From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1792706696; b=xvZMX2jaKkipyZduTQhbuaRsRLI4yitH0Wo60k2Z1b191nxkuU2zwU1/BLRVvoxI8268 laxGD95zcLxJfmAT/MlX8CKqv0Sm6pS0MAaTWN6sgyN6fycRrnsoTaYn4cEFUTbzDDRzl uxvUCaJq9nHG6PuRnSyVZ0x04KIID1tv/uyruWfgwiUnLlMXTaJbwmrcI8d418EaN8uv+ lEILxPteKP2uZWgcKDrlal3y7Psm/5BezA5g0KD7EuI2V9N5mRxNAQSR7iLAwadGYbwzB 7oQ9Vm2B/3JDg25p+lHN/4vb9SzqotQu1Ko88icmt16R7WdvEFsImUgPtJbppY/s8nw== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1792706696; h=Received:DKIM-Signature:From:To:Cc:Subject:Date:Message-ID:X-Mailer: In-Reply-To:References:MIME-Version:Content-Transfer-Encoding; bh=V+pp4HsGF8MG7/4fXk97ZD5VQnE/u6EXSEWiageyILI=; b=q9bav7e0Rz/viLUrPAjQ08d9Gs0D+hZyABB/0P+1aJN/89OEW3Aay8cyzjW46+k4FAS5 osCODGUWM4SMUM58DsB437YDvllV49lLOcS1JQtSESznjxi0dBPosfDkm6gyMOIrYnOfx mR/LDXc8R/WX0Kkuq7RaVYeOBUoom+KZx1xSRGoQZrTKNuSR7POQo2iSpwy48pdeDWMvR YyTCk0UlOY0QFROjpvOfMWQLNq2wkYiCumC4TH3ghJsAXoQPkqx0qZAuiGPFFfptOtc9Y fkLkVlJDVcIm1cWpkECrjKaZFxYmtxbnqMdsH6Gg0mLDGGRFWr2FNvsqtZvUqnQOnZA== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=LdaFiZEd; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 01181606A319 for ; Thu, 22 Oct 2026 22:04:55 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=LdaFiZEd Received: by smtp.kernel.org (Postfix) with ESMTPSA id 0C4961F01825; Thu, 22 Oct 2026 22:04:54 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1792706695; bh=V+pp4HsGF8MG7/4fXk97ZD5VQnE/u6EXSEWiageyILI=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=LdaFiZEdA8e03VToXJ1wNjb6eYSwgeSGC6wZVp2vZuPU0JJxeCUfVTuGQRpQd4G39 +GDru3OKN/0v5e7CnLJExAXKt5+JYQ5QyYNVsckdmPcHB2vC324rbRaun8D8uBA7BI o4sJObP9wk/ZLV2J/8EK58MTCMDSxvXElM1iqOV5m8JcXhLovVYgGTtpIrTu0vebGf ap/mLRLQHeSNxQSLBL7sfKU6rwdRz5jExEYGJgKUyEvtzhIQoPzbSq61cNqBrh2mJr BjUCjpWz6gdHdgeaiTx65yd7FdHabLT40nMWFh1nYPkg0PKPQgvM5rmag4FyO2SEbf fkMoz19wPA7RQ== From: Sasha Levin To: stable@vger.kernel.org Cc: Ido Schimmel , Amit Klein , Noam Caspi , David Ahern , Jakub Kicinski , Sasha Levin Subject: [PATCH 5.15.y] ipv4: udp: Create exceptions before socket matching Date: Thu, 22 Oct 2026 18:04:53 -0400 Message-ID: <20261022220234.2082806-1-sashal@kernel.org> X-Mailer: git-send-email 2.53.0 In-Reply-To: <2026102125-annuity-hardcore-06cd@gregkh> References: <2026102125-annuity-hardcore-06cd@gregkh> X-Mailing-List: stable@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Ido Schimmel [ Upstream commit 4c3499f79f8c7e8561266bcc220a18538cec0458 ] Currently, when ICMP Fragmentation Needed and Redirect Message packets are locally delivered and quote a UDP packet, a FIB nexthop exception (FNHE) is only created if the kernel can match the UDP packet to an existing socket. This behavior allows off-path attackers to conduct a side-channel attack on the FNHE cache in order to discover the ephemeral port used by a connected UDP socket. Commit 6457378fe796 ("ipv4: use siphash instead of Jenkins in fnhe_hashfun()") and commit 67d6d681e15b ("ipv4: make exception cache less predictible") tried to mitigate such attacks by making it harder for attackers to discover hash collisions in the FNHE cache and by randomizing the number of exceptions a hash bucket can hold, respectively. Unfortunately, both of the mitigations can be bypassed. Instead, mitigate such attacks by always creating a FNHE, even before trying to find a matching socket. Do that by calling ipv4_update_pmtu() and ipv4_redirect(), the helpers used when the quoted packet did not originate from a socket. This means that guesses (right or wrong) from an off-path attacker will always result in a FNHE being created or updated in the cache that the attacker can observe. Pass an oif of 0, in a similar fashion to icmp_err(). This is also the oif used by the socket path for sockets that are not bound to a device. Note that this does not allow attackers to create FNHEs that they could not create before, as both helpers can already be reached with little to no validation. For example, by sending an ICMP error that quotes an ICMP Echo Reply or one that quotes a UDP source port that matches a wildcard socket. Also note that in the good case (matched socket) the above scheme comes at the cost of an extra route lookup, as the no socket helpers perform their own lookup before the one performed by ipv4_sk_update_pmtu() / ipv4_sk_redirect(). When the two resolve to different nexthops, it also results in two exceptions being created for the same destination IP. One in the FNHE cache of the nexthop resolved by the no socket helpers and another in the FNHE cache of the nexthop used by the socket. Fixes: 4895c771c7f0 ("ipv4: Add FIB nexthop exceptions.") Cc: stable@vger.kernel.org Reported-by: Amit Klein Reported-by: Noam Caspi Signed-off-by: Ido Schimmel Reviewed-by: David Ahern Link: https://patch.msgid.link/20260828192344.2596928-3-idosch@nvidia.com Signed-off-by: Jakub Kicinski [ sashal: inlined udp_err_update_exception() into the existing udp_err() wrapper ] Signed-off-by: Sasha Levin --- net/ipv4/udp.c | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c index 3ebd5765fb9f3..65780cf993eac 100644 --- a/net/ipv4/udp.c +++ b/net/ipv4/udp.c @@ -821,6 +821,15 @@ int __udp4_lib_err(struct sk_buff *skb, u32 info, struct udp_table *udptable) int udp_err(struct sk_buff *skb, u32 info) { + const int type = icmp_hdr(skb)->type; + const int code = icmp_hdr(skb)->code; + struct net *net = dev_net(skb->dev); + + if (type == ICMP_DEST_UNREACH && code == ICMP_FRAG_NEEDED) + ipv4_update_pmtu(skb, net, info, 0, IPPROTO_UDP); + else if (type == ICMP_REDIRECT) + ipv4_redirect(skb, net, 0, IPPROTO_UDP); + return __udp4_lib_err(skb, info, &udp_table); } -- 2.53.0