Subject: CVE-2021-47593: mptcp: clear 'kern' flag from fallback sockets

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

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

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

mptcp: clear 'kern' flag from fallback sockets

The mptcp ULP extension relies on sk->sk_sock_kern being set correctly:
It prevents setsockopt(fd, IPPROTO_TCP, TCP_ULP, "mptcp", 6); from
working for plain tcp sockets (any userspace-exposed socket).

But in case of fallback, accept() can return a plain tcp sk.
In such case, sk is still tagged as 'kernel' and setsockopt will work.

This will crash the kernel, The subflow extension has a NULL ctx->conn
mptcp socket:

BUG: KASAN: null-ptr-deref in subflow_data_ready+0x181/0x2b0
Call Trace:
 tcp_data_ready+0xf8/0x370
 [..]

The Linux kernel CVE team has assigned CVE-2021-47593 to this issue.

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

	Issue introduced in 5.6 with commit cf7da0d66cc1 and fixed in 5.10.88 with commit 451f1eded7f5
	Issue introduced in 5.6 with commit cf7da0d66cc1 and fixed in 5.15.11 with commit c26ac0ea3a91
	Issue introduced in 5.6 with commit cf7da0d66cc1 and fixed in 5.16 with commit d6692b3b97bd

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-2021-47593
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/mptcp/protocol.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/451f1eded7f56e93aaf52eb547ba97742d9c0e97
	https://git.kernel.org/stable/c/c26ac0ea3a91c210cf90452e625dc441adf3e549
	https://git.kernel.org/stable/c/d6692b3b97bdc165d150f4c1505751a323a80717