Subject: Re: [PATCH net] ipv4: validate ip_forward_options() option fields against skb tail

From: Jiayuan Chen <[email-masked]>
Date: Thu, 28 May 2026 21:48:41 +0800

On 5/28/26 7:12 PM, Qi Tang wrote:
> ip_forward_options() re-reads the RR/SRR/TS option length byte
> optptr[1] and pointer byte optptr[2] from the skb on the forwarding
> path and uses them as indexes for 4-byte writes via
> ip_rt_get_source() (and a memcmp walk in the SRR branch).
>
> __ip_options_compile() validates those bytes at parse time but stores
> only the option's offset into IPCB(skb)->opt.{rr,srr,ts}.  An nftables
> FORWARD-chain payload mutation between parse and consume can rewrite
> the bytes, driving the indexed writes out of bounds and overlapping
> skb_shared_info.  With optptr[2] mutated the write can land in
> skb_shared_info.frag_list; the next time the skb is dropped
> kfree_skb_list_reason() walks the forged list and frees an
> attacker-controlled pointer, an arbitrary-free primitive (R15 below
> is the corrupted frag_list):
>
>    BUG: unable to handle page fault for address: ffffed10195fd757
>    Oops: 0000 [#1] SMP KASAN NOPTI
>    RIP: 0010:kfree_skb_list_reason+0x167/0x5f0
>    RAX: 1ffff110195fd757 RBX: dffffc0000000000
>    R15: ffff8880cafebabe
>    CR2: ffffed10195fd757
>    Call Trace:
>     skb_release_data+0x565/0x820
>     sk_skb_reason_drop+0xc1/0x350
>     ip_rcv_core+0x7a8/0xcd0
>     ip_rcv+0x97/0x270
>     __netif_receive_skb_one_core+0x161/0x1b0
>     process_backlog+0x1c4/0x5b0
>     net_rx_action+0x934/0xfa0

The bug is real, but I'm curious what kernel version and driver you're on.
On my side the skb falls into SKB_SMALL_HEAD_CACHE_SIZE (704), so the 
linear area
is pretty long, and optptr[2] maxes out at 255, which doesn't look like 
it can reach frag_list.

May the driver use alloc_skb to allocate small liner buffer?

> Bound optptr[2] within optptr[1] before the RR and TS writes, and
> clamp the SRR walk to the bytes actually present in the skb.  Match
> the existing error handling in this function: skip the malformed
> option in place rather than returning, so the single ip_send_check()
> at the end still recomputes the checksum for any option that was
> updated earlier.
>
> Cc: [email-masked]
> Reported-by: Qi Tang <[email-masked]>
> Reported-by: Tong Liu <[email-masked]>
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Signed-off-by: Qi Tang <[email-masked]>
> ---
>   net/ipv4/ip_options.c | 27 +++++++++++++++++++--------
>   1 file changed, 19 insertions(+), 8 deletions(-)
>
> diff --git a/net/ipv4/ip_options.c b/net/ipv4/ip_options.c
> index be8815ce3ac24..36a4e3cc39dd1 100644
> --- a/net/ipv4/ip_options.c
> +++ b/net/ipv4/ip_options.c
> @@ -544,18 +544,26 @@ void ip_forward_options(struct sk_buff *skb)
>   
>   	if (opt->rr_needaddr) {
>   		optptr = (unsigned char *)raw + opt->rr;
> -		ip_rt_get_source(&optptr[optptr[2]-5], skb, rt);
> -		opt->is_changed = 1;
> +		if (optptr + optptr[1] <= skb_tail_pointer(skb) &&
> +		    optptr[2] >= 5 && optptr[2] <= optptr[1] + 1) {
> +			ip_rt_get_source(&optptr[optptr[2] - 5], skb, rt);
> +			opt->is_changed = 1;
> +		}
>   	}
>   	if (opt->srr_is_hit) {
>   		int srrptr, srrspace;
>   
>   		optptr = raw + opt->srr;
>   
> -		for ( srrptr = optptr[2], srrspace = optptr[1];
> -		     srrptr <= srrspace;
> -		     srrptr += 4
> -		     ) {
> +		/* optptr[1] (option length) may have been rewritten after the
> +		 * parse-time check; if it now runs past the skb the option is
> +		 * malformed, so skip the source-route rewrite below.
> +		 */
> +		srrspace = optptr[1];
> +		if (optptr + srrspace > skb_tail_pointer(skb))
> +			srrspace = 0;
> +
> +		for (srrptr = optptr[2]; srrptr <= srrspace; srrptr += 4) {
>   			if (srrptr + 3 > srrspace)
>   				break;
>   			if (memcmp(&opt->nexthop, &optptr[srrptr-1], 4) == 0)
> @@ -572,8 +580,11 @@ void ip_forward_options(struct sk_buff *skb)
>   		}
>   		if (opt->ts_needaddr) {
>   			optptr = raw + opt->ts;
> -			ip_rt_get_source(&optptr[optptr[2]-9], skb, rt);
> -			opt->is_changed = 1;
> +			if (optptr + optptr[1] <= skb_tail_pointer(skb) &&
> +			    optptr[2] >= 9 && optptr[2] <= optptr[1] + 5) {
> +				ip_rt_get_source(&optptr[optptr[2] - 9], skb, rt);
> +				opt->is_changed = 1;
> +			}
>   		}
>   	}
>   	if (opt->is_changed) {