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CVE-2026-99120 json

KVM: x86/mmu: Cope with the SS bit being set in #NPF exit codes

published 14d ago modified 14d ago also EUVD-2026-112020

cvss
—
not scored
cisa kev
Not listed
not in CISA's catalog
epss
0.2%
EPSS 0.2% chance of exploitation in 30 days, percentile 6%
public exploits
0
none known

Summary

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

KVM: x86/mmu: Cope with the SS bit being set in #NPF exit codes

When running Hyper-V from Windows 11, KVM is seeing the SS bit set in nested page fault error codes produced for shadowed NPT. The AMD manual is unclear about when SS is set, including whether it can be set when SupervisorShadowStackEn (bit 6) is not set in the MISC_CTL field of the VMCB.

Nevertheless, the unexpected SS bit causes out of bounds accesses in permission_fault(), seen as either WARNs or UBSAN reports:

kernel: UBSAN: array-index-out-of-bounds in arch/x86/kvm/mmu.h:225:27 kernel: index 34 is out of range for type 'u16 [16]'

(in this case, the page fault error code is 34*2=0x44, i.e. SS|U; the report shows SS|U|W happening as well). Fix the handling of unknown error codes by dropping SS and (with a WARN) any bits not among those that permission_fault() expects to receive.

At the same time, given the uncertainty about when hardware sets SS, follow the processor's behavior when constructing the fault's error code, and preserve the SS bit as passed to FNAME(walk_addr_generic). While the behavior is surprising, there's a possibility that Hyper-V expects it (Hyper-V does not even enable CET unless it finds MBEC/GMET!), so retain the information when reflecting the fault to L1 instead of unconditionally discarding it.

Bisection shows that this started happening with commit 687ee95c1b6d ("KVM: nSVM: enable GMET for guests"), even though SHSTK was available to guests since Linux 6.18. However, this is not because bit 1 of MISC_CTL (GMET) somehow enables reporting the SS bit; rather, it's simply that Hyper-V never uses CET unless GMET is also present. The WARN is reproducible even with gmet=0, with some hacking of kvm-unit-tests's cet.c testcase to ensure that the first access to the shadow stack triggers a page fault.

Patch / fix

  • Linux Linux 7.3
  • linux Debian 14 7.2.13-1

debian advisories DEBIAN-CVE-2026-99120

  • linux Ubuntu 26.10 7.3.0-10.10
  • linux-aws Ubuntu 26.10 7.3.0-1004.4
  • linux-gcp Ubuntu 26.10 7.3.0-1004.4
  • linux-raspi Ubuntu 26.10 7.3.0-1004.4
  • linux-riscv Ubuntu 26.10 7.3.0-10.10.1
  • linux-oracle Ubuntu 26.10 7.3.0-1004.4
  • linux-realtime Ubuntu 26.10 7.3.0-10.10.1

ubuntu advisories UBUNTU-CVE-2026-99120

Ubuntu security data © Canonical Ltd., from https://github.com/canonical/ubuntu-security-notices via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

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Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-99120 updated cvss 4.7

kernel: KVM: x86/mmu: Cope with the SS bit being set in #NPF exit codes

  • Red Hat Enterprise Linux 10 kernel
    affected: no fix available “Fix deferred”
  • Red Hat Enterprise Linux 6 perf, kernel, python-perf, kernel-kdump, kernel-firmware, kernel-bootwrapper, +1 more Red Hat Enterprise Linux 7 perf, kernel, bpftool, kernel-rt, python-perf, kernel-kdump, +8 more Red Hat Enterprise Linux 8 perf, kernel, bpftool, kernel-rt, kernel-core, kernel-tools, +29 more Red Hat Enterprise Linux 9 rv, perf, rtla, kernel, bpftool, libperf, +83 more Red Hat OpenShift Container Platform 4 openshift/ose-rhel-coreos-8, openshift/ose-rhel-coreos-9 Red Hat OpenShift Container Platform 5 openshift/ose-rhel-coreos-9
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

Exploit prediction

0.17% chance of exploitation in the next 30 days, higher than 6% of all scored CVEs (FIRST EPSS, 2026-11-11).

Affected products

  • Linux Linux

Classification

Technology feeds

References

Sources

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