CVE-2026-99120 KVM: x86/mmu: Cope with the SS bit being set in #NPF exit codes
- Exploited in the wild
- Not known
- Fix
- Available
What is CVE-2026-99120?
CVE-2026-99120 is a vulnerability in Linux, published on 28 Oct 2026.
Official description
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.
Which versions are affected by CVE-2026-99120?
How do I fix CVE-2026-99120?
Upgrade to a fixed version.
Is CVE-2026-99120 being exploited?
There is no known exploitation of CVE-2026-99120 in the wild and no public exploit code.
Sources
Also known as BELL-CVE-2026-99120, DEBIAN-CVE-2026-99120, GHSA-r4wm-h5f5-6qhj, UBUNTU-CVE-2026-99120.