Subject: Re: [PATCH v2] netfilter: nfnetlink_osf: fix null-ptr-deref in nf_osf_ttl

From: Florian Westphal <[email-masked]>
Date: Tue, 14 Apr 2026 13:14:11 +0200

Kito Xu (veritas501) <[email-masked]> wrote:
> nf_osf_ttl() calls __in_dev_get_rcu(skb->dev) and passes the result
> to in_dev_for_each_ifa_rcu() without checking for NULL. When the
> receiving device has no IPv4 configuration (ip_ptr is NULL),
> __in_dev_get_rcu() returns NULL and in_dev_for_each_ifa_rcu()
> dereferences it unconditionally, causing a kernel crash.
> 
> This can happen when a packet arrives on a device that has had its
> IPv4 configuration removed (e.g., MTU set below IPV4_MIN_MTU causing
> inetdev_destroy) or on a device that was never assigned an IPv4
> address, while an xt_osf or nft_osf rule with TTL_LESS mode is
> active and the packet TTL exceeds the fingerprint TTL.
> 
> Add a NULL check for in_dev before using it. When in_dev is NULL,
> return 0 (no match) since source-address locality cannot be
> determined without IPv4 addresses on the device.
> 
> KASAN: null-ptr-deref in range [0x0000000000000010-0x0000000000000017]
> RIP: 0010:nf_osf_match_one+0x204/0xa70
> Call Trace:
>  <IRQ>
>  nf_osf_match+0x2f8/0x780
>  xt_osf_match_packet+0x11c/0x1f0
>  ipt_do_table+0x7fe/0x12b0
>  nf_hook_slow+0xac/0x1e0
>  ip_rcv+0x123/0x370
>  __netif_receive_skb_one_core+0x166/0x1b0
>  process_backlog+0x197/0x590
>  __napi_poll+0xa1/0x540
>  net_rx_action+0x401/0xd80
>  handle_softirqs+0x19f/0x610
>  </IRQ>
> 
> Fixes: a218dc82f0b5 ("netfilter: nft_osf: Add ttl option support")
> Suggested-by: Pablo Neira Ayuso <[email-masked]>
> Signed-off-by: Kito Xu (veritas501) <[email-masked]>

The other __in_dev_get_rcu() callers in netfilter check return value, so:

Reviewed-by: Florian Westphal <[email-masked]>