[PATCH] xfrm: Duplicate SPI Handling – IPsec-v3 Compliance Concern

[Aakash Kumar S <saakashkumar@marvell.com>]

The issue originates when Strongswan initiates an XFRM_MSG_ALLOCSPI
Netlink message, which triggers the kernel function xfrm_alloc_spi().
This function is expected to ensure uniqueness of the Security Parameter
Index (SPI) for inbound Security Associations (SAs). However, it can
return success even when the requested SPI is already in use, leading
to duplicate SPIs assigned to multiple inbound SAs, differentiated
only by their destination addresses.

This behavior causes inconsistencies during SPI lookups for inbound packets.
Since the lookup may return an arbitrary SA among those with the same SPI,
packet processing can fail, resulting in packet drops.

According to RFC 6071, in IPsec-v3, a unicast SA is uniquely identified
by the SPI alone. Therefore, relying on additional fields
(such as destination addresses, proto) to disambiguate SPIs contradicts
the RFC and undermines protocol correctness.

Hence, the change is necessary to enforce strict SPI uniqueness for inbound SAs,
ensuring deterministic lookup behavior and compliance with the IPsec specification.

Signed-off-by: Aakash Kumar S <saakashkumar@marvell.com>
---
 net/xfrm/xfrm_hash.h | 15 ++++-----------
 1 file changed, 4 insertions(+), 11 deletions(-)

diff --git a/net/xfrm/xfrm_hash.h b/net/xfrm/xfrm_hash.h
index d12bb906c9c9..a71b6dbdf532 100644
--- a/net/xfrm/xfrm_hash.h
+++ b/net/xfrm/xfrm_hash.h
@@ -116,18 +116,11 @@ static inline unsigned int __xfrm_src_hash(const xfrm_address_t *daddr,
 }
 
 static inline unsigned int
-__xfrm_spi_hash(const xfrm_address_t *daddr, __be32 spi, u8 proto,
-		unsigned short family, unsigned int hmask)
+__xfrm_spi_hash(const xfrm_address_t * __maybe_unused daddr, __be32 spi,
+		u8 __maybe_unused proto, unsigned short __maybe_unused family,
+		unsigned int hmask)
 {
-	unsigned int h = (__force u32)spi ^ proto;
-	switch (family) {
-	case AF_INET:
-		h ^= __xfrm4_addr_hash(daddr);
-		break;
-	case AF_INET6:
-		h ^= __xfrm6_addr_hash(daddr);
-		break;
-	}
+	unsigned int h = (__force u32)spi;
 	return (h ^ (h >> 10) ^ (h >> 20)) & hmask;
 }
 
-- 
2.48.1