netdev - Re: [PATCH net 1/3] ipv6: exthdrs: fix potential use-after-free in ipv6_rpl_srh

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Date: Thu, 18 May 2023 19:05:00 +0200
From: Simon Horman <simon.horman@...igine.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: "David S . Miller" <davem@...emloft.net>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Alexander Aring <alex.aring@...il.com>,
	David Lebrun <david.lebrun@...ouvain.be>, netdev@...r.kernel.org,
	eric.dumazet@...il.com
Subject: Re: [PATCH net 1/3] ipv6: exthdrs: fix potential use-after-free in
 ipv6_rpl_srh_rcv()

On Wed, May 17, 2023 at 09:31:16PM +0000, Eric Dumazet wrote:
> After calls to pskb_may_pull() we need to reload @hdr variable,
> because skb->head might have changed.
> 
> We need to move up first pskb_may_pull() call right after
> looped_back label.
> 
> Fixes: 8610c7c6e3bd ("net: ipv6: add support for rpl sr exthdr")
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: Alexander Aring <alex.aring@...il.com>
> Cc: David Lebrun <david.lebrun@...ouvain.be>
> ---
>  net/ipv6/exthdrs.c | 11 +++++------
>  1 file changed, 5 insertions(+), 6 deletions(-)
> 
> diff --git a/net/ipv6/exthdrs.c b/net/ipv6/exthdrs.c
> index a8d961d3a477f6516f542025dfbcfc6f47407a70..b129e982205ee43cbf74f4900c3031827d962dc2 100644
> --- a/net/ipv6/exthdrs.c
> +++ b/net/ipv6/exthdrs.c
> @@ -511,6 +511,10 @@ static int ipv6_rpl_srh_rcv(struct sk_buff *skb)
>  	}
>  
>  looped_back:
> +	if (!pskb_may_pull(skb, sizeof(*hdr))) {
> +		kfree_skb(skb);
> +		return -1;
> +	}
>  	hdr = (struct ipv6_rpl_sr_hdr *)skb_transport_header(skb);
>  
>  	if (hdr->segments_left == 0) {

Hi Eric,

Not far below this line there is a call to pskb_pull():

                if (hdr->nexthdr == NEXTHDR_IPV6) {
                        int offset = (hdr->hdrlen + 1) << 3;

                        skb_postpull_rcsum(skb, skb_network_header(skb),
                                           skb_network_header_len(skb));

                        if (!pskb_pull(skb, offset)) {
                                kfree_skb(skb);
                                return -1;
                        }
                        skb_postpull_rcsum(skb, skb_transport_header(skb),
                                           offset);

Should hdr be reloaded after the call to pskb_pull() too?

> @@ -544,12 +548,6 @@ static int ipv6_rpl_srh_rcv(struct sk_buff *skb)
>  
>  		return 1;
>  	}
> -
> -	if (!pskb_may_pull(skb, sizeof(*hdr))) {
> -		kfree_skb(skb);
> -		return -1;
> -	}
> -
>  	n = (hdr->hdrlen << 3) - hdr->pad - (16 - hdr->cmpre);
>  	r = do_div(n, (16 - hdr->cmpri));
>  	/* checks if calculation was without remainder and n fits into
> @@ -592,6 +590,7 @@ static int ipv6_rpl_srh_rcv(struct sk_buff *skb)
>  		kfree_skb(skb);
>  		return -1;
>  	}
> +	hdr = (struct ipv6_rpl_sr_hdr *)skb_transport_header(skb);
>  
>  	hdr->segments_left--;
>  	i = n - hdr->segments_left;
> -- 
> 2.40.1.606.ga4b1b128d6-goog
> 
>