[PATCH net 4/6] seg6: discard CHECKSUM_COMPLETE after the End.BPF program

[Andrea Mayer <andrea.mayer@uniroma2.it>]

The End.BPF program can change the packet through the seg6 BPF helpers,
for instance the SRH Tag with bpf_lwt_seg6_store_bytes(). After the
program, seg6_bpf_has_valid_srh() can also rewrite the SRH Hdr Ext Len.

With CHECKSUM_COMPLETE, the program does not always update skb->csum for
the bytes it writes, and seg6_bpf_has_valid_srh() does not either. If the
packet is then delivered on the same node and skb->csum was computed in
software, for instance by conntrack, UDP can drop the packet as a checksum
error. With skb->csum computed by the device, TCP can instead log a
"hw csum failure", as if the device had computed a wrong skb->csum.

Discard CHECKSUM_COMPLETE with skb_checksum_complete_unset() after the
program, since input_action_end_bpf() does not know which bytes the program
changed. The checksum is then verified in software.

Found by manual code inspection and reproduced in a VM without kernel
changes or error injection: with skb->csum computed by a tc ct action, an
End.BPF program that writes the SRH Tag, followed by End.B6.Encaps and a
local End.DT6 SID, makes UDP drop the packet. With this patch, the packet
is delivered. Also tested against the lwt_seg6local BPF selftest, and with
a ConnectX-6 Lx NIC (mlx5) that provides CHECKSUM_COMPLETE.

Fixes: 004d4b274e2a ("ipv6: sr: Add seg6local action End.BPF")
Signed-off-by: Andrea Mayer <andrea.mayer@uniroma2.it>
---
 net/ipv6/seg6_local.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/net/ipv6/seg6_local.c b/net/ipv6/seg6_local.c
index 57a82477ce3f..3045a351f4e9 100644
--- a/net/ipv6/seg6_local.c
+++ b/net/ipv6/seg6_local.c
@@ -1582,6 +1582,12 @@ static int input_action_end_bpf(struct sk_buff *skb,
 		goto drop;
 	}
 
+	/* The program may have changed the packet and seg6_bpf_has_valid_srh()
+	 * may rewrite the SRH Hdr Ext Len below.
+	 * skb->csum may then no longer be the checksum of the full packet.
+	 */
+	skb_checksum_complete_unset(skb);
+
 	if (srh_state->srh && !seg6_bpf_has_valid_srh(skb))
 		goto drop;
 	local_unlock_nested_bh(&seg6_bpf_srh_states.bh_lock);
-- 
2.43.0