ID

VAR-201906-1176

CVE

CVE-2019-11478

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 TCP retransmission queue implementation in tcp_fragment in the Linux kernel could be fragmented when handling certain TCP Selective Acknowledgment (SACK) sequences. 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 commit f070ef2ac66716357066b683fb0baf55f8191a2e. 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. Bug Fix(es): * Mistmach between 'tty->termios->c_lflag' and 'ldata->icanon' for 'ICANON' (BZ#1708061) * RHEL7: rwsem reader/writer mutual exclusion guarantee may not work (BZ#1709702) * hardened usercopy is causing crash (BZ#1712311) * [RHEL7] md_clear flag missing from /proc/cpuinfo on late microcode update (BZ#1712991) * [RHEL7] MDS mitigations are not enabled after double microcode update (BZ#1712996) * WARNING: CPU: 0 PID: 0 at kernel/jump_label.c:90 __static_key_slow_dec+0xa6/0xb0 (BZ#1713002) * [debug kernel] [x86_64]INFO: possible circular locking dependency detected (BZ#1715326) * RHEL-7.7: tty: termios_rwsem possible deadlock (BZ#1715329) Enhancement(s): * [MCHP 7.7 FEAT] Update smartpqi driver to latest upstream (BZ#1709467) 4. ========================================================================== Kernel Live Patch Security Notice 0055-1 September 06, 2019 linux vulnerability ========================================================================== A security issue affects these releases of Ubuntu: | Series | Base kernel | Arch | flavors | |------------------+--------------+----------+------------------| | Ubuntu 18.04 LTS | 4.15.0 | amd64 | generic | | Ubuntu 18.04 LTS | 4.15.0 | amd64 | lowlatency | | Ubuntu 16.04 LTS | 4.4.0 | amd64 | generic | | Ubuntu 16.04 LTS | 4.4.0 | amd64 | lowlatency | | Ubuntu 16.04 LTS | 4.15.0 | amd64 | generic | | Ubuntu 16.04 LTS | 4.15.0 | amd64 | lowlatency | Summary: Several security issues were fixed in the kernel. (CVE-2018-20856) It was discovered that the Bluetooth UART implementation in the Linux kernel did not properly check for missing tty operations. (CVE-2019-13233) It was discovered that the floppy driver in the Linux kernel did not properly validate meta data, leading to a buffer overread. (CVE-2019-14283) It was discovered that the floppy driver in the Linux kernel did not properly validate ioctl() calls, leading to a division-by-zero. (CVE-2019-14284) Update instructions: The problem can be corrected by updating your livepatches to the following versions: | Kernel | Version | flavors | |--------------------------+----------+--------------------------| | 4.4.0-148.174 | 55.1 | lowlatency, generic | | 4.4.0-150.176 | 55.1 | generic, lowlatency | | 4.4.0-151.178 | 55.1 | lowlatency, generic | | 4.4.0-154.181 | 55.1 | lowlatency, generic | | 4.4.0-157.185 | 55.1 | lowlatency | | 4.4.0-159.187 | 55.1 | lowlatency, generic | | 4.15.0-50.54 | 55.1 | generic, lowlatency | | 4.15.0-50.54~16.04.1 | 55.1 | generic, lowlatency | | 4.15.0-51.55 | 55.1 | generic, lowlatency | | 4.15.0-51.55~16.04.1 | 55.2 | generic, lowlatency | | 4.15.0-52.56 | 55.1 | lowlatency, generic | | 4.15.0-52.56~16.04.1 | 55.1 | generic, lowlatency | | 4.15.0-54.58 | 55.1 | generic, lowlatency | | 4.15.0-54.58~16.04.1 | 55.1 | generic, lowlatency | | 4.15.0-55.60 | 55.1 | generic, lowlatency | | 4.15.0-58.64 | 55.1 | generic, lowlatency | | 4.15.0-58.64~16.04.1 | 55.1 | lowlatency, generic | Support Information: Kernels older than the levels listed below do not receive livepatch updates. Please upgrade your kernel as soon as possible. | Series | Version | Flavors | |------------------+------------------+--------------------------| | Ubuntu 18.04 LTS | 4.15.0-50 | generic lowlatency | | Ubuntu 16.04 LTS | 4.15.0-50 | generic lowlatency | | Ubuntu 16.04 LTS | 4.4.0-148 | generic lowlatency | | Ubuntu 14.04 LTS | 4.4.0-148 | generic lowlatency | References: CVE-2018-20856, CVE-2019-10207, CVE-2019-11478, CVE-2019-13233, CVE-2019-14283, CVE-2019-14284 -- ubuntu-security-announce mailing list ubuntu-security-announce@lists.ubuntu.com Modify settings or unsubscribe at: https://lists.ubuntu.com/mailman/listinfo/ubuntu-security-announce . Description: The kernel-rt packages provide the Real Time Linux Kernel, which enables fine-tuning for systems with extremely high determinism requirements. Bug Fix(es): * kernel-rt: update to the RHEL8.0.z batch#1 source tree (BZ#1704955) 4. These packages include redhat-release-virtualization-host, ovirt-node, and rhev-hypervisor. RHVH features a Cockpit user interface for monitoring the host's resources and performing administrative tasks. The following packages have been upgraded to a later upstream version: redhat-release-virtualization-host (4.3.4), redhat-virtualization-host (4.3.4). -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA256 ==================================================================== Red Hat Security Advisory Synopsis: Important: kernel security update Advisory ID: RHSA-2019:1482-01 Product: Red Hat Enterprise Linux Advisory URL: https://access.redhat.com/errata/RHSA-2019:1482 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 7.5 Extended Update Support. 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 ComputeNode EUS (v. 7.5) - noarch, x86_64 Red Hat Enterprise Linux ComputeNode Optional EUS (v. 7.5) - x86_64 Red Hat Enterprise Linux Server EUS (v. 7.5) - noarch, ppc64, ppc64le, s390x, x86_64 Red Hat Enterprise Linux Server Optional EUS (v. 7.5) - ppc64, ppc64le, 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 ComputeNode EUS (v. 7.5): Source: kernel-3.10.0-862.34.2.el7.src.rpm noarch: kernel-abi-whitelists-3.10.0-862.34.2.el7.noarch.rpm kernel-doc-3.10.0-862.34.2.el7.noarch.rpm x86_64: kernel-3.10.0-862.34.2.el7.x86_64.rpm kernel-debug-3.10.0-862.34.2.el7.x86_64.rpm kernel-debug-debuginfo-3.10.0-862.34.2.el7.x86_64.rpm kernel-debug-devel-3.10.0-862.34.2.el7.x86_64.rpm kernel-debuginfo-3.10.0-862.34.2.el7.x86_64.rpm kernel-debuginfo-common-x86_64-3.10.0-862.34.2.el7.x86_64.rpm kernel-devel-3.10.0-862.34.2.el7.x86_64.rpm kernel-headers-3.10.0-862.34.2.el7.x86_64.rpm kernel-tools-3.10.0-862.34.2.el7.x86_64.rpm kernel-tools-debuginfo-3.10.0-862.34.2.el7.x86_64.rpm kernel-tools-libs-3.10.0-862.34.2.el7.x86_64.rpm perf-3.10.0-862.34.2.el7.x86_64.rpm perf-debuginfo-3.10.0-862.34.2.el7.x86_64.rpm python-perf-3.10.0-862.34.2.el7.x86_64.rpm python-perf-debuginfo-3.10.0-862.34.2.el7.x86_64.rpm Red Hat Enterprise Linux ComputeNode Optional EUS (v. 7.5): x86_64: kernel-debug-debuginfo-3.10.0-862.34.2.el7.x86_64.rpm kernel-debuginfo-3.10.0-862.34.2.el7.x86_64.rpm kernel-debuginfo-common-x86_64-3.10.0-862.34.2.el7.x86_64.rpm kernel-tools-debuginfo-3.10.0-862.34.2.el7.x86_64.rpm kernel-tools-libs-devel-3.10.0-862.34.2.el7.x86_64.rpm perf-debuginfo-3.10.0-862.34.2.el7.x86_64.rpm python-perf-debuginfo-3.10.0-862.34.2.el7.x86_64.rpm Red Hat Enterprise Linux Server EUS (v. 7.5): Source: kernel-3.10.0-862.34.2.el7.src.rpm noarch: kernel-abi-whitelists-3.10.0-862.34.2.el7.noarch.rpm kernel-doc-3.10.0-862.34.2.el7.noarch.rpm ppc64: kernel-3.10.0-862.34.2.el7.ppc64.rpm kernel-bootwrapper-3.10.0-862.34.2.el7.ppc64.rpm kernel-debug-3.10.0-862.34.2.el7.ppc64.rpm kernel-debug-debuginfo-3.10.0-862.34.2.el7.ppc64.rpm kernel-debug-devel-3.10.0-862.34.2.el7.ppc64.rpm kernel-debuginfo-3.10.0-862.34.2.el7.ppc64.rpm kernel-debuginfo-common-ppc64-3.10.0-862.34.2.el7.ppc64.rpm kernel-devel-3.10.0-862.34.2.el7.ppc64.rpm kernel-headers-3.10.0-862.34.2.el7.ppc64.rpm kernel-tools-3.10.0-862.34.2.el7.ppc64.rpm kernel-tools-debuginfo-3.10.0-862.34.2.el7.ppc64.rpm kernel-tools-libs-3.10.0-862.34.2.el7.ppc64.rpm perf-3.10.0-862.34.2.el7.ppc64.rpm perf-debuginfo-3.10.0-862.34.2.el7.ppc64.rpm python-perf-3.10.0-862.34.2.el7.ppc64.rpm python-perf-debuginfo-3.10.0-862.34.2.el7.ppc64.rpm ppc64le: kernel-3.10.0-862.34.2.el7.ppc64le.rpm kernel-bootwrapper-3.10.0-862.34.2.el7.ppc64le.rpm kernel-debug-3.10.0-862.34.2.el7.ppc64le.rpm kernel-debug-debuginfo-3.10.0-862.34.2.el7.ppc64le.rpm kernel-debuginfo-3.10.0-862.34.2.el7.ppc64le.rpm kernel-debuginfo-common-ppc64le-3.10.0-862.34.2.el7.ppc64le.rpm kernel-devel-3.10.0-862.34.2.el7.ppc64le.rpm kernel-headers-3.10.0-862.34.2.el7.ppc64le.rpm kernel-tools-3.10.0-862.34.2.el7.ppc64le.rpm kernel-tools-debuginfo-3.10.0-862.34.2.el7.ppc64le.rpm kernel-tools-libs-3.10.0-862.34.2.el7.ppc64le.rpm perf-3.10.0-862.34.2.el7.ppc64le.rpm perf-debuginfo-3.10.0-862.34.2.el7.ppc64le.rpm python-perf-3.10.0-862.34.2.el7.ppc64le.rpm python-perf-debuginfo-3.10.0-862.34.2.el7.ppc64le.rpm s390x: kernel-3.10.0-862.34.2.el7.s390x.rpm kernel-debug-3.10.0-862.34.2.el7.s390x.rpm kernel-debug-debuginfo-3.10.0-862.34.2.el7.s390x.rpm kernel-debug-devel-3.10.0-862.34.2.el7.s390x.rpm kernel-debuginfo-3.10.0-862.34.2.el7.s390x.rpm kernel-debuginfo-common-s390x-3.10.0-862.34.2.el7.s390x.rpm kernel-devel-3.10.0-862.34.2.el7.s390x.rpm kernel-headers-3.10.0-862.34.2.el7.s390x.rpm kernel-kdump-3.10.0-862.34.2.el7.s390x.rpm kernel-kdump-debuginfo-3.10.0-862.34.2.el7.s390x.rpm kernel-kdump-devel-3.10.0-862.34.2.el7.s390x.rpm perf-3.10.0-862.34.2.el7.s390x.rpm perf-debuginfo-3.10.0-862.34.2.el7.s390x.rpm python-perf-3.10.0-862.34.2.el7.s390x.rpm python-perf-debuginfo-3.10.0-862.34.2.el7.s390x.rpm x86_64: kernel-3.10.0-862.34.2.el7.x86_64.rpm kernel-debug-3.10.0-862.34.2.el7.x86_64.rpm kernel-debug-debuginfo-3.10.0-862.34.2.el7.x86_64.rpm kernel-debug-devel-3.10.0-862.34.2.el7.x86_64.rpm kernel-debuginfo-3.10.0-862.34.2.el7.x86_64.rpm kernel-debuginfo-common-x86_64-3.10.0-862.34.2.el7.x86_64.rpm kernel-devel-3.10.0-862.34.2.el7.x86_64.rpm kernel-headers-3.10.0-862.34.2.el7.x86_64.rpm kernel-tools-3.10.0-862.34.2.el7.x86_64.rpm kernel-tools-debuginfo-3.10.0-862.34.2.el7.x86_64.rpm kernel-tools-libs-3.10.0-862.34.2.el7.x86_64.rpm perf-3.10.0-862.34.2.el7.x86_64.rpm perf-debuginfo-3.10.0-862.34.2.el7.x86_64.rpm python-perf-3.10.0-862.34.2.el7.x86_64.rpm python-perf-debuginfo-3.10.0-862.34.2.el7.x86_64.rpm Red Hat Enterprise Linux Server Optional EUS (v. 7.5): ppc64: kernel-debug-debuginfo-3.10.0-862.34.2.el7.ppc64.rpm kernel-debuginfo-3.10.0-862.34.2.el7.ppc64.rpm kernel-debuginfo-common-ppc64-3.10.0-862.34.2.el7.ppc64.rpm kernel-tools-debuginfo-3.10.0-862.34.2.el7.ppc64.rpm kernel-tools-libs-devel-3.10.0-862.34.2.el7.ppc64.rpm perf-debuginfo-3.10.0-862.34.2.el7.ppc64.rpm python-perf-debuginfo-3.10.0-862.34.2.el7.ppc64.rpm ppc64le: kernel-debug-debuginfo-3.10.0-862.34.2.el7.ppc64le.rpm kernel-debug-devel-3.10.0-862.34.2.el7.ppc64le.rpm kernel-debuginfo-3.10.0-862.34.2.el7.ppc64le.rpm kernel-debuginfo-common-ppc64le-3.10.0-862.34.2.el7.ppc64le.rpm kernel-tools-debuginfo-3.10.0-862.34.2.el7.ppc64le.rpm kernel-tools-libs-devel-3.10.0-862.34.2.el7.ppc64le.rpm perf-debuginfo-3.10.0-862.34.2.el7.ppc64le.rpm python-perf-debuginfo-3.10.0-862.34.2.el7.ppc64le.rpm x86_64: kernel-debug-debuginfo-3.10.0-862.34.2.el7.x86_64.rpm kernel-debuginfo-3.10.0-862.34.2.el7.x86_64.rpm kernel-debuginfo-common-x86_64-3.10.0-862.34.2.el7.x86_64.rpm kernel-tools-debuginfo-3.10.0-862.34.2.el7.x86_64.rpm kernel-tools-libs-devel-3.10.0-862.34.2.el7.x86_64.rpm perf-debuginfo-3.10.0-862.34.2.el7.x86_64.rpm python-perf-debuginfo-3.10.0-862.34.2.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 iQIVAwUBXQfk3NzjgjWX9erEAQiytA/+LYuWZPA+XAzTGUtoSanvsTMhe3fdVmEs uC08ljrlPeR/Wd6nDM6/kA31tXy0DuL23ocXTXoz+eBQNzpv7wAV3BNegLB+EbaF lalFLRhneQHOPqVQm8NRM/+aQoUSPZGroINcBCxC+RRDCnzgd1IYYOPJeq3N3che 3/o1whqn8af/uYrE49Lg+qimRFPIDkUg/r+pTv8M/cWdskrDlv3ir8Dn3rczrg9C /MjRxlqayPLN60RsdQO1MJPIq/5/4avHbmlqYGvzWyqdMYZckJWtJbRcCNg4MaII P8sx7pW1h6MdC23tGdqdk1VsgPl5ap1qYDjIJdHkp2wOeih6Lk1ezBViRuOfWpx1 Wy2EpmGRy8IUA/biIHedki48cfBHE2T3Bd7dE0V1HHWP5c9iZIov5OTG6dvy5OMV bQA5+BVrSdrOPKj6VDwrbcDWoeSR8O0eMghoj7o3aP6cPzAMmQ2xxj8G4hrZuP0P 1eVZrMPgE+N6Brd9k/o70Gw90Q7N2o8ONJFrz+gFdIGxI+m3dNGhne5hwxou1Q+k 5/O4S3Eu2afic3WNh6K3Zbk8K5rzfqFGdOFEGYw0/lZblRgr9scVQdfo4ohoxabf ojkr1xsbNIcSxHYBm9LRM4cMXblFr+yx+mYZsCA539F2DKID42Y89msdcjGhOUEL SkhJ47gvUQ4=+jJe -----END PGP SIGNATURE----- -- RHSA-announce mailing list RHSA-announce@redhat.com https://www.redhat.com/mailman/listinfo/rhsa-announce . 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. 6.6) - x86_64 3. 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