linux-kernel - Re: [PATCH] net: fix wrong skb_get() usage / crash in IGMP/MLD parsing code

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Date:	Thu, 13 Aug 2015 17:11:01 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	linus.luessing@...3.blue
Cc:	netdev@...r.kernel.org, bridge@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, stephen@...workplumber.org,
	herbert@...dor.apana.org.au, bblanco@...mgrid.com,
	alexei.starovoitov@...il.com
Subject: Re: [PATCH] net: fix wrong skb_get() usage / crash in IGMP/MLD
 parsing code

From: Linus Lüssing <linus.luessing@...3.blue>
Date: Thu, 13 Aug 2015 05:54:07 +0200

> The recent refactoring of the IGMP and MLD parsing code into
> ipv6_mc_check_mld() / ip_mc_check_igmp() introduced a potential crash /
> BUG() invocation for bridges:
> 
> I wrongly assumed that skb_get() could be used as a simple reference
> counter for an skb which is not the case. skb_get() bears additional
> semantics, a user count. This leads to a BUG() invocation in
> pskb_expand_head() / kernel panic if pskb_may_pull() is called on an skb
> with a user count greater than one - unfortunately the refactoring did
> just that.
> 
> Fixing this by removing the skb_get() call and changing the API: The
> caller of ipv6_mc_check_mld() / ip_mc_check_igmp() now needs to
> additionally check whether the returned skb_trimmed is a clone.
> 
> Fixes: 9afd85c9e455 ("net: Export IGMP/MLD message validation code")
> Reported-by: Brenden Blanco <bblanco@...mgrid.com>
> Signed-off-by: Linus Lüssing <linus.luessing@...3.blue>

Applied, thanks.
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