linux-cve-announce - CVE-2021-46978: KVM: nVMX: Always make an attempt to map eVMCS after migration

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Date: Wed, 28 Feb 2024 09:14:28 +0100
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: gregkh@...nel.org
Subject: CVE-2021-46978: KVM: nVMX: Always make an attempt to map eVMCS after migration

From: gregkh@...nel.org

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

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

KVM: nVMX: Always make an attempt to map eVMCS after migration

When enlightened VMCS is in use and nested state is migrated with
vmx_get_nested_state()/vmx_set_nested_state() KVM can't map evmcs
page right away: evmcs gpa is not 'struct kvm_vmx_nested_state_hdr'
and we can't read it from VP assist page because userspace may decide
to restore HV_X64_MSR_VP_ASSIST_PAGE after restoring nested state
(and QEMU, for example, does exactly that). To make sure eVMCS is
mapped /vmx_set_nested_state() raises KVM_REQ_GET_NESTED_STATE_PAGES
request.

Commit f2c7ef3ba955 ("KVM: nSVM: cancel KVM_REQ_GET_NESTED_STATE_PAGES
on nested vmexit") added KVM_REQ_GET_NESTED_STATE_PAGES clearing to
nested_vmx_vmexit() to make sure MSR permission bitmap is not switched
when an immediate exit from L2 to L1 happens right after migration (caused
by a pending event, for example). Unfortunately, in the exact same
situation we still need to have eVMCS mapped so
nested_sync_vmcs12_to_shadow() reflects changes in VMCS12 to eVMCS.

As a band-aid, restore nested_get_evmcs_page() when clearing
KVM_REQ_GET_NESTED_STATE_PAGES in nested_vmx_vmexit(). The 'fix' is far
from being ideal as we can't easily propagate possible failures and even if
we could, this is most likely already too late to do so. The whole
'KVM_REQ_GET_NESTED_STATE_PAGES' idea for mapping eVMCS after migration
seems to be fragile as we diverge too much from the 'native' path when
vmptr loading happens on vmx_set_nested_state().

The Linux kernel CVE team has assigned CVE-2021-46978 to this issue.

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

	Issue introduced in 5.10.13 with commit 0faceb7d6dda and fixed in 5.10.38 with commit c8bf64e3fb77
	Issue introduced in 5.11 with commit f2c7ef3ba955 and fixed in 5.11.22 with commit 200a45649ab7
	Issue introduced in 5.11 with commit f2c7ef3ba955 and fixed in 5.12.5 with commit bd0e8455b85b
	Issue introduced in 5.11 with commit f2c7ef3ba955 and fixed in 5.13 with commit f5c7e8425f18

Please see https://www.kernel.org or 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-2021-46978
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/kvm/vmx/nested.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/c8bf64e3fb77cc19bad146fbe26651985b117194
	https://git.kernel.org/stable/c/200a45649ab7361bc80c70aebf7165b64f9a6c9f
	https://git.kernel.org/stable/c/bd0e8455b85b651a4c77de9616e307129b15aaa7
	https://git.kernel.org/stable/c/f5c7e8425f18fdb9bdb7d13340651d7876890329