Subject: CVE-2021-47010: net: Only allow init netns to set default tcp cong to a restricted algo

From: Greg Kroah-Hartman <[email-masked]>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

net: Only allow init netns to set default tcp cong to a restricted algo

tcp_set_default_congestion_control() is netns-safe in that it writes
to &net->ipv4.tcp_congestion_control, but it also sets
ca->flags |= TCP_CONG_NON_RESTRICTED which is not namespaced.
This has the unintended side-effect of changing the global
net.ipv4.tcp_allowed_congestion_control sysctl, despite the fact that it
is read-only: 97684f0970f6 ("net: Make tcp_allowed_congestion_control
readonly in non-init netns")

Resolve this netns "leak" by only allowing the init netns to set the
default algorithm to one that is restricted. This restriction could be
removed if tcp_allowed_congestion_control were namespace-ified in the
future.

This bug was uncovered with
https://github.com/JonathonReinhart/linux-netns-sysctl-verify

The Linux kernel CVE team has assigned CVE-2021-47010 to this issue.

Affected and fixed versions
===========================

	Issue introduced in 4.15 with commit 6670e1524477 and fixed in 4.19.191 with commit 992de06308d9
	Issue introduced in 4.15 with commit 6670e1524477 and fixed in 5.4.119 with commit 9884f745108f
	Issue introduced in 4.15 with commit 6670e1524477 and fixed in 5.10.37 with commit 6c1ea8bee75d
	Issue introduced in 4.15 with commit 6670e1524477 and fixed in 5.11.21 with commit efe1532a6e1a
	Issue introduced in 4.15 with commit 6670e1524477 and fixed in 5.12.4 with commit e7d7bedd507b
	Issue introduced in 4.15 with commit 6670e1524477 and fixed in 5.13 with commit 8d432592f30f

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2021-47010
will be updated if fixes are backported, please check that for the most
up to date information about this issue.

Affected files
==============

The file(s) affected by this issue are:
	net/ipv4/tcp_cong.c

Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/992de06308d9a9584d59b96d294ac676f924e437
	https://git.kernel.org/stable/c/9884f745108f7d25b189bbcd6754e284fb29ab68
	https://git.kernel.org/stable/c/6c1ea8bee75df8fe2184a50fcd0f70bf82986f42
	https://git.kernel.org/stable/c/efe1532a6e1a8e3c343d04fff510f0ed80328f9c
	https://git.kernel.org/stable/c/e7d7bedd507bb732e600403b7a96f9fe48d0ca31
	https://git.kernel.org/stable/c/8d432592f30fcc34ef5a10aac4887b4897884493