linux-kernel - Re: [PATCH 5.10 024/252] mac80211: prevent mixed key and fragment cache attacks

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Message-ID: <20210601092619.GA30422@amd>
Date:   Tue, 1 Jun 2021 11:26:19 +0200
From:   Pavel Machek <pavel@....cz>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Mathy Vanhoef <Mathy.Vanhoef@...euven.be>,
        Johannes Berg <johannes.berg@...el.com>
Subject: Re: [PATCH 5.10 024/252] mac80211: prevent mixed key and fragment
 cache attacks

Hi!

> From: Mathy Vanhoef <Mathy.Vanhoef@...euven.be>
> 
> commit 94034c40ab4a3fcf581fbc7f8fdf4e29943c4a24 upstream.
> 
> Simultaneously prevent mixed key attacks (CVE-2020-24587) and fragment
> cache attacks (CVE-2020-24586). This is accomplished by assigning a
> unique color to every key (per interface) and using this to track which
> key was used to decrypt a fragment. When reassembling frames, it is
> now checked whether all fragments were decrypted using the same key.
> 
> To assure that fragment cache attacks are also prevented, the ID that is
> assigned to keys is unique even over (re)associations and (re)connects.
> This means fragments separated by a (re)association or (re)connect will
> not be reassembled. Because mac80211 now also prevents the reassembly of
> mixed encrypted and plaintext fragments, all cache attacks are
> prevented.

> --- a/net/mac80211/key.c
> +++ b/net/mac80211/key.c
> @@ -799,6 +799,7 @@ int ieee80211_key_link(struct ieee80211_
>  		       struct ieee80211_sub_if_data *sdata,
>  		       struct sta_info *sta)
>  {
> +	static atomic_t key_color = ATOMIC_INIT(0);
>  	struct ieee80211_key *old_key;

This is nice and simple, but does not include any kind of overflow
handling. sparc32 moved away from 24-bit atomics, which is good I
guess. OTOH if this is incremented 10 times a second, we'll still
overflow in 6 years or so. Can attacker make it overflow?

Should this have a note why overflow is not possible / why it is not a
problem?

Best regards,
								Pavel

-- 
http://www.livejournal.com/~pavelmachek

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