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

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Message-ID: <20150813045549.GA60201@Alexeis-MBP.westell.com>
Date:	Wed, 12 Aug 2015 21:55:50 -0700
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	Linus Lüssing <linus.luessing@...3.blue>
Cc:	netdev@...r.kernel.org, bridge@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org,
	Stephen Hemminger <stephen@...workplumber.org>,
	"David S. Miller" <davem@...emloft.net>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Brenden Blanco <bblanco@...mgrid.com>
Subject: Re: [PATCH] net: fix wrong skb_get() usage / crash in IGMP/MLD
 parsing code

On Thu, Aug 13, 2015 at 05:54:07AM +0200, Linus Lüssing wrote:
> 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>

I think the fix actually made the code easier to read.
Thank you. Looks good to me.
Acked-by: Alexei Starovoitov <ast@...mgrid.com>

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