[PATCH ipsec 1/7] xfrm: esp6: use the current sequence number for the IV

["Jérémy Jean" <Jeremy.Jean@oss.cyber.gouv.fr>]

When a GSO packet is split in software for IPv6 ESP,
validate_xmit_xfrm() calls esp6_xmit() for each segment. These
segments have XFRM_GSO_SEGMENT set, but after the split, skb_is_gso()
returns false for each skb, so each call increments xo->seq.low by one
after writing the ESP sequence number.

When encryption is done in software, the AES-GCM IV is generated from
the incremented value: if the last segment has sequence number 100, it
uses 101 to generate its IV. A bug arises when the next ordinary
packet is processed: it also goes through esp6_xmit() with sequence
number 101, but since it does not have XFRM_GSO_SEGMENT, no increment
happens and the same IV is reused.

Save the full sequence number before incrementing xo->seq, and use
that value to generate the IV.

Fixes: 3dca3f38cfb8 ("xfrm: Separate ESP handling from segmentation for GRO packets.")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Jérémy Jean <Jeremy.Jean@oss.cyber.gouv.fr>
---
 net/ipv6/esp6_offload.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/net/ipv6/esp6_offload.c b/net/ipv6/esp6_offload.c
index 22895521a57d..05d13cce22e4 100644
--- a/net/ipv6/esp6_offload.c
+++ b/net/ipv6/esp6_offload.c
@@ -346,6 +346,7 @@ static int esp6_xmit(struct xfrm_state *x, struct sk_buff *skb,  netdev_features
 	}
 
 	seq = xo->seq.low;
+	esp.seqno = cpu_to_be64(seq + ((u64)xo->seq.hi << 32));
 
 	esp.esph = ip_esp_hdr(skb);
 	esp.esph->spi = x->id.spi;
@@ -364,8 +365,6 @@ static int esp6_xmit(struct xfrm_state *x, struct sk_buff *skb,  netdev_features
 	if (xo->seq.low < seq)
 		xo->seq.hi++;
 
-	esp.seqno = cpu_to_be64(xo->seq.low + ((u64)xo->seq.hi << 32));
-
 	len = skb->len - sizeof(struct ipv6hdr);
 	if (len > IPV6_MAXPLEN)
 		len = 0;
-- 
2.47.3