exe_file.

Anyway, I plan to get rid of mm->num_exe_file_vmas and VM_EXECUTABLE.

>
> 	Cyrill

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