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Thu, 20 Aug 2026 21:42:40 -0700 (PDT) Received: from localhost.localdomain ([2603:8001:5f01:8bab:b111:8435:d2e3:35de]) by smtp.gmail.com with ESMTPSA id 5a478bee46e88-327bef77fc0sm40321253eec.8.2026.08.20.21.42.39 (version=TLS1_3 cipher=TLS_CHACHA20_POLY1305_SHA256 bits=256/256); Thu, 20 Aug 2026 21:42:40 -0700 (PDT) From: Artem Dinaburg To: stable@vger.kernel.org Cc: Xiang Mei , Steffen Klassert , Herbert Xu , linux-kernel@vger.kernel.org, AutonomousCodeSecurity@microsoft.com, Artem Dinaburg Subject: [PATCH 6.1.y] xfrm: fix sk_dst_cache double-free in xfrm_user_policy() Date: Fri, 21 Aug 2026 00:42:00 -0400 Message-ID: <20260821044210.10076-1-artem@trailofbits.com> X-Mailer: git-send-email 2.55.0 Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Xiang Mei (Microsoft) [ Upstream commit c283e9ada7fcb7dd4b10592623086b2e6d2f9925 ] 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. Fixes: 2b06cdf3e688 ("xfrm: Clear sk_dst_cache when applying per-socket policy.") Fixes: be8f8284cd89 ("net: xfrm: allow clearing socket xfrm policies.") Reported-by: AutonomousCodeSecurity@microsoft.com Signed-off-by: Xiang Mei (Microsoft) Signed-off-by: Steffen Klassert Assisted-by: Codex:GPT-5 Signed-off-by: Artem Dinaburg --- Please queue this unchanged upstream fix for CVE-2026-64581 in 6.1.y. An unprivileged namespace user can race UDP transmit with per-socket XFRM policy replacement and double-release the cached destination. Reproduced immediately on KASAN v6.1.182; the patched target completed 50,000 rounds. The fix is in 7.1.6 but absent from 6.1.y. net/xfrm/xfrm_state.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/net/xfrm/xfrm_state.c b/net/xfrm/xfrm_state.c index 6f70aeaafe14..4b0835cb58f5 100644 --- a/net/xfrm/xfrm_state.c +++ b/net/xfrm/xfrm_state.c @@ -2428,7 +2428,7 @@ int xfrm_user_policy(struct sock *sk, int optname, sockptr_t optval, int optlen) if (sockptr_is_null(optval) && !optlen) { xfrm_sk_policy_insert(sk, XFRM_POLICY_IN, NULL); xfrm_sk_policy_insert(sk, XFRM_POLICY_OUT, NULL); - __sk_dst_reset(sk); + sk_dst_reset(sk); return 0; } @@ -2468,7 +2468,7 @@ int xfrm_user_policy(struct sock *sk, int optname, sockptr_t optval, int optlen) if (err >= 0) { xfrm_sk_policy_insert(sk, err, pol); xfrm_pol_put(pol); - __sk_dst_reset(sk); + sk_dst_reset(sk); err = 0; } -- 2.39.5