CVE-2026-63807: KVM: x86/mmu: Ensure hugepage is in by slot before checking max mapping level

CVE-2026-63807: KVM: x86/mmu: Ensure hugepage is in by slot before checking max mapping level

[Greg Kroah-Hartman]

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

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

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

KVM: x86/mmu: Ensure hugepage is in by slot before checking max mapping level

When recovering hugepages in the shadow MMU, verify that the base gfn of
the shadow page is actually contained within the target memslot, *before*
querying the max mapping level given the shadow page's gfn.  Failure to
pre-check the validity of the gfn can lead to an out-of-bounds access to
the slot's lpage_info (which typically manifests as a host #PF because the
lpage_info is vmalloc'd) if the guest creates a hugepage mapping (in its
PTEs) that extends "below" the bounds of a memslot.

When faulting in memory for a guest, and the size of the guest mapping is
greater than KVM's (current) max mapping, then KVM will create a "direct"
shadow page (direct in that there are no gPTEs to shadow, and so the target
gfn is a direct calculation given the base gfn of the shadow page).  The
hugepage recovery flow looks for such direct shadow pages, as forcing 4KiB
mappings when dirty logging generates the guest > host mapping size case.
When the 4KiB restriction is lifted, then KVM can replace the shadow page
with a hugepage.

But if KVM originally used a smaller mapping than the guest because the
range of memory covered by the guest hugepage exceeds the bounds of a
memslot, then KVM will link a direct shadow page with a gfn that is outside
the bounds of the memslot being used to fault in memory.  The rmap entry
added for the leaf mapping is correct and within bounds, but the gfn of the
leaf SPTE's parent shadow page will be out of bounds.

  BUG: unable to handle page fault for address: ffffc90000806ffc
  #PF: supervisor read access in kernel mode
  #PF: error_code(0x0000) - not-present page
  PGD 100000067 P4D 100000067 PUD 1002a7067 PMD 10612f067 PTE 0
  Oops: Oops: 0000 [#1] SMP
  CPU: 13 UID: 1000 PID: 757 Comm: mmu_stress_test Not tainted 7.1.0-rc1-48ce1e26eace-x86_pir_to_irr_comments-vm #341 PREEMPT
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
  RIP: 0010:kvm_mmu_max_mapping_level+0x79/0x2b0 [kvm]
  Call Trace:
   <TASK>
   kvm_mmu_recover_huge_pages+0x21b/0x320 [kvm]
   kvm_set_memslot+0x1ee/0x590 [kvm]
   kvm_set_memory_region.part.0+0x3a1/0x4d0 [kvm]
   kvm_vm_ioctl+0x9bf/0x15d0 [kvm]
   __x64_sys_ioctl+0x8a/0xd0
   do_syscall_64+0xb7/0xbb0
   entry_SYSCALL_64_after_hwframe+0x4b/0x53
  RIP: 0033:0x7f21c0f1a9bf
   </TASK>

Don't bother pre-checking the bounds of the potential hugepage, i.e. don't
check that e.g. sp->gfn + KVM_PAGES_PER_HPAGE(sp->role.level + 1) is also
within the memslot, as the checks performed by kvm_mmu_max_mapping_level()
are a superset of the basic bounds checks.  I.e. pre-checking the full
range would be a dubious micro-optimization.

The Linux kernel CVE team has assigned CVE-2026-63807 to this issue.


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

	Issue introduced in 5.12 with commit 9eba50f8d7fcb61774f160890f98239fa3ab68a6 and fixed in 5.15.211 with commit 7b52008023b7facf40fba3ebe92449bda8ea53b9
	Issue introduced in 5.12 with commit 9eba50f8d7fcb61774f160890f98239fa3ab68a6 and fixed in 6.1.177 with commit 5cab1c989f938f5e1b9a0de66486f1fc2c28479b
	Issue introduced in 5.12 with commit 9eba50f8d7fcb61774f160890f98239fa3ab68a6 and fixed in 6.6.144 with commit 48b91ed7e22bb82571c34f8b80b6ecdc90a6fab8
	Issue introduced in 5.12 with commit 9eba50f8d7fcb61774f160890f98239fa3ab68a6 and fixed in 6.12.95 with commit c5c29b3c268332afe67d598a034c58344540ed92
	Issue introduced in 5.12 with commit 9eba50f8d7fcb61774f160890f98239fa3ab68a6 and fixed in 6.18.38 with commit 18587f9831612e24cd8f24be1ec15478feff7abc
	Issue introduced in 5.12 with commit 9eba50f8d7fcb61774f160890f98239fa3ab68a6 and fixed in 7.1.3 with commit b2ae3245ea44dccaa9af676b6747476951883318
	Issue introduced in 5.12 with commit 9eba50f8d7fcb61774f160890f98239fa3ab68a6 and fixed in 7.2-rc1 with commit ef057cbf825e03b63f6edf5980f96abf3c53089d

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-2026-63807
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
	include/linux/kvm_host.h


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/7b52008023b7facf40fba3ebe92449bda8ea53b9
	https://git.kernel.org/stable/c/5cab1c989f938f5e1b9a0de66486f1fc2c28479b
	https://git.kernel.org/stable/c/48b91ed7e22bb82571c34f8b80b6ecdc90a6fab8
	https://git.kernel.org/stable/c/c5c29b3c268332afe67d598a034c58344540ed92
	https://git.kernel.org/stable/c/18587f9831612e24cd8f24be1ec15478feff7abc
	https://git.kernel.org/stable/c/b2ae3245ea44dccaa9af676b6747476951883318
	https://git.kernel.org/stable/c/ef057cbf825e03b63f6edf5980f96abf3c53089d