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Subject: Re: [PATCH v6 1/1] ns: add binfmt_misc to the user namespace
To:     Laurent Vivier <laurent@vivier.eu>, linux-kernel@vger.kernel.org
Cc:     Jann Horn <jannh@google.com>,
        James Bottomley <James.Bottomley@HansenPartnership.com>,
        linux-api@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        Andrei Vagin <avagin@gmail.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Eric Biederman <ebiederm@xmission.com>,
        containers@lists.linux-foundation.org,
        Dmitry Safonov <dima@arista.com>
References: <20181010161430.11633-1-laurent@vivier.eu>
 <20181010161430.11633-2-laurent@vivier.eu>
From:   Rasmus Villemoes <linux@rasmusvillemoes.dk>
Message-ID: <97280bb2-933e-9281-bd91-99748e1dd653@rasmusvillemoes.dk>
Date:   Tue, 16 Oct 2018 12:13:47 +0200
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On 2018-10-10 18:14, Laurent Vivier wrote:

> +	/* create a new binfmt namespace
> +	 * if we are not in the first user namespace
> +	 * but the binfmt namespace is the first one
> +	 */
> +	if (READ_ONCE(ns->binfmt_ns) == NULL) {
> +		struct binfmt_namespace *new_ns;
> +
> +		new_ns = kmalloc(sizeof(struct binfmt_namespace),
> +				 GFP_KERNEL);
> +		if (new_ns == NULL)
> +			return -ENOMEM;
> +		INIT_LIST_HEAD(&new_ns->entries);
> +		new_ns->enabled = 1;
> +		rwlock_init(&new_ns->entries_lock);
> +		new_ns->bm_mnt = NULL;
> +		new_ns->entry_count = 0;
> +		/* ensure new_ns is completely initialized before sharing it */
> +		smp_wmb();
> +		WRITE_ONCE(ns->binfmt_ns, new_ns);
> +	}

If ns->binfmt_ns can really change under us (given you use READ_ONCE),
what prevents two instances of this code running at the same time, in
which case one of them would leak its new_ns instance? Also, there
doesn't seem to be any smp_rmb() buddy to that wmb(), I don't think
that's implied by READ_ONCE() in binfmt_ns().

Rasmus