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CVE-2026-64561: KVM: x86: check for invalid/obsolete root after making MMU pages available

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Taken verbatim from CentOS Stream 10 merge request 3068, which cherry-picks
upstream commit 2abd5287f083 ("KVM: x86: Check for invalid/obsolete root
*after* making MMU pages available") by Sean Christopherson and Paolo
Bonzini.

If reclaiming shadow pages zaps an in-use root, KVM would go on to map
memory into a root already marked invalid. Child shadow pages inherit
their parent's role, so any children created during the map/fetch land on
the list of active MMU pages while flagged invalid, breaking the invariant
introduced in 5.9 that invalid pages are never on that list. The fix moves
the is_page_fault_stale() check to after make_mmu_pages_available().

Applies with --fuzz=0 against arch/x86/kvm/mmu/{mmu.c,paging_tmpl.h}, no
fuzz and no offsets. Verified against the centos-stream-10 main branch: the
el10_2 z-stream tree this kernel is built from (6.12.0-211.38.1.el10_2) is
not published as a branch or tag, but both hunks sit in direct_page_fault()
and FNAME(page_fault)(), which are unchanged across 6.12 z-streams.

Note that MR 3068 is still a Draft upstream. If Red Hat revises it before
merge, this patch needs to be re-synced.

JIRA: RHEL-224013
This commit is contained in:
Andrew Lukoshko 2026-08-06 13:44:37 +02:00
parent 5cbbe2cb09
commit 048f1efd0c
2 changed files with 102 additions and 0 deletions
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5
config.yaml
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@ -56,6 +56,10 @@ actions:
name: "2012-gve-Enable-reading-max-ring-size-in-DQO-QPL-mode.patch"
number: 2012
- type: "patch"
name: "2013-CVE-2026-64561-KVM-x86-Check-for-invalid-obsolete-root.patch"
number: 2013
- replace:
- target: "kernel*.config"
find: |
@ -466,6 +470,7 @@ actions:
- name: "Andrew Lukoshko"
email: "alukoshko@almalinux.org"
line:
- "KVM: x86: check for invalid/obsolete root after making MMU pages available {CVE-2026-64561}"
- "hpsa: bring back deprecated PCI ids #CFHack #CFHack2024"
- "mptsas: bring back deprecated PCI ids #CFHack #CFHack2024"
- "megaraid_sas: bring back deprecated PCI ids #CFHack #CFHack2024"

97
files/2013-CVE-2026-64561-KVM-x86-Check-for-invalid-obsolete-root.patch Normal file
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@ -0,0 +1,97 @@
From 884d1cab4bfadf53f05ca36f35f06d3cc1f916a2 Mon Sep 17 00:00:00 2001
From: CKI Backport Bot <cki-ci-bot+cki-gitlab-backport-bot@redhat.com>
Date: Tue, 4 Aug 2026 10:08:07 +0000
Subject: [PATCH] KVM: x86: Check for invalid/obsolete root *after* making MMU
pages available
JIRA: https://redhat.atlassian.net/browse/RHEL-224013
CVE: CVE-2026-64561
Backported from tree(s): linux
KVM: x86: Check for invalid/obsolete root *after* making MMU pages available
Check for a "stale" page fault, i.e. for an invalid and/or obsolete root,
after making MMU pages available for the shadow MMU. If reclaiming shadow
pages zaps an in-use root, i.e. marks it invalid, then KVM will attempt to
map memory into an invalid root. On its own, populating an invalid root is
"fine", but because child shadow pages inherit their parent's role, any
children created during the map/fetch will be created as invalid pages,
thus violating KVM's invariant that invalid pages are never on the list of
active MMU pages.
Note, the underlying flaw has existed since KVM first started tracking
invalid roots in 2008 (commit 2e53d63acba7, "KVM: MMU: ignore zapped root
pagetables"), but the true badness only came along in 2020 (Linux 5.9)
with the invariant that invalid shadow pages can't be on the list of
active pages.
Note #2, inheriting role.invalid when creating child shadow pages is also
far from ideal; that flaw will be addressed separately.
Reported-by: Hyunwoo Kim <imv4bel@gmail.com>
Fixes: f95eec9bed76 ("KVM: x86/mmu: Don't put invalid SPs back on the list of active pages")
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit 2abd5287f08319fa35764566b15c6e22cb1068db)
Signed-off-by: CKI Backport Bot <cki-ci-bot+cki-gitlab-backport-bot@redhat.com>
---
arch/x86/kvm/mmu/mmu.c | 9 +++++----
arch/x86/kvm/mmu/paging_tmpl.h | 10 ++++++----
2 files changed, 11 insertions(+), 8 deletions(-)
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index ab4bbb692064..0c4553e57731 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -4814,16 +4814,17 @@ static int direct_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
if (r != RET_PF_CONTINUE)
return r;
- r = RET_PF_RETRY;
write_lock(&vcpu->kvm->mmu_lock);
- if (is_page_fault_stale(vcpu, fault))
- goto out_unlock;
-
r = make_mmu_pages_available(vcpu);
if (r)
goto out_unlock;
+ if (is_page_fault_stale(vcpu, fault)) {
+ r = RET_PF_RETRY;
+ goto out_unlock;
+ }
+
r = direct_map(vcpu, fault);
out_unlock:
diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
index 901cd2bd40b8..6465de820e70 100644
--- a/arch/x86/kvm/mmu/paging_tmpl.h
+++ b/arch/x86/kvm/mmu/paging_tmpl.h
@@ -827,15 +827,17 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
}
#endif
- r = RET_PF_RETRY;
write_lock(&vcpu->kvm->mmu_lock);
- if (is_page_fault_stale(vcpu, fault))
- goto out_unlock;
-
r = make_mmu_pages_available(vcpu);
if (r)
goto out_unlock;
+
+ if (is_page_fault_stale(vcpu, fault)) {
+ r = RET_PF_RETRY;
+ goto out_unlock;
+ }
+
r = FNAME(fetch)(vcpu, fault, &walker);
out_unlock:
--
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