Subject: CVE-2021-47217: x86/hyperv: Fix NULL deref in set_hv_tscchange_cb() if Hyper-V setup fails

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

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

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

x86/hyperv: Fix NULL deref in set_hv_tscchange_cb() if Hyper-V setup fails

Check for a valid hv_vp_index array prior to derefencing hv_vp_index when
setting Hyper-V's TSC change callback.  If Hyper-V setup failed in
hyperv_init(), the kernel will still report that it's running under
Hyper-V, but will have silently disabled nearly all functionality.

  BUG: kernel NULL pointer dereference, address: 0000000000000010
  #PF: supervisor read access in kernel mode
  #PF: error_code(0x0000) - not-present page
  PGD 0 P4D 0
  Oops: 0000 [#1] SMP
  CPU: 4 PID: 1 Comm: swapper/0 Not tainted 5.15.0-rc2+ #75
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
  RIP: 0010:set_hv_tscchange_cb+0x15/0xa0
  Code: <8b> 04 82 8b 15 12 17 85 01 48 c1 e0 20 48 0d ee 00 01 00 f6 c6 08
  ...
  Call Trace:
   kvm_arch_init+0x17c/0x280
   kvm_init+0x31/0x330
   vmx_init+0xba/0x13a
   do_one_initcall+0x41/0x1c0
   kernel_init_freeable+0x1f2/0x23b
   kernel_init+0x16/0x120
   ret_from_fork+0x22/0x30

The Linux kernel CVE team has assigned CVE-2021-47217 to this issue.

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

	Issue introduced in 4.16 with commit 93286261de1b and fixed in 4.19.218 with commit b20ec58f8a6f
	Issue introduced in 4.16 with commit 93286261de1b and fixed in 5.4.162 with commit b0e44dfb4e4c
	Issue introduced in 4.16 with commit 93286261de1b and fixed in 5.10.82 with commit 9c177eee116c
	Issue introduced in 4.16 with commit 93286261de1b and fixed in 5.15.5 with commit 8823ea27fff6
	Issue introduced in 4.16 with commit 93286261de1b and fixed in 5.16 with commit daf972118c51

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-2021-47217
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:
	arch/x86/hyperv/hv_init.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/b20ec58f8a6f4fef32cc71480ddf824584e24743
	https://git.kernel.org/stable/c/b0e44dfb4e4c699cca33ede431b8d127e6e8d661
	https://git.kernel.org/stable/c/9c177eee116cf888276d3748cb176e72562cfd5c
	https://git.kernel.org/stable/c/8823ea27fff6084bbb4bc71d15378fae0220b1d8
	https://git.kernel.org/stable/c/daf972118c517b91f74ff1731417feb4270625a4