linux-kernel - Re: Oops (request_key_auth_describe) while running cve-2016-7042 from LTP

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Message-ID: <4384.1567174383@warthog.procyon.org.uk>
Date:   Fri, 30 Aug 2019 15:13:03 +0100
From:   David Howells <dhowells@...hat.com>
To:     Hillf Danton <hdanton@...a.com>
Cc:     dhowells@...hat.com, Sachin Sant <sachinp@...ux.vnet.ibm.com>,
        linuxppc-dev@...abs.org, keyrings@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: Oops (request_key_auth_describe) while running cve-2016-7042 from LTP

Hillf Danton <hdanton@...a.com> wrote:

> -	struct request_key_auth *rka = dereference_key_rcu(key);
> +	struct request_key_auth *rka;
> +
> +	rcu_read_lock();
> +	rka = dereference_key_rcu(key);

This shouldn't help as the caller, proc_keys_show(), is holding the RCU read
lock across the call.  The end of the function reads:

		if (key->type->describe)
			key->type->describe(key, m);
		seq_putc(m, '\n');

		rcu_read_unlock();
		return 0;
	}

and the documentation says "This method will be called with the RCU read lock
held".

I suspect the actual bugfix is this bit:

> +	if (!rka)
> +		goto out;

David