Re: [PATCH net] net: initialize ICMP error queue opt_stats

[Hangbin Liu]

On Wed, Nov 11, 2026 at 09:05:30AM -0800, Weiming Shi wrote:
> ip_icmp_error() and ipv6_icmp_error() clone a received ICMP skb before
> building its error queue entry.  skb_clone() preserves the complete control
> buffer, while the IP receive paths initialize only the portion aliased by
> sock_exterr_skb::header.  The two functions initialize ee, addr_offset, and
> port, but leave the final byte containing opt_stats and unused untouched.
> A TCX classifier can leave bpf_skb_data_end::data_end across that byte,
> forging opt_stats and making __sock_recv_timestamp() pass skb->len bytes
> from skb->data to put_cmsg().  For a non-linear ICMP skb, this reads beyond
> its linear head.
> 
> Initialize both bitfields in the two clone-based producers.  Do not clear
> the whole sock_exterr_skb because the error receive paths consume the saved
> IP control-buffer header.
> 
>   usercopy: Kernel memory exposure attempt detected from SLUB object
>             'skbuff_small_head' (offset 72, size 59964)!
>   kernel BUG at mm/usercopy.c:102!
>   Oops: invalid opcode: 0000 [#1] SMP KASAN NOPTI
>   RIP: usercopy_abort (mm/usercopy.c:102)
>   Call Trace:
>    __check_heap_object (mm/slub.c:8146)
>    __check_object_size (mm/usercopy.c:258)
>    put_cmsg (net/core/scm.c:276)
>    __sock_recv_timestamp (net/socket.c:1061)
>    ip_recv_error (net/ipv4/ip_sockglue.c:550)
>    udp_recvmsg (net/ipv4/udp.c:2034)
>    inet_recvmsg (net/ipv4/af_inet.c:896)
>    sock_recvmsg (net/socket.c:1159)
>    ____sys_recvmsg (net/socket.c:2918)
>    ___sys_recvmsg (net/socket.c:2961)
>    __sys_recvmsg (net/socket.c:2993)
>    do_syscall_64 (arch/x86/entry/syscall_64.c:94)
>    entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)
>   Kernel panic - not syncing: Fatal exception
> 
> With HARDENED_USERCOPY enabled, the reproducer aborted the unpatched kernel
> on its first packet.  With runtime usercopy checks disabled and KASAN left
> in report mode, the same recvmsg() returned 59,964 control-message data
> bytes sourced past a 704-byte skb head.  Beyond offset 1,024, 58,932 bytes
> differed from the prefilled userspace marker, including 6,288 nonzero
> bytes.  The patched build completed 2,000 packets under both configurations
> without a forged SCM_TIMESTAMPING_OPT_STATS message or sanitizer report.
> Networking selftests were not run.
> 
> Fixes: 4ef1b2869447 ("tcp: mark skbs with SCM_TIMESTAMPING_OPT_STATS")
> Cc: stable@xxxxxxxxxxxxxxx
> Reported-by: <co+beaa1ed2d7aaec16@xxxxxxx>
> Signed-off-by: Weiming Shi <bestswngs@xxxxxxxxx>
> 
> ---
>  net/ipv4/ip_sockglue.c | 2 ++
>  net/ipv6/datagram.c    | 2 ++
>  2 files changed, 4 insertions(+)
> 
> diff --git a/net/ipv4/ip_sockglue.c b/net/ipv4/ip_sockglue.c
> index a55ef327ec93..3ab84a4c31e5 100644
> --- a/net/ipv4/ip_sockglue.c
> +++ b/net/ipv4/ip_sockglue.c
> @@ -429,6 +429,8 @@ void ip_icmp_error(struct sock *sk, struct sk_buff *skb, int err,
>  	serr->addr_offset = (u8 *)&(((struct iphdr *)(icmp_hdr(skb) + 1))->daddr) -
>  				   skb_network_header(skb);
>  	serr->port = port;
> +	serr->opt_stats = 0;
> +	serr->unused = 0;
>  
>  	if (skb_pull(skb, payload - skb->data)) {
>  		if (inet_test_bit(RECVERR_RFC4884, sk))
> diff --git a/net/ipv6/datagram.c b/net/ipv6/datagram.c
> index ca3605acb433..85f646f8d059 100644
> --- a/net/ipv6/datagram.c
> +++ b/net/ipv6/datagram.c
> @@ -330,6 +330,8 @@ void ipv6_icmp_error(struct sock *sk, struct sk_buff *skb, int err,
>  	serr->addr_offset = (u8 *)&(((struct ipv6hdr *)(icmph + 1))->daddr) -
>  				  skb_network_header(skb);
>  	serr->port = port;
> +	serr->opt_stats = 0;
> +	serr->unused = 0;
>  
>  	__skb_pull(skb, payload - skb->data);
>  
> -- 
> 2.56.0
> 

Reviewed-by: Hangbin Liu <liuhangbin@xxxxxxxxxx>