[<prev] [next>] [thread-next>] [day] [month] [year] [list]
Message-Id: <84db7fe5-446a-4445-96db-8445fd43395c@saasmail.netflix.com>
Date: Mon, 17 Jun 2019 10:33:38 -0700 (PDT)
From: Security Report <security-report@...smail.netflix.com>
To: Security Report <security-report@...smail.netflix.com>
Cc: security-report@...flix.com, oss-security@...ts.openwall.com
Subject: Linux and FreeBSD Kernel: Multiple TCP-based remote denial of
service issues
Netflix has identified several TCP networking vulnerabilities in FreeBSD
and Linux kernels.
The vulnerabilities specifically relate to the minimum segment size (MSS)
and TCP Selective Acknowledgement (SACK) capabilities. 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.
#1: 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 attached patch (“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”).
Workaround #1: Block connections with a low MSS using one of the attached
filters. (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: 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 attached patch (“PATCH_net_2_4.patch”)
Workaround #1: Block connections with a low MSS using one of the attached
filters. (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: 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 attached patch (“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: 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 attached patches (“PATCH_net_3_4.patch” and “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 attached
filters. (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.
An advisory has been published
at https://github.com/Netflix/security-bulletins/blob/master/advisories/third-party/2019-001.md
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.
Regards,
Netflix Information Security
Content of type "text/html" skipped
View attachment "PATCH_net_1_4.patch" of type "text/x-diff" (5885 bytes)
View attachment "PATCH_net_1a.patch" of type "text/x-diff" (2192 bytes)
Download attachment "block-low-mss.tgz" of type "application/x-gtar" (1682 bytes)
View attachment "PATCH_net_2_4.patch" of type "text/x-diff" (2739 bytes)
View attachment "split_limit.patch" of type "text/x-diff" (6530 bytes)
View attachment "PATCH_net_3_4.patch" of type "text/x-diff" (5118 bytes)
View attachment "PATCH_net_4_4.patch" of type "text/x-diff" (1374 bytes)
Powered by blists - more mailing lists
Please check out the
Open Source Software Security Wiki, which is counterpart to this
mailing list.
Confused about mailing lists and their use?
Read about mailing lists on Wikipedia
and check out these
guidelines on proper formatting of your messages.
