[PATCH ipsec 7/7] xfrm: leave the sequence counter unchanged on ESN overflow

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

When the 64-bit ESN counter overflows, xfrm_replay_overflow_offload_esn()
rejects the packet and rolls back the stored counter. It decrements
replay_esn->oseq even though only the local oseq has advanced, which
can later induce a reuse of the last sequence number and the
corresponding AES-GCM nonce. Both IPv4 and IPv6 are affected, yet,
processing about 2^64 packets under a single key is required to
trigger this bug, which is highly unlikely in regular use cases.

Leave the stored low word unchanged on overflow. The high word still
needs to be restored because it was already incremented.

Fixes: d7dbefc45cf5 ("xfrm: Add xfrm_replay_overflow functions for offloading")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Jérémy Jean <Jeremy.Jean@oss.cyber.gouv.fr>
---
 net/xfrm/xfrm_replay.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/net/xfrm/xfrm_replay.c b/net/xfrm/xfrm_replay.c
index dbdf8a39dffe..12457d97c819 100644
--- a/net/xfrm/xfrm_replay.c
+++ b/net/xfrm/xfrm_replay.c
@@ -721,7 +721,6 @@ static int xfrm_replay_overflow_offload_esn(struct xfrm_state *x, struct sk_buff
 				xo->seq.hi = oseq_hi;
 			}
 			if (replay_esn->oseq_hi == 0) {
-				replay_esn->oseq--;
 				replay_esn->oseq_hi--;
 				xfrm_audit_state_replay_overflow(x, skb);
 				err = -EOVERFLOW;
-- 
2.47.3