CVE-2026-98717: KVM: x86/mmu: Bail from shadow walks if the root is invalid or a dummy

CVE-2026-98717: KVM: x86/mmu: Bail from shadow walks if the root is invalid or a dummy

[Greg Kroah-Hartman]

From: Greg Kroah-Hartman <gregkh@kernel.org>

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

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

KVM: x86/mmu: Bail from shadow walks if the root is invalid or a dummy

When walking shadow page tables, immediately terminate the walk if the root
is a "dummy" root, i.e. a root whose top-level page table is backed by the
zero page, but otherwise doesn't exist.  If memslot creation races with a
stage-2 page fault (EPT violation or #NPF) from L2, then if the stars align,
KVM will attempt to walk shadow page tables using the zero page and hit a
NULL pointer deref.

  BUG: kernel NULL pointer dereference, address: 0000000000000021
  #PF: supervisor read access in kernel mode
  #PF: error_code(0x0000) - not-present page
  PGD 0 P4D 0
  Oops: Oops: 0000 [#1] SMP
  CPU: 30 UID: 1000 PID: 941 Comm: qemu Not tainted 7.2.0-rc2-1b731e5ded48-next-vm #1741 PREEMPTLAZY
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
  RIP: 0010:__kvm_mmu_invalidate_addr+0xea/0x210 [kvm]
  Call Trace:
   <TASK>
   kvm_mmu_invalidate_addr+0x92/0xe0 [kvm]
   __kvm_inject_emulated_page_fault+0x67/0x80 [kvm]
   ept_page_fault+0x160/0x850 [kvm]
   kvm_mmu_do_page_fault+0x102/0x1f0 [kvm]
   kvm_mmu_page_fault+0x8e/0x6b0 [kvm]
   vmx_handle_exit+0x163/0x640 [kvm_intel]
   kvm_arch_vcpu_ioctl_run+0x960/0x2120 [kvm]
   kvm_vcpu_ioctl+0x2c7/0x970 [kvm]
   __x64_sys_ioctl+0x90/0xd0
   do_syscall_64+0x67/0x5f0
   entry_SYSCALL_64_after_hwframe+0x4b/0x53
  RIP: 0033:0x7f606d0b53bb

Opportunistically harden the shadow walks against fully invalid roots, but
WARN, as all callers are expected/required to pre-check for a valid root.
Don't WARN in the dummy root case as the whole point of using a dummy root
is to provide a root that's valid enough to enter the guest, i.e. it should
Just Work for all flows except those that *need* to know about dummy roots.

Alternatively, KVM could allocate a dedicated page and associated shadow
page structure for the dummy root, which is very tempting as it such an
approach should be more resilient against unexpected behavior.  But that
would be a much larger and thus riskier change than simply terminating
walks of dummy roots.

The Linux kernel CVE team has assigned CVE-2026-98717 to this issue.


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

	Issue introduced in 6.6 with commit 0e3223d8d00ac05849661f1b8b476d3caca251cf and fixed in 6.12.113 with commit e1bcabc07efd029ab414894127411129ffdb8b48
	Issue introduced in 6.6 with commit 0e3223d8d00ac05849661f1b8b476d3caca251cf and fixed in 6.18.56 with commit 97c1dcce17ef27853860c781f9912257e6adbf7c
	Issue introduced in 6.6 with commit 0e3223d8d00ac05849661f1b8b476d3caca251cf and fixed in 7.2.10 with commit 681e31f03ec9219099a66a0f6e1bc48e1b5aa914
	Issue introduced in 6.6 with commit 0e3223d8d00ac05849661f1b8b476d3caca251cf and fixed in 7.3-rc6 with commit b6249c1d5665098c8bf12f8d3e4712bf04c775bc

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-2026-98717
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/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/e1bcabc07efd029ab414894127411129ffdb8b48
	https://git.kernel.org/stable/c/97c1dcce17ef27853860c781f9912257e6adbf7c
	https://git.kernel.org/stable/c/681e31f03ec9219099a66a0f6e1bc48e1b5aa914
	https://git.kernel.org/stable/c/b6249c1d5665098c8bf12f8d3e4712bf04c775bc