Subject: CVE-2021-46912: net: Make tcp_allowed_congestion_control readonly in non-init netns

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

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

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

net: Make tcp_allowed_congestion_control readonly in non-init netns

Currently, tcp_allowed_congestion_control is global and writable;
writing to it in any net namespace will leak into all other net
namespaces.

tcp_available_congestion_control and tcp_allowed_congestion_control are
the only sysctls in ipv4_net_table (the per-netns sysctl table) with a
NULL data pointer; their handlers (proc_tcp_available_congestion_control
and proc_allowed_congestion_control) have no other way of referencing a
struct net. Thus, they operate globally.

Because ipv4_net_table does not use designated initializers, there is no
easy way to fix up this one "bad" table entry. However, the data pointer
updating logic shouldn't be applied to NULL pointers anyway, so we
instead force these entries to be read-only.

These sysctls used to exist in ipv4_table (init-net only), but they were
moved to the per-net ipv4_net_table, presumably without realizing that
tcp_allowed_congestion_control was writable and thus introduced a leak.

Because the intent of that commit was only to know (i.e. read) "which
congestion algorithms are available or allowed", this read-only solution
should be sufficient.

The logic added in recent commit
31c4d2f160eb: ("net: Ensure net namespace isolation of sysctls")
does not and cannot check for NULL data pointers, because
other table entries (e.g. /proc/sys/net/netfilter/nf_log/) have
.data=NULL but use other methods (.extra2) to access the struct net.

The Linux kernel CVE team has assigned CVE-2021-46912 to this issue.

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

	Issue introduced in 5.7 with commit 9cb8e048e5d9 and fixed in 5.10.32 with commit 35d7491e2f77
	Issue introduced in 5.7 with commit 9cb8e048e5d9 and fixed in 5.11.16 with commit 1ccdf1bed140
	Issue introduced in 5.7 with commit 9cb8e048e5d9 and fixed in 5.12 with commit 97684f0970f6

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-2021-46912
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/sysctl_net_ipv4.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/35d7491e2f77ce480097cabcaf93ed409e916e12
	https://git.kernel.org/stable/c/1ccdf1bed140820240e383ba0accc474ffc7f006
	https://git.kernel.org/stable/c/97684f0970f6e112926de631fdd98d9693c7e5c1