Re: [PATCH net v2] net: openvswitch: make transport checksums writable

[netdev-bot+sashiko@kernel.org]

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- [Medium] The patch handles every skb_ensure_writable() failure in
  update_ip_l4_checksum() and update_ipv6_checksum()…

--

net: openvswitch: make transport checksums writable

This patch changes the IPv4 and IPv6 address rewrite helpers in Open
vSwitch. Each L4 checksum update now calls skb_ensure_writable() first,
instead of checking skb->len. A failure stays nonfatal, and packet
pointers are reloaded afterwards because the skb head may have moved.

Link: https://lore.kernel.org/r/20261030042304.74291-1-kylebot@openai.com

> diff --git a/net/openvswitch/actions.c b/net/openvswitch/actions.c
> index 12e339cbf7912..fe472e950cb1a 100644
> --- a/net/openvswitch/actions.c
> +++ b/net/openvswitch/actions.c
> @@ -319,20 +319,22 @@ static int pop_nsh(struct sk_buff *skb, struct sw_flow_key *key)
>  	return 0;
>  }
>  
> -static void update_ip_l4_checksum(struct sk_buff *skb, struct iphdr *nh,
> -				  __be32 addr, __be32 new_addr)
> +static void update_ip_l4_checksum(struct sk_buff *skb, __be32 addr,
> +				  __be32 new_addr)
>  {
> -	int transport_len = skb->len - skb_transport_offset(skb);
> +	struct iphdr *nh = ip_hdr(skb);
>  
>  	if (nh->frag_off & htons(IP_OFFSET))
>  		return;
>  
>  	if (nh->protocol == IPPROTO_TCP) {
> -		if (likely(transport_len >= sizeof(struct tcphdr)))
> +		if (likely(!skb_ensure_writable(skb, skb_transport_offset(skb) +
> +					       sizeof(struct tcphdr))))
>  			inet_proto_csum_replace4(&tcp_hdr(skb)->check, skb,
>  						 addr, new_addr, true);

[Severity: Medium]
Is it intended that every skb_ensure_writable() failure here is handled
the same way as a truncated transport header?

skb_ensure_writable() can fail even when the full TCP header is in the
packet. pskb_may_pull()->__pskb_pull_tail() returns NULL in three cases:
pskb_expand_head(GFP_ATOMIC) fails, a frag_list skb_clone() fails, or the
frags are unreadable:

net/core/skbuff.c:__pskb_pull_tail() {
    ...
	if (!skb_frags_readable(skb))
		return NULL;

	if (eat > 0 || skb_cloned(skb)) {
		if (pskb_expand_head(skb, 0, eat > 0 ? eat + 128 : 0,
				     GFP_ATOMIC))
			return NULL;
	}
    ...
}

pskb_expand_head(GFP_ATOMIC) can also fail later for a cloned skb.

In those cases the L4 checksum update is skipped. set_ip_addr() still
updates the IP header checksum and writes the new address:

	update_ip_l4_checksum(skb, old_addr, new_addr);
	addr = (__be32 *)(skb_network_header(skb) + addr_offset);
	csum_replace4(&ip_hdr(skb)->check, old_addr, new_addr);
	...
	*addr = new_addr;

set_ipv6_addr() does the same with memcpy(addr, new_addr, ...). Then
set_ipv4() or set_ipv6() returns 0.

Would the packet then be forwarded with a rewritten pseudo-header and a
stale TCP/UDP/ICMPv6 checksum?

With CHECKSUM_NONE or CHECKSUM_COMPLETE the checksum on the wire would be
wrong. With CHECKSUM_PARTIAL (including GSO), the stale seed in th->check
or uh->check would give a wrong final checksum for every segment.

The commit message says the reproducer relies on "natural allocation
pressure". That is the same condition that would make this new
skb_ensure_writable() call fail.

Other writers in the same file treat this failure as fatal. For example,
set_tcp():

	err = skb_ensure_writable(skb, skb_transport_offset(skb) +
				  sizeof(struct tcphdr));
	if (unlikely(err))
		return err;

set_udp() and set_sctp() do the same, so do_execute_actions() drops the
skb with OVS_DROP_ACTION_ERROR.

Could the update be skipped only when skb->len - skb_transport_offset(skb)
is smaller than the header size, with the skb_ensure_writable() error
returned in all other cases? That would need set_ip_addr() and
set_ipv6_addr() to return int.

The same pattern applies to the UDP branch here, and to the TCP, UDP and
ICMPv6 branches in update_ipv6_checksum().

[ ... ]

-- 
Sashiko AI review · https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20261107040238.27606-1-kylebot%40openai.com