linux-kernel - Re: PATCH? fix unshare(NEWPID) && vfork()

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Date:	Tue, 20 Aug 2013 12:00:18 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Brad Spengler <spender@...ecurity.net>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Colin Walters <walters@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: PATCH? fix unshare(NEWPID) && vfork()

On Tue, Aug 20, 2013 at 11:50 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> On 08/20, Andy Lutomirski wrote:
>>
>> On Mon, Aug 19, 2013 at 11:43 AM, Oleg Nesterov <oleg@...hat.com> wrote:
>> > On 08/19, Andy Lutomirski wrote:
>> >>
>> >> On Mon, Aug 19, 2013 at 11:33 AM, Oleg Nesterov <oleg@...hat.com> wrote:
>> >> >
>> >> > So do you think this change is fine or not (ignoring the fact it needs
>> >> > cleanups) ?
>> >>
>> >> I think that removing the CLONE_VM check is fine (although there are
>> >> some other ones that should probably be removed as well), but I'm not
>> >> sure if that check needs replacing with something else.
>> >
>> > OK, thanks... but I still can't understand.
>> >
>> > The patch I sent is equivalent to the new one below. I just tried to
>> > unify it with another check in do_fork().
>>
>> I was confused.
>
> Andy, I do not know how much you were confused, but I bet I am confused
> much more ;)
>
>> Currently (with or without your patch), vfork() followed by
>> unshare(CLONE_NEWUSER) or unshare(CLONE_NEWPID) will unshare the VM.
>
> Could you spell please?
>
> We never unshare the VM. CLONE_VM in sys_unshare() paths just means
> "fail unless ->mm is not shared".
>

Argh.  In that case this is probably buggy, and I am just as confused
as you.  This stuff is serious spaghetti code.

sys_unshare will see CLONE_NEWPID or CLONE_NEWUSER and set
CLONE_THREAD.  Then it will see CLONE_THREAD and set CLONE_VM.  Then
check_unshare_flags will see CLONE_VM and fail if we just called
vfork.

Could this be made much more comprehensible by having a single list of
shareable things are allowed to be shared across namespaces and
enforcing the *same* list in clone and unshare?

--Andy



> Oleg.
>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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