Re: [RFC v5 1/1] ns: add binfmt_misc to the user namespace

From: Aleksa Sarai <asarai@suse.de>
To: unlisted-recipients:; (no To-header on input)
Cc: Tycho Andersen <tycho@tycho.ws>, Jann Horn <jannh@google.com>,
	linux-api@vger.kernel.org, containers@lists.linux-foundation.org,
	Dmitry Safonov <dima@arista.com>,
	linux-kernel@vger.kernel.org,
	James Bottomley <James.Bottomley@HansenPartnership.com>,
	Eric Biederman <ebiederm@xmission.com>,
	linux-fsdevel@vger.kernel.org,
	Alexander Viro <viro@zeniv.linux.org.uk>
Subject: Re: [RFC v5 1/1] ns: add binfmt_misc to the user namespace
Date: Wed, 10 Oct 2018 18:14:47 +1100	[thread overview]
Message-ID: <20181010071447.whwwxjdv54qfnvsr@mikami> (raw)
In-Reply-To: <409c22e3-1df8-cf7f-2462-ead2bb3020cf@vivier.eu>

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On 2018-10-09, Laurent Vivier <laurent@vivier.eu> wrote:
> Le 09/10/2018 à 17:16, Tycho Andersen a écrit :
> > On Tue, Oct 09, 2018 at 12:37:52PM +0200, Laurent Vivier wrote:
> >> @@ -80,18 +74,32 @@ static int entry_count;
> >>   */
> >>  #define MAX_REGISTER_LENGTH 1920
> >>  
> >> +static struct binfmt_namespace *binfmt_ns(struct user_namespace *ns)
> >> +{
> >> +	struct binfmt_namespace *b_ns;
> >> +
> >> +	while (ns) {
> >> +		b_ns = READ_ONCE(ns->binfmt_ns);
> >> +		if (b_ns)
> >> +			return b_ns;
> >> +		ns = ns->parent;
> >> +	}
> >> +	WARN_ON_ONCE(1);
> > 
> > It looks like we warn here,
> > 
> >> @@ -133,17 +141,18 @@ static int load_misc_binary(struct linux_binprm *bprm)
> >>  	struct file *interp_file = NULL;
> >>  	int retval;
> >>  	int fd_binary = -1;
> >> +	struct binfmt_namespace *ns = binfmt_ns(current_user_ns());
> >>  
> >>  	retval = -ENOEXEC;
> >> -	if (!enabled)
> >> +	if (!ns->enabled)
> > 
> > ...but then in cases like this we immediately dereference the pointer
> > anyways and crash. Can we return some other error code here in the !ns
> > case so we don't crash?
> 
> My concern here is I don't want to add code to check an error case that
> cannot happen. The first namespace binfmt_ns pointer is initialized with
> &init_binfmt_ns, so the return value cannot be NULL.

I'd argue that BUG() is a better thing to do then -- if doing a dummy
error path makes no sense. Though IIRC BUG() is no longer a popular
thing to do.

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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