Re: [PATCH v2] KVM: x86/mmu: Cope with the SS bit being set in #NPF exit codes

[Sean Christopherson]

On Mon, Oct 12, 2026, Paolo Bonzini wrote:
> 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.
> 
> Reported-by: SimonP <simonp.git@xxxxxxxxxxx>

Can you add a Closes when applying/pushing?  It really does help archeologists
piece things together.

Closes: https://lore.kernel.org/all/asUqYEisPp9MJ8mQ@xxxxxxxxxxxxxxxx