netdev - Re: [PATCH net v2] ipvs: fix ipv4 null-ptr-deref in route error path

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Message-ID: <e0ecac84-29b4-477e-8e40-2603ca7b1154@ssi.bg>
Date: Mon, 24 Nov 2025 17:34:47 +0200 (EET)
From: Julian Anastasov <ja@....bg>
To: Slavin Liu <slavin452@...il.com>
cc: horms@...ge.net.au, pablo@...filter.org, kadlec@...filter.org,
        fw@...len.de, phil@....cc, davem@...emloft.net, edumazet@...gle.com,
        kuba@...nel.org, pabeni@...hat.com, netdev@...r.kernel.org,
        lvs-devel@...r.kernel.org, netfilter-devel@...r.kernel.org,
        coreteam@...filter.org
Subject: Re: [PATCH net v2] ipvs: fix ipv4 null-ptr-deref in route error
 path


	Hello,

On Fri, 21 Nov 2025, Slavin Liu wrote:

> The IPv4 code path in __ip_vs_get_out_rt() calls dst_link_failure()
> without ensuring skb->dev is set, leading to a NULL pointer dereference
> in fib_compute_spec_dst() when ipv4_link_failure() attempts to send
> ICMP destination unreachable messages.
> 
> The issue emerged after commit ed0de45a1008 ("ipv4: recompile ip options
> in ipv4_link_failure") started calling __ip_options_compile() from
> ipv4_link_failure(). This code path eventually calls fib_compute_spec_dst()
> which dereferences skb->dev. An attempt was made to fix the NULL skb->dev
> dereference in commit 0113d9c9d1cc ("ipv4: fix null-deref in
> ipv4_link_failure"), but it only addressed the immediate dev_net(skb->dev)
> dereference by using a fallback device. The fix was incomplete because
> fib_compute_spec_dst() later in the call chain still accesses skb->dev
> directly, which remains NULL when IPVS calls dst_link_failure().
> 
> The crash occurs when:
> 1. IPVS processes a packet in NAT mode with a misconfigured destination
> 2. Route lookup fails in __ip_vs_get_out_rt() before establishing a route
> 3. The error path calls dst_link_failure(skb) with skb->dev == NULL
> 4. ipv4_link_failure() → ipv4_send_dest_unreach() →
>    __ip_options_compile() → fib_compute_spec_dst()
> 5. fib_compute_spec_dst() dereferences NULL skb->dev
> 
> Apply the same fix used for IPv6 in commit 326bf17ea5d4 ("ipvs: fix
> ipv6 route unreach panic"): set skb->dev from skb_dst(skb)->dev before
> calling dst_link_failure().
> 
> KASAN: null-ptr-deref in range [0x0000000000000328-0x000000000000032f]
> CPU: 1 PID: 12732 Comm: syz.1.3469 Not tainted 6.6.114 #2
> RIP: 0010:__in_dev_get_rcu include/linux/inetdevice.h:233
> RIP: 0010:fib_compute_spec_dst+0x17a/0x9f0 net/ipv4/fib_frontend.c:285
> Call Trace:
>   <TASK>
>   spec_dst_fill net/ipv4/ip_options.c:232
>   spec_dst_fill net/ipv4/ip_options.c:229
>   __ip_options_compile+0x13a1/0x17d0 net/ipv4/ip_options.c:330
>   ipv4_send_dest_unreach net/ipv4/route.c:1252
>   ipv4_link_failure+0x702/0xb80 net/ipv4/route.c:1265
>   dst_link_failure include/net/dst.h:437
>   __ip_vs_get_out_rt+0x15fd/0x19e0 net/netfilter/ipvs/ip_vs_xmit.c:412
>   ip_vs_nat_xmit+0x1d8/0xc80 net/netfilter/ipvs/ip_vs_xmit.c:764
> 
> Fixes: ed0de45a1008 ("ipv4: recompile ip options in ipv4_link_failure")
> Signed-off-by: Slavin Liu <slavin452@...il.com>

	Looks good to me for the nf tree, thanks!

Acked-by: Julian Anastasov <ja@....bg>

> ---
>  net/netfilter/ipvs/ip_vs_xmit.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/net/netfilter/ipvs/ip_vs_xmit.c b/net/netfilter/ipvs/ip_vs_xmit.c
> index 95af252b2939..618fbe1240b5 100644
> --- a/net/netfilter/ipvs/ip_vs_xmit.c
> +++ b/net/netfilter/ipvs/ip_vs_xmit.c
> @@ -409,6 +409,9 @@ __ip_vs_get_out_rt(struct netns_ipvs *ipvs, int skb_af, struct sk_buff *skb,
>  	return -1;
>  
>  err_unreach:
> +	if (!skb->dev)
> +		skb->dev = skb_dst(skb)->dev;
> +
>  	dst_link_failure(skb);
>  	return -1;
>  }
> -- 
> 2.43.0

Regards

--
Julian Anastasov <ja@....bg>