cve/published/2021/CVE-2021-47639.mbox

From bippy-5f407fcff5a0 Mon Sep 17 00:00:00 2001
From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: <linux-cve-announce@vger.kernel.org>
Reply-to: <cve@kernel.org>, <linux-kernel@vger.kernel.org>
Subject: CVE-2021-47639: KVM: x86/mmu: Zap _all_ roots when unmapping gfn range in TDP MMU

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

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

KVM: x86/mmu: Zap _all_ roots when unmapping gfn range in TDP MMU

Zap both valid and invalid roots when zapping/unmapping a gfn range, as
KVM must ensure it holds no references to the freed page after returning
from the unmap operation.  Most notably, the TDP MMU doesn't zap invalid
roots in mmu_notifier callbacks.  This leads to use-after-free and other
issues if the mmu_notifier runs to completion while an invalid root
zapper yields as KVM fails to honor the requirement that there must be
_no_ references to the page after the mmu_notifier returns.

The bug is most easily reproduced by hacking KVM to cause a collision
between set_nx_huge_pages() and kvm_mmu_notifier_release(), but the bug
exists between kvm_mmu_notifier_invalidate_range_start() and memslot
updates as well.  Invalidating a root ensures pages aren't accessible by
the guest, and KVM won't read or write page data itself, but KVM will
trigger e.g. kvm_set_pfn_dirty() when zapping SPTEs, and thus completing
a zap of an invalid root _after_ the mmu_notifier returns is fatal.

  WARNING: CPU: 24 PID: 1496 at arch/x86/kvm/../../../virt/kvm/kvm_main.c:173 [kvm]
  RIP: 0010:kvm_is_zone_device_pfn+0x96/0xa0 [kvm]
  Call Trace:
   <TASK>
   kvm_set_pfn_dirty+0xa8/0xe0 [kvm]
   __handle_changed_spte+0x2ab/0x5e0 [kvm]
   __handle_changed_spte+0x2ab/0x5e0 [kvm]
   __handle_changed_spte+0x2ab/0x5e0 [kvm]
   zap_gfn_range+0x1f3/0x310 [kvm]
   kvm_tdp_mmu_zap_invalidated_roots+0x50/0x90 [kvm]
   kvm_mmu_zap_all_fast+0x177/0x1a0 [kvm]
   set_nx_huge_pages+0xb4/0x190 [kvm]
   param_attr_store+0x70/0x100
   module_attr_store+0x19/0x30
   kernfs_fop_write_iter+0x119/0x1b0
   new_sync_write+0x11c/0x1b0
   vfs_write+0x1cc/0x270
   ksys_write+0x5f/0xe0
   do_syscall_64+0x38/0xc0
   entry_SYSCALL_64_after_hwframe+0x44/0xae
   </TASK>

The Linux kernel CVE team has assigned CVE-2021-47639 to this issue.


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

	Issue introduced in 5.13 with commit b7cccd397f310739fb85383033e95580f99927e0 and fixed in 5.15.33 with commit af47248407c0c5ae52a752af1ab5ce5b0db91502
	Issue introduced in 5.13 with commit b7cccd397f310739fb85383033e95580f99927e0 and fixed in 5.16.19 with commit 0c8a8da182d4333d9bbb9131d765145568c847b2
	Issue introduced in 5.13 with commit b7cccd397f310739fb85383033e95580f99927e0 and fixed in 5.17.2 with commit 8cf6f98ab1d16d5e607635a0c21c4231eb15367e
	Issue introduced in 5.13 with commit b7cccd397f310739fb85383033e95580f99927e0 and fixed in 5.18 with commit d62007edf01f5c11f75d0f4b1e538fc52a5b1982

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-2021-47639
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/mmu/tdp_mmu.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/af47248407c0c5ae52a752af1ab5ce5b0db91502
	https://git.kernel.org/stable/c/0c8a8da182d4333d9bbb9131d765145568c847b2
	https://git.kernel.org/stable/c/8cf6f98ab1d16d5e607635a0c21c4231eb15367e
	https://git.kernel.org/stable/c/d62007edf01f5c11f75d0f4b1e538fc52a5b1982