Re: [ovs-dev] [PATCH net] net: openvswitch: validate transport header presence in set_ipv6_addr

On 10/1/26 3:29 PM, Fernando Fernandez Mancera wrote:

On 10/1/26 2:00 PM, Ilya Maximets wrote:

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On 10/1/26 1:20 PM, Fernando Fernandez Mancera via dev wrote:

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On 10/1/26 1:15 PM, Ilya Maximets wrote:

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On 10/1/26 10:52 AM, Fernando Fernandez Mancera wrote:

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When executing IPv6 address rewrite actions on IPv6 fragments,
set_ipv6_addr() might attempt to recalculate the L4 header checksum
calling skb_transport_header() with an uninitialized transport header.
This can potentially lead to a OOB write.

Not really.  later fragments have l4_proto set to NEXTHDR_FRAGMENT
and so no writes will be performed.  See parse_ipv6hdr().

Oh, I didn't notice that. In that case, this is harmless. I will update
the commit message, thanks!

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diff --git a/net/openvswitch/actions.c b/net/openvswitch/actions.c
index dc5ff859f114..a14668565ec4 100644
--- a/net/openvswitch/actions.c
+++ b/net/openvswitch/actions.c

@@ -395,7 +395,7 @@ static void set_ipv6_addr(struct sk_buff *skb, u8 l4_proto,
   			  __be32 addr[4], const __be32 new_addr[4],
   			  bool recalculate_csum)
   {
-	if (recalculate_csum)
+	if (recalculate_csum && skb_transport_header_was_set(skb))
   		update_ipv6_checksum(skb, l4_proto, addr, new_addr);

I'd suggest moving the check into update_ipv6_checksum() and maybe add
a small comment that the check is only to avoid warning on reading the
offset that wasn't set, offset must always be set for any l4_proto for
which the checksum is actually getting updated.

Will do that. Thanks Ilya.

Actually, it might be better to check for NEXTHDR_FRAGMENT and return
early only in this case.  This way if we ever have the offset missing
for any l4_ptoto that needs it, we'll still get a warning instead of
silently not updating the checksum.  This will be close to what the
update_ip_l4_checksum() is doing.

At the same time, update_ip_l4_checksum() is able to get the transport
offset without a warning, so another alternative is to actually set the
transport offset in parse_ipv6hdr() to something like frag_off, which
would closer match the behavior of ipv4 where check_iphdr() sets the
transport header unconditionally to the next byte after the network header.
This is fine because transport headers are never accessed when they are
not set in the key.  The network checksum update cases are the only ones
where the actions are even allowed to execute and touch higher level
headers that may not be present in the packet.  At the same time the
frag_off is kind of an arbitrary value, so I'm not sure.

WDYT?

I noticed this but IMHO it is a worse pattern. In the sense that one 
persone in the future might use the transport header without noticing 
this and the warning won't show up because the transport header will be 
set incorrectly. This could cause an actual OOB.

Yeah, that's true.  It's better to keep the transport header unset if
there is actually no transport header.  We may also adjust the ipv4 path
to avoid having it set for later fragments, but that's unrelated for the
issue at hand.

I will trust your taste in this matter since I am not that familiar with 
the code.

Let's just go with the check inside update_ipv6_checksum() before reading
the value.  Seems like the best option.

Best regards, Ilya Maximets.

Thanks,
Fernando.

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Eelco, Aaron, do you maybe have a preference?

Best regards, Ilya Maximets.