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

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Message-ID: <20130820192340.GA25441@redhat.com>
Date:	Tue, 20 Aug 2013 21:23:40 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>
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 08/20, Andy Lutomirski wrote:
>
> On Tue, Aug 20, 2013 at 12:05 PM, Oleg Nesterov <oleg@...hat.com> wrote:
> > On 08/20, Andy Lutomirski wrote:
> >>
> >> On Tue, Aug 20, 2013 at 11:50 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> >> >
> >> >> 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,
> >
> > I don't think so. Just we can't really unshare ->mm

this looks confusing, sorry. Afaics it is possible to implement
unshare(CLONE_VM), but

> or implement
> > unshare(CLONE_THREAD).

this is unlikely.

but this doesn't matter,

> We simply pretend it works if there is nothing
> > to unshare.
> >
> >> sys_unshare will see CLONE_NEWPID or CLONE_NEWUSER and set
> >> CLONE_THREAD.  Then it will see CLONE_THREAD and set CLONE_VM.
> >
> > This matches copy_process() to some degree... but looks confusing,
> > I agree.

I only mean that copy_process() requires CLONE_VM if CLONE_THREAD.
But, unlike unshare(), it fails if CLONE_VM is not set.

> Huh?  Doesn't this mean that unshare(CLONE_NEWPID); vfork() will work
> with your patches,

I hope,

> but vfork(); unshare(CLONE_NEWPID) will fail?  (I
> admit I haven't tested it.)

Do you mean that the child does unshare(CLONE_NEWPID) before exec?
It should fail with or without this patch.

Oleg.

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