linux-kernel - Re: [PATCH] livepatch: add load/unload hooks to objects

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Message-ID: <alpine.LNX.2.00.1608301130040.22028@cbobk.fhfr.pm>
Date:   Tue, 30 Aug 2016 11:41:28 +0200 (CEST)
From:   Jiri Kosina <jikos@...nel.org>
To:     Christopher Arges <chris.j.arges@...onical.com>
cc:     Petr Mladek <pmladek@...e.com>, live-patching@...r.kernel.org,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jessica Yu <jeyu@...hat.com>, Miroslav Benes <mbenes@...e.cz>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] livepatch: add load/unload hooks to objects

On Mon, 29 Aug 2016, Christopher Arges wrote:

> Another example is CVE-2016-2117. Here we need to unset NETIF_F_SG on a 
> particular device. If the device is already loaded we need a way to 
> fixup hw_features on an already allocated network device. Again this 
> could be done in the init code of the patch, but a nicer solution would 
> be to do this on a load/unload hook appropriately.

I am afraid this is more complicated than what you describe. You can't 
just unset NETIF_F_SG and be done with it; look for example what might 
happen if you clear the flag while skb_segment() is running and gcc is 
refetching netdev_features_t (there is no READ_ONCE() for that). The same 
holds for __ip6_append_data().
I am not saying this can't be worked around, but it's way much more 
complicated than just clearing a bit in a callback.

-- 
Jiri Kosina
SUSE Labs