ID

VAR-201906-1174

CVE

CVE-2019-11479

TITLE

Multiple TCP Selective Acknowledgement (SACK) and Maximum Segment Size (MSS) networking vulnerabilities may cause denial-of-service conditions in Linux and FreeBSD kernels

DESCRIPTION

Jonathan Looney discovered that the Linux kernel default MSS is hard-coded to 48 bytes. This allows a remote peer to fragment TCP resend queues significantly more than if a larger MSS were enforced. A remote attacker could use this to cause a denial of service. This has been fixed in stable kernel releases 4.4.182, 4.9.182, 4.14.127, 4.19.52, 5.1.11, and is fixed in commits 967c05aee439e6e5d7d805e195b3a20ef5c433d6 and 5f3e2bf008c2221478101ee72f5cb4654b9fc363. Multiple TCP Selective Acknowledgement (SACK) and Maximum Segment Size (MSS) networking vulnerabilities may cause denial-of-service conditions in Linux and FreeBSD kernels. Linux Kernel Contains a resource exhaustion vulnerability.Service operation interruption (DoS) There is a possibility of being put into a state. Linux Kernel is prone to a denial-of-service vulnerability. This vulnerability stems from improper management of system resources (such as memory, disk space, files, etc.) by network systems or products. -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA256 ==================================================================== Red Hat Security Advisory Synopsis: Important: kernel security update Advisory ID: RHSA-2019:1481-01 Product: Red Hat Enterprise Linux Advisory URL: https://access.redhat.com/errata/RHSA-2019:1481 Issue date: 2019-06-17 CVE Names: CVE-2019-11477 CVE-2019-11478 CVE-2019-11479 ==================================================================== 1. Summary: An update for kernel is now available for Red Hat Enterprise Linux 7. Red Hat Product Security has rated this update as having a security impact of Important. A Common Vulnerability Scoring System (CVSS) base score, which gives a detailed severity rating, is available for each vulnerability from the CVE link(s) in the References section. 2. Relevant releases/architectures: Red Hat Enterprise Linux Client (v. 7) - noarch, x86_64 Red Hat Enterprise Linux Client Optional (v. 7) - x86_64 Red Hat Enterprise Linux ComputeNode (v. 7) - noarch, x86_64 Red Hat Enterprise Linux ComputeNode Optional (v. 7) - x86_64 Red Hat Enterprise Linux Server (v. 7) - noarch, ppc64, ppc64le, s390x, x86_64 Red Hat Enterprise Linux Server Optional (v. 7) - ppc64, ppc64le, x86_64 Red Hat Enterprise Linux Workstation (v. 7) - noarch, x86_64 Red Hat Enterprise Linux Workstation Optional (v. 7) - x86_64 3. While processing SACK segments, the Linux kernel's socket buffer (SKB) data structure becomes fragmented. To efficiently process SACK blocks, the Linux kernel merges multiple fragmented SKBs into one, potentially overflowing the variable holding the number of segments. A remote attacker could use this flaw to crash the Linux kernel by sending a crafted sequence of SACK segments on a TCP connection with small value of TCP MSS, resulting in a denial of service (DoS). (CVE-2019-11477) * Kernel: tcp: excessive resource consumption while processing SACK blocks allows remote denial of service (CVE-2019-11478) * Kernel: tcp: excessive resource consumption for TCP connections with low MSS allows remote denial of service (CVE-2019-11479) For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and other related information, refer to the CVE page(s) listed in the References section. 4. Solution: For details on how to apply this update, which includes the changes described in this advisory, refer to: https://access.redhat.com/articles/11258 The system must be rebooted for this update to take effect. 5. Bugs fixed (https://bugzilla.redhat.com/): 1719123 - CVE-2019-11477 Kernel: tcp: integer overflow while processing SACK blocks allows remote denial of service 1719128 - CVE-2019-11478 Kernel: tcp: excessive resource consumption while processing SACK blocks allows remote denial of service 1719129 - CVE-2019-11479 Kernel: tcp: excessive resource consumption for TCP connections with low MSS allows remote denial of service 6. Package List: Red Hat Enterprise Linux Client (v. 7): Source: kernel-3.10.0-957.21.3.el7.src.rpm noarch: kernel-abi-whitelists-3.10.0-957.21.3.el7.noarch.rpm kernel-doc-3.10.0-957.21.3.el7.noarch.rpm x86_64: bpftool-3.10.0-957.21.3.el7.x86_64.rpm kernel-3.10.0-957.21.3.el7.x86_64.rpm kernel-debug-3.10.0-957.21.3.el7.x86_64.rpm kernel-debug-debuginfo-3.10.0-957.21.3.el7.x86_64.rpm kernel-debug-devel-3.10.0-957.21.3.el7.x86_64.rpm kernel-debuginfo-3.10.0-957.21.3.el7.x86_64.rpm kernel-debuginfo-common-x86_64-3.10.0-957.21.3.el7.x86_64.rpm kernel-devel-3.10.0-957.21.3.el7.x86_64.rpm kernel-headers-3.10.0-957.21.3.el7.x86_64.rpm kernel-tools-3.10.0-957.21.3.el7.x86_64.rpm kernel-tools-debuginfo-3.10.0-957.21.3.el7.x86_64.rpm kernel-tools-libs-3.10.0-957.21.3.el7.x86_64.rpm perf-3.10.0-957.21.3.el7.x86_64.rpm perf-debuginfo-3.10.0-957.21.3.el7.x86_64.rpm python-perf-3.10.0-957.21.3.el7.x86_64.rpm python-perf-debuginfo-3.10.0-957.21.3.el7.x86_64.rpm Red Hat Enterprise Linux Client Optional (v. 7): x86_64: kernel-debug-debuginfo-3.10.0-957.21.3.el7.x86_64.rpm kernel-debuginfo-3.10.0-957.21.3.el7.x86_64.rpm kernel-debuginfo-common-x86_64-3.10.0-957.21.3.el7.x86_64.rpm kernel-tools-debuginfo-3.10.0-957.21.3.el7.x86_64.rpm kernel-tools-libs-devel-3.10.0-957.21.3.el7.x86_64.rpm perf-debuginfo-3.10.0-957.21.3.el7.x86_64.rpm python-perf-debuginfo-3.10.0-957.21.3.el7.x86_64.rpm Red Hat Enterprise Linux ComputeNode (v. 7): Source: kernel-3.10.0-957.21.3.el7.src.rpm noarch: kernel-abi-whitelists-3.10.0-957.21.3.el7.noarch.rpm kernel-doc-3.10.0-957.21.3.el7.noarch.rpm x86_64: bpftool-3.10.0-957.21.3.el7.x86_64.rpm kernel-3.10.0-957.21.3.el7.x86_64.rpm kernel-debug-3.10.0-957.21.3.el7.x86_64.rpm kernel-debug-debuginfo-3.10.0-957.21.3.el7.x86_64.rpm kernel-debug-devel-3.10.0-957.21.3.el7.x86_64.rpm kernel-debuginfo-3.10.0-957.21.3.el7.x86_64.rpm kernel-debuginfo-common-x86_64-3.10.0-957.21.3.el7.x86_64.rpm kernel-devel-3.10.0-957.21.3.el7.x86_64.rpm kernel-headers-3.10.0-957.21.3.el7.x86_64.rpm kernel-tools-3.10.0-957.21.3.el7.x86_64.rpm kernel-tools-debuginfo-3.10.0-957.21.3.el7.x86_64.rpm kernel-tools-libs-3.10.0-957.21.3.el7.x86_64.rpm perf-3.10.0-957.21.3.el7.x86_64.rpm perf-debuginfo-3.10.0-957.21.3.el7.x86_64.rpm python-perf-3.10.0-957.21.3.el7.x86_64.rpm python-perf-debuginfo-3.10.0-957.21.3.el7.x86_64.rpm Red Hat Enterprise Linux ComputeNode Optional (v. 7): x86_64: kernel-debug-debuginfo-3.10.0-957.21.3.el7.x86_64.rpm kernel-debuginfo-3.10.0-957.21.3.el7.x86_64.rpm kernel-debuginfo-common-x86_64-3.10.0-957.21.3.el7.x86_64.rpm kernel-tools-debuginfo-3.10.0-957.21.3.el7.x86_64.rpm kernel-tools-libs-devel-3.10.0-957.21.3.el7.x86_64.rpm perf-debuginfo-3.10.0-957.21.3.el7.x86_64.rpm python-perf-debuginfo-3.10.0-957.21.3.el7.x86_64.rpm Red Hat Enterprise Linux Server (v. 7): Source: kernel-3.10.0-957.21.3.el7.src.rpm noarch: kernel-abi-whitelists-3.10.0-957.21.3.el7.noarch.rpm kernel-doc-3.10.0-957.21.3.el7.noarch.rpm ppc64: kernel-3.10.0-957.21.3.el7.ppc64.rpm kernel-bootwrapper-3.10.0-957.21.3.el7.ppc64.rpm kernel-debug-3.10.0-957.21.3.el7.ppc64.rpm kernel-debug-debuginfo-3.10.0-957.21.3.el7.ppc64.rpm kernel-debug-devel-3.10.0-957.21.3.el7.ppc64.rpm kernel-debuginfo-3.10.0-957.21.3.el7.ppc64.rpm kernel-debuginfo-common-ppc64-3.10.0-957.21.3.el7.ppc64.rpm kernel-devel-3.10.0-957.21.3.el7.ppc64.rpm kernel-headers-3.10.0-957.21.3.el7.ppc64.rpm kernel-tools-3.10.0-957.21.3.el7.ppc64.rpm kernel-tools-debuginfo-3.10.0-957.21.3.el7.ppc64.rpm kernel-tools-libs-3.10.0-957.21.3.el7.ppc64.rpm perf-3.10.0-957.21.3.el7.ppc64.rpm perf-debuginfo-3.10.0-957.21.3.el7.ppc64.rpm python-perf-3.10.0-957.21.3.el7.ppc64.rpm python-perf-debuginfo-3.10.0-957.21.3.el7.ppc64.rpm ppc64le: kernel-3.10.0-957.21.3.el7.ppc64le.rpm kernel-bootwrapper-3.10.0-957.21.3.el7.ppc64le.rpm kernel-debug-3.10.0-957.21.3.el7.ppc64le.rpm kernel-debug-debuginfo-3.10.0-957.21.3.el7.ppc64le.rpm kernel-debuginfo-3.10.0-957.21.3.el7.ppc64le.rpm kernel-debuginfo-common-ppc64le-3.10.0-957.21.3.el7.ppc64le.rpm kernel-devel-3.10.0-957.21.3.el7.ppc64le.rpm kernel-headers-3.10.0-957.21.3.el7.ppc64le.rpm kernel-tools-3.10.0-957.21.3.el7.ppc64le.rpm kernel-tools-debuginfo-3.10.0-957.21.3.el7.ppc64le.rpm kernel-tools-libs-3.10.0-957.21.3.el7.ppc64le.rpm perf-3.10.0-957.21.3.el7.ppc64le.rpm perf-debuginfo-3.10.0-957.21.3.el7.ppc64le.rpm python-perf-3.10.0-957.21.3.el7.ppc64le.rpm python-perf-debuginfo-3.10.0-957.21.3.el7.ppc64le.rpm s390x: kernel-3.10.0-957.21.3.el7.s390x.rpm kernel-debug-3.10.0-957.21.3.el7.s390x.rpm kernel-debug-debuginfo-3.10.0-957.21.3.el7.s390x.rpm kernel-debug-devel-3.10.0-957.21.3.el7.s390x.rpm kernel-debuginfo-3.10.0-957.21.3.el7.s390x.rpm kernel-debuginfo-common-s390x-3.10.0-957.21.3.el7.s390x.rpm kernel-devel-3.10.0-957.21.3.el7.s390x.rpm kernel-headers-3.10.0-957.21.3.el7.s390x.rpm kernel-kdump-3.10.0-957.21.3.el7.s390x.rpm kernel-kdump-debuginfo-3.10.0-957.21.3.el7.s390x.rpm kernel-kdump-devel-3.10.0-957.21.3.el7.s390x.rpm perf-3.10.0-957.21.3.el7.s390x.rpm perf-debuginfo-3.10.0-957.21.3.el7.s390x.rpm python-perf-3.10.0-957.21.3.el7.s390x.rpm python-perf-debuginfo-3.10.0-957.21.3.el7.s390x.rpm x86_64: bpftool-3.10.0-957.21.3.el7.x86_64.rpm kernel-3.10.0-957.21.3.el7.x86_64.rpm kernel-debug-3.10.0-957.21.3.el7.x86_64.rpm kernel-debug-debuginfo-3.10.0-957.21.3.el7.x86_64.rpm kernel-debug-devel-3.10.0-957.21.3.el7.x86_64.rpm kernel-debuginfo-3.10.0-957.21.3.el7.x86_64.rpm kernel-debuginfo-common-x86_64-3.10.0-957.21.3.el7.x86_64.rpm kernel-devel-3.10.0-957.21.3.el7.x86_64.rpm kernel-headers-3.10.0-957.21.3.el7.x86_64.rpm kernel-tools-3.10.0-957.21.3.el7.x86_64.rpm kernel-tools-debuginfo-3.10.0-957.21.3.el7.x86_64.rpm kernel-tools-libs-3.10.0-957.21.3.el7.x86_64.rpm perf-3.10.0-957.21.3.el7.x86_64.rpm perf-debuginfo-3.10.0-957.21.3.el7.x86_64.rpm python-perf-3.10.0-957.21.3.el7.x86_64.rpm python-perf-debuginfo-3.10.0-957.21.3.el7.x86_64.rpm Red Hat Enterprise Linux Server Optional (v. 7): ppc64: kernel-debug-debuginfo-3.10.0-957.21.3.el7.ppc64.rpm kernel-debuginfo-3.10.0-957.21.3.el7.ppc64.rpm kernel-debuginfo-common-ppc64-3.10.0-957.21.3.el7.ppc64.rpm kernel-tools-debuginfo-3.10.0-957.21.3.el7.ppc64.rpm kernel-tools-libs-devel-3.10.0-957.21.3.el7.ppc64.rpm perf-debuginfo-3.10.0-957.21.3.el7.ppc64.rpm python-perf-debuginfo-3.10.0-957.21.3.el7.ppc64.rpm ppc64le: kernel-debug-debuginfo-3.10.0-957.21.3.el7.ppc64le.rpm kernel-debug-devel-3.10.0-957.21.3.el7.ppc64le.rpm kernel-debuginfo-3.10.0-957.21.3.el7.ppc64le.rpm kernel-debuginfo-common-ppc64le-3.10.0-957.21.3.el7.ppc64le.rpm kernel-tools-debuginfo-3.10.0-957.21.3.el7.ppc64le.rpm kernel-tools-libs-devel-3.10.0-957.21.3.el7.ppc64le.rpm perf-debuginfo-3.10.0-957.21.3.el7.ppc64le.rpm python-perf-debuginfo-3.10.0-957.21.3.el7.ppc64le.rpm x86_64: kernel-debug-debuginfo-3.10.0-957.21.3.el7.x86_64.rpm kernel-debuginfo-3.10.0-957.21.3.el7.x86_64.rpm kernel-debuginfo-common-x86_64-3.10.0-957.21.3.el7.x86_64.rpm kernel-tools-debuginfo-3.10.0-957.21.3.el7.x86_64.rpm kernel-tools-libs-devel-3.10.0-957.21.3.el7.x86_64.rpm perf-debuginfo-3.10.0-957.21.3.el7.x86_64.rpm python-perf-debuginfo-3.10.0-957.21.3.el7.x86_64.rpm Red Hat Enterprise Linux Workstation (v. 7): Source: kernel-3.10.0-957.21.3.el7.src.rpm noarch: kernel-abi-whitelists-3.10.0-957.21.3.el7.noarch.rpm kernel-doc-3.10.0-957.21.3.el7.noarch.rpm x86_64: bpftool-3.10.0-957.21.3.el7.x86_64.rpm kernel-3.10.0-957.21.3.el7.x86_64.rpm kernel-debug-3.10.0-957.21.3.el7.x86_64.rpm kernel-debug-debuginfo-3.10.0-957.21.3.el7.x86_64.rpm kernel-debug-devel-3.10.0-957.21.3.el7.x86_64.rpm kernel-debuginfo-3.10.0-957.21.3.el7.x86_64.rpm kernel-debuginfo-common-x86_64-3.10.0-957.21.3.el7.x86_64.rpm kernel-devel-3.10.0-957.21.3.el7.x86_64.rpm kernel-headers-3.10.0-957.21.3.el7.x86_64.rpm kernel-tools-3.10.0-957.21.3.el7.x86_64.rpm kernel-tools-debuginfo-3.10.0-957.21.3.el7.x86_64.rpm kernel-tools-libs-3.10.0-957.21.3.el7.x86_64.rpm perf-3.10.0-957.21.3.el7.x86_64.rpm perf-debuginfo-3.10.0-957.21.3.el7.x86_64.rpm python-perf-3.10.0-957.21.3.el7.x86_64.rpm python-perf-debuginfo-3.10.0-957.21.3.el7.x86_64.rpm Red Hat Enterprise Linux Workstation Optional (v. 7): x86_64: kernel-debug-debuginfo-3.10.0-957.21.3.el7.x86_64.rpm kernel-debuginfo-3.10.0-957.21.3.el7.x86_64.rpm kernel-debuginfo-common-x86_64-3.10.0-957.21.3.el7.x86_64.rpm kernel-tools-debuginfo-3.10.0-957.21.3.el7.x86_64.rpm kernel-tools-libs-devel-3.10.0-957.21.3.el7.x86_64.rpm perf-debuginfo-3.10.0-957.21.3.el7.x86_64.rpm python-perf-debuginfo-3.10.0-957.21.3.el7.x86_64.rpm These packages are GPG signed by Red Hat for security. Our key and details on how to verify the signature are available from https://access.redhat.com/security/team/key/ 7. References: https://access.redhat.com/security/cve/CVE-2019-11477 https://access.redhat.com/security/cve/CVE-2019-11478 https://access.redhat.com/security/cve/CVE-2019-11479 https://access.redhat.com/security/updates/classification/#important https://access.redhat.com/security/vulnerabilities/tcpsack 8. Contact: The Red Hat security contact is <secalert@redhat.com>. More contact details at https://access.redhat.com/security/team/contact/ Copyright 2019 Red Hat, Inc. -----BEGIN PGP SIGNATURE----- Version: GnuPG v1 iQIVAwUBXQfc5dzjgjWX9erEAQiHvA/9H8ok5idW5wQXIYe9k2vrUnDV0EilyBeN ybKZlww2bhuHKYwmUKD2WSbggNXvxOR0uzuLTEqM5R0od4ALa4SSxdNMTVO1ac8V 4nogSc7quxNGoYdhXyWPFqRTWIGMMV7mXRifTqGskcB1xB5JwMc2mGGvFA3xXGDu lxvNK70J+HjlTFf4wodaRCZdmyEQxYB8rOxQ42U+yFcbPmPqlyvLJV/6ULHU2utZ K/Ifmd5g9B0NbDR8hceVIdWFD6XIUbiyItn3Ubx0IHaBDVE9j+fo3FQ+ciPG73CD kl7nfHU0ebOrooxZQxFmbVltPUlVwyZQ6H/d1a31ZNdWDu/LQkG94bY7YARfDFLT xsJKMW+pQjAlFAbEVEBbesMXBwb6qdRgpugyr0o8cZbOYAEH2vkxbf1rcz1wAGgv dMRxnvUbgNML50jSuAh80vdzCjrkmhJxO2KSZXwEoLyDiiwfs4MILf20W+jn8tdG p6AGgFkENInQtAuAbcQHpjCmJPUNLoSKuzpQAry4RCNfklFElKhJxlHoxUPqvc42 SUN5BS7w8SF70ANWiW/V73+nYcDKJfV/3oMndJInVaCER6GF6NwpGQb7DjLM4krh gxhcCHeA0dHKVL8udjSrrEbdi4c3OmXz53p36Ww/Yh8k3BhVid7Wzt3HPBWFF5qs rjP0JW27B5U=rNYS -----END PGP SIGNATURE----- -- RHSA-announce mailing list RHSA-announce@redhat.com https://www.redhat.com/mailman/listinfo/rhsa-announce . Bug Fix(es): * update the MRG 2.5.z 3.10 kernel-rt sources (BZ#1711010) 4. Description: The kernel-rt packages provide the Real Time Linux Kernel, which enables fine-tuning for systems with extremely high determinism requirements. 7.2) - noarch, x86_64 3. Bug Fix(es): * THP: Race between MADV_DONTNEED and NUMA hinting node migration code (BZ#1698103) * [RHEL7] md_clear flag missing from /proc/cpuinfo on late microcode update (BZ#1712989) * [RHEL7] MDS mitigations are not enabled after double microcode update (BZ#1712994) * WARNING: CPU: 0 PID: 0 at kernel/jump_label.c:90 __static_key_slow_dec+0xa6/0xb0 (BZ#1713000) 4. 7) - aarch64, noarch, ppc64le 3. ========================================================================== Ubuntu Security Notice USN-4041-2 June 29, 2019 linux-lts-xenial, linux-aws, linux-azure update ========================================================================== A security issue affects these releases of Ubuntu and its derivatives: - Ubuntu 14.04 ESM Summary: Several security issues were fixed in the Linux kernel. Unfortunately, the update introduced a regression that interfered with networking applications that setup very low SO_SNDBUF values. This update fixes the problem. We apologize for the inconvenience. (CVE-2019-11479) Update instructions: The problem can be corrected by updating your system to the following package versions: Ubuntu 14.04 ESM: linux-image-4.15.0-1049-azure 4.15.0-1049.54~14.04.1 linux-image-4.4.0-1048-aws 4.4.0-1048.52 linux-image-4.4.0-154-generic 4.4.0-154.181~14.04.1 linux-image-4.4.0-154-generic-lpae 4.4.0-154.181~14.04.1 linux-image-4.4.0-154-lowlatency 4.4.0-154.181~14.04.1 linux-image-4.4.0-154-powerpc-e500mc 4.4.0-154.181~14.04.1 linux-image-4.4.0-154-powerpc-smp 4.4.0-154.181~14.04.1 linux-image-4.4.0-154-powerpc64-emb 4.4.0-154.181~14.04.1 linux-image-4.4.0-154-powerpc64-smp 4.4.0-154.181~14.04.1 linux-image-aws 4.4.0.1048.49 linux-image-azure 4.15.0.1049.36 linux-image-generic-lpae-lts-xenial 4.4.0.154.135 linux-image-generic-lts-xenial 4.4.0.154.135 linux-image-lowlatency-lts-xenial 4.4.0.154.135 linux-image-powerpc-e500mc-lts-xenial 4.4.0.154.135 linux-image-powerpc-smp-lts-xenial 4.4.0.154.135 linux-image-powerpc64-emb-lts-xenial 4.4.0.154.135 linux-image-powerpc64-smp-lts-xenial 4.4.0.154.135 linux-image-virtual-lts-xenial 4.4.0.154.135 After a standard system update you need to reboot your computer to make all the necessary changes. ATTENTION: Due to an unavoidable ABI change the kernel updates have been given a new version number, which requires you to recompile and reinstall all third party kernel modules you might have installed. Unless you manually uninstalled the standard kernel metapackages (e.g. linux-generic, linux-generic-lts-RELEASE, linux-virtual, linux-powerpc), a standard system upgrade will automatically perform this as well. The most serious, dubbed _“SACK Panic_,” allows a remotely-triggered kernel panic on recent Linux kernels. There are patches that address most of these vulnerabilities. If patches can not be applied, certain mitigations will be effective. We recommend that affected parties enact one of those described below, based on their environment. ### Details: #### 1: [CVE-2019-11477](https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2019-11477): SACK Panic (Linux >= 2.6.29) __Description:__ A sequence of SACKs may be crafted such that one can trigger an integer overflow, leading to a kernel panic. __Fix:__ Apply the patch [PATCH_net_1_4.patch](2019-001/PATCH_net_1_4.patch). Additionally, versions of the Linux kernel up to, and including, 4.14 require a second patch [PATCH_net_1a.patch](2019-001/PATCH_net_1a.patch). __Workaround #1:__ Block connections with a low MSS using one of the supplied [filters](2019-001/block-low-mss/README.md). (The values in the filters are examples. You can apply a higher or lower limit, as appropriate for your environment.) Note that these filters may break legitimate connections which rely on a low MSS. Also, note that this mitigation is only effective if TCP probing is disabled (that is, the `net.ipv4.tcp_mtu_probing` sysctl is set to 0, which appears to be the default value for that sysctl). __Workaround #2:__ Disable SACK processing (`/proc/sys/net/ipv4/tcp_sack` set to 0). (Note that either workaround should be sufficient on its own. It is not necessary to apply both workarounds.) ### 2: [CVE-2019-11478](https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2019-11478): SACK Slowness (Linux < 4.15) or Excess Resource Usage (all Linux versions) __Description:__ It is possible to send a crafted sequence of SACKs which will fragment the TCP retransmission queue. On Linux kernels prior to 4.15, an attacker may be able to further exploit the fragmented queue to cause an expensive linked-list walk for subsequent SACKs received for that same TCP connection. __Fix:__ Apply the patch [PATCH_net_2_4.patch](2019-001/PATCH_net_2_4.patch) __Workaround #1:__ Block connections with a low MSS using one of the supplied [filters](2019-001/block-low-mss/README.md). (The values in the filters are examples. You can apply a higher or lower limit, as appropriate for your environment.) Note that these filters may break legitimate connections which rely on a low MSS. Also, note that this mitigation is only effective if TCP probing is disabled (that is, the `net.ipv4.tcp_mtu_probing` sysctl is set to 0, which appears to be the default value for that sysctl). __Workaround #2:__ Disable SACK processing (`/proc/sys/net/ipv4/tcp_sack` set to 0). (Note that either workaround should be sufficient on its own. It is not necessary to apply both workarounds.) ### 3: [CVE-2019-5599](https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2019-5599): SACK Slowness (FreeBSD 12 using the RACK TCP Stack) __Description:__ It is possible to send a crafted sequence of SACKs which will fragment the RACK send map. An attacker may be able to further exploit the fragmented send map to cause an expensive linked-list walk for subsequent SACKs received for that same TCP connection. __Workaround #1:__ Apply the patch [split_limit.patch](2019-001/split_limit.patch) and set the `net.inet.tcp.rack.split_limit` sysctl to a reasonable value to limit the size of the SACK table. __Workaround #2:__ Temporarily disable the RACK TCP stack. (Note that either workaround should be sufficient on its own. It is not necessary to apply both workarounds.) ### 4: [CVE-2019-11479](https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2019-11479): Excess Resource Consumption Due to Low MSS Values (all Linux versions) __Description:__ An attacker can force the Linux kernel to segment its responses into multiple TCP segments, each of which contains only 8 bytes of data. This drastically increases the bandwidth required to deliver the same amount of data. Further, it consumes additional resources (CPU and NIC processing power). This attack requires continued effort from the attacker and the impacts will end shortly after the attacker stops sending traffic. __Fix:__ Two patches [PATCH_net_3_4.patch](2019-001/PATCH_net_3_4.patch) and [PATCH_net_4_4.patch](2019-001/PATCH_net_4_4.patch) add a sysctl which enforces a minimum MSS, set by the `net.ipv4.tcp_min_snd_mss` sysctl. This lets an administrator enforce a minimum MSS appropriate for their applications. __Workaround:__ Block connections with a low MSS using one of the supplied [filters](2019-001/block-low-mss/README.md). (The values in the filters are examples. You can apply a higher or lower limit, as appropriate for your environment.) Note that these filters may break legitimate connections which rely on a low MSS. Also, note that this mitigation is only effective if TCP probing is disabled (that is, the `net.ipv4.tcp_mtu_probing` sysctl is set to 0, which appears to be the default value for that sysctl). ### Note: Good system and application coding and configuration practices (limiting write buffers to the necessary level, monitoring connection memory consumption via SO_MEMINFO, and aggressively closing misbehaving connections) can help to limit the impact of attacks against these kinds of vulnerabilities. ## Acknowledgments: Originally reported by Jonathan Looney. We thank Eric Dumazet for providing Linux fixes and support. We thank Bruce Curtis for providing the Linux filters. We thank Jonathan Lemon and Alexey Kodanev for helping to improve the Linux patches. We gratefully acknowledge the assistance of Tyler Hicks in testing fixes, refining the information about vulnerable versions, and providing assistance during the disclosure process. Bug Fix(es): * MDS mitigations not enabled on Intel Skylake CPUs (BZ#1713026) * [RHEL6] md_clear flag missing from /proc/cpuinfo (BZ#1713029) * RHEL6 kernel does not disable SMT with mds=full,nosmt (BZ#1713044) 4

AFFECTED PRODUCTS