CVE-2026-64581 in Linux
Summary
by MITRE • 08/05/2026
In the Linux kernel, the following vulnerability has been resolved:
xfrm: fix sk_dst_cache double-free in xfrm_user_policy()
xfrm_user_policy() clears the socket dst cache with __sk_dst_reset(), i.e. the non-atomic __sk_dst_set(sk, NULL): it reads sk_dst_cache with rcu_dereference_protected(), stores NULL and dst_release()s the old dst. That is only safe if no other thread modifies sk_dst_cache concurrently.
For a connected UDP socket that does not hold: the transmit fast path (udp_sendmsg -> sk_dst_check -> sk_dst_reset) resets the cache locklessly with an atomic xchg(). A per-socket policy change racing a send can make both sides observe the same old dst and each dst_release() it, dropping the socket's single reference twice and freeing the xfrm_dst bundle while it is still referenced:
BUG: KASAN: slab-use-after-free in dst_release Write of size 4 at addr ffff88801897b6c0 by task exploit/155 Call Trace: ... dst_release (... ./include/linux/rcuref.h:109) xfrm_user_policy (./include/net/sock.h:2239 ./include/net/sock.h:2256 net/xfrm/xfrm_state.c:3053) do_ip_setsockopt (net/ipv4/ip_sockglue.c:1347) ip_setsockopt (net/ipv4/ip_sockglue.c:1417) do_sock_setsockopt (net/socket.c:2368) __sys_setsockopt (net/socket.c:2393) __x64_sys_setsockopt (net/socket.c:2396) do_syscall_64 (arch/x86/entry/syscall_64.c:94) entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)
Reachable by an unprivileged user via a user+network namespace.
Use the atomic sk_dst_reset() so the cache is cleared and released with a single xchg(): whichever side wins releases the dst once, the other sees NULL and does nothing. Behaviour is otherwise unchanged.
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Analysis
by VulDB Data Team • 08/05/2026
The vulnerability described affects the Linux kernel's IPsec framework, specifically within the xfrm_user_policy() function where a double-free condition can occur due to improper handling of the socket destination cache. This issue stems from a race condition between concurrent operations that modify the socket's destination cache, leading to potential memory corruption and system instability.
The technical flaw manifests when a connected UDP socket undergoes a policy change while simultaneously experiencing network transmission. The xfrm_user_policy() function employs __sk_dst_reset() which performs a non-atomic operation to clear the socket's destination cache by reading it with rcu_dereference_protected(), storing NULL, and then calling dst_release() on the old destination. This approach assumes exclusive access to the socket's destination cache, but concurrent operations can violate this assumption.
When a per-socket policy change occurs concurrently with a send operation, both sides may observe the same old destination entry. The transmit fast path for UDP sockets uses atomic xchg() operations to reset the cache locklessly, which ensures proper reference counting. However, the non-atomic approach in xfrm_user_policy() creates a scenario where each side independently releases the same destination object twice, causing a use-after-free error as evidenced by the KASAN report showing dst_release attempting to write to freed memory at address 0xffff88801897b6c0.
This vulnerability is classified as a double-free condition under CWE-415 and represents a memory safety issue in kernel space that can lead to arbitrary code execution or system crashes. The attack vector requires an unprivileged user with access to user and network namespaces, making it particularly concerning for containerized environments where such privileges might be available. The exploit path follows the sequence of setting socket options through ip_setsockopt which eventually calls xfrm_user_policy, creating the race condition between policy modification and packet transmission.
The fix implements atomic sk_dst_reset() operations throughout the codebase to ensure that cache clearing and release occur as a single atomic operation. This approach guarantees that whichever thread wins the race will release the destination object exactly once while the loser observes NULL and performs no further action. The solution maintains identical behavior while preventing the double-free scenario, aligning with defensive programming practices recommended in the ATT&CK framework for kernel-level security hardening. The fix ensures proper synchronization between concurrent socket operations and prevents the use-after-free condition that could lead to privilege escalation or denial of service attacks.
The vulnerability demonstrates how seemingly isolated kernel subsystems can interact in unexpected ways when concurrent access patterns are not properly synchronized, highlighting the importance of atomic operations and proper memory management in kernel space. This type of race condition represents a classic example of how improper locking mechanisms can lead to critical security flaws that allow unprivileged users to potentially compromise system integrity through carefully crafted network operations.