linux-kernel - Re: [PATCH 2/5] KEYS: user_defined: sanitize key payloads

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Date:   Fri, 21 Apr 2017 11:34:45 -0700
From:   Eric Biggers <ebiggers3@...il.com>
To:     David Howells <dhowells@...hat.com>
Cc:     keyrings@...r.kernel.org, linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, Eric Biggers <ebiggers@...gle.com>
Subject: Re: [PATCH 2/5] KEYS: user_defined: sanitize key payloads

On Fri, Apr 21, 2017 at 02:57:17PM +0100, David Howells wrote:
> Eric Biggers <ebiggers3@...il.com> wrote:
> 
> > -		kfree_rcu(zap, rcu);
> > +		call_rcu(&zap->rcu, user_free_payload_rcu);
> 
> Add kzfree_rcu()?
> 
> David

We could, but it's not trivial because the way kfree_rcu() works is to store the
offset of the rcu_head as the callback function, then have a special case in RCU
reclaim that recognizes "function pointers" with value < 4096 and call kfree()
rather than the function.  To support kzfree_rcu() we'd need to reserve another
4096 bytes of the address space (maybe at the end?), then check for the special
kzfree value on every RCU reclaim.  Or equivalently it could be a flag.  It's
possible, but it may be best to just use a custom callback for now.  Then if it
can be shown later that there are a lot of users who would like a
"kzfree_rcu()", it can be added.

- Eric