linux-kernel - Re: [PATCH] net: bridge: mcast: read ngrec once in igmp3/mld2 report

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Date:   Tue, 20 Dec 2022 12:13:12 +0200
From:   Nikolay Aleksandrov <razor@...ckwall.org>
To:     Joy Gu <jgu@...estorage.com>, bridge@...ts.linux-foundation.org
Cc:     roopa@...dia.com, davem@...emloft.net, edumazet@...gle.com,
        kuba@...nel.org, pabeni@...hat.com, joern@...estorage.com,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net: bridge: mcast: read ngrec once in igmp3/mld2 report

On 20/12/2022 04:48, Joy Gu wrote:
> In br_ip4_multicast_igmp3_report() and br_ip6_multicast_mld2_report(),
> "ih" or "mld2r" is a pointer into the skb header. It's dereferenced to
> get "num", which is used in the for-loop condition that follows.
> 
> Compilers are free to not spend a register on "num" and dereference that
> pointer every time "num" would be used, i.e. every loop iteration. Which
> would be a bug if pskb_may_pull() (called by ip_mc_may_pull() or
> ipv6_mc_may_pull() in the loop body) were to change pointers pointing
> into the skb header, e.g. by freeing "skb->head".
> 
> We can avoid this by using READ_ONCE().
> 
> Suggested-by: Joern Engel <joern@...estorage.com>
> Signed-off-by: Joy Gu <jgu@...estorage.com>
> ---
>  net/bridge/br_multicast.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 

I doubt any compiler would do that (partly due to the ntohs()). If you have hit a bug or
seen this with some compiler please provide more details, disassembly of the resulting
code would be best.

Thanks.