linux-kernel - Re: [PATCH v3 1/2] fork: extend clone3() to support CLONE_SET

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Date:   Wed, 7 Aug 2019 20:05:35 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Adrian Reber <areber@...hat.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        Pavel Emelianov <xemul@...tuozzo.com>,
        Jann Horn <jannh@...gle.com>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        linux-kernel@...r.kernel.org, Andrei Vagin <avagin@...il.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Radostin Stoyanov <rstoyanov1@...il.com>
Subject: Re: [PATCH v3 1/2] fork: extend clone3() to support CLONE_SET_TID

On Wed, Aug 07, 2019 at 06:08:56PM +0200, Oleg Nesterov wrote:
> On 08/06, Adrian Reber wrote:
> >
> > @@ -2573,6 +2575,14 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
> >  		.tls		= args.tls,
> >  	};
> >  
> > +	if (size == sizeof(struct clone_args)) {
> > +		/* Only check permissions if set_tid is actually set. */
> > +		if (args.set_tid &&
> > +			!ns_capable(pid_ns->user_ns, CAP_SYS_ADMIN))
> 
> and I just noticed this uses pid_ns = task_active_pid_ns() ...
> 
> is it correct?
> 
> I feel I am totally confused, but should we use the same
> p->nsproxy->pid_ns_for_children passed to alloc_pid?

We need to have CAP_SYS_ADMIN in the owning user namespace of the target
pidns for the pidns in which we spawn the new process. The value for
pid_ns_for_children could've been altered by either passing CLONE_NEWPID
or by having called unshare(CLONE_NEWPID) before. So yes,
pid_ns_for_children is what we want.

Sorry again for the delay in my responses. On vacation atm.

Christian