CVE-2025-39797: kernel: xfrm: Duplicate SPI Handling
Component: kernel
Severity: Moderate
CWE: CWE-703 (Improper Check or Handling of Exceptional Conditions)
CVSS v3: 5.3 (CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H)
Public date: 2025-09-12T00:00:00+00:00

In the Linux kernel, the following vulnerability has been resolved:

xfrm: Duplicate SPI Handling

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 4301 section 4.4.2 , for inbound processing a unicast SA
is uniquely identified by the SPI and optionally protocol.

Reproducing the Issue Reliably:
To consistently reproduce the problem, restrict the available SPI range in
charon.conf : spi_min = 0x10000000 spi_max = 0x10000002
This limits the system to only 2 usable SPI values.
Next, create more than 2 Child SA. each using unique pair of src/dst address.
As soon as the 3rd Child SA is initiated, it will be assigned a duplicate
SPI, since the SPI pool is already exhausted.
With a narrow SPI range, the issue is consistently reproducible.
With a broader/default range, it becomes rare and unpredictable.

Current implementation:
xfrm_spi_hash() lookup function computes hash using daddr, proto, and family.
So if two SAs have the same SPI but different destination addresses, then
they will:
a. Hash into different buckets
b. Be stored in different linked lists (byspi + h)
c. Not be seen in the same hlist_for_each_entry_rcu() iteration.
As a result, the lookup will result in NULL and kernel allows that Duplicate SPI

Proposed Change:
xfrm_state_lookup_spi_proto() does a truly global search - across all states,
regardless of hash bucket and matches SPI and proto.

Statement: The patch fixes a logic flaw in XFRM SPI allocation where xfrm_alloc_spi() could succeed even if the requested SPI was already in use on another inbound SA (different dst). That lets multiple inbound SAs share the same SPI+proto, so inbound lookups may hit the “wrong” SA or none at all, causing IPsec packet drops (DoS). It’s reliably reproducible by constraining the SPI range (e.g., 0x10000000–0x10000002) and creating more Child SAs than available SPIs. The main risk is remote DoS.
Triggering this issue requires interaction at the IKE/IPsec control plane — it can only be exercised when an IKE daemon (for example strongSwan, LibreSwan, racoon or OpenSwan) is present and issues XFRM (netlink) requests to create SAs.

Affected upstream versions: 2.6.12
Fixed upstream versions: 0, 6.12.43, 6.15.11, 6.16.2, 6.17, 6.6.103
Upstream repository: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
Upstream fix commit: 3d8090bb53424432fa788fe9a49e8ceca74f0544