From e90ab3669181c6469e3647ef2b30ae408275f496 Mon Sep 17 00:00:00 2001 From: Paolo Bonzini Date: Fri, 9 Oct 2026 14:37:10 +0200 Subject: [PATCH] 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. Reported-by: SimonP Fixes: 8db428fd5229 ("KVM: SVM: Enable shadow stack virtualization for SVM") Cc: stable@vger.kernel.org Reviewed-by: Sean Christopherson Closes: https://lore.kernel.org/all/asUqYEisPp9MJ8mQ@rika.bernkas.tel Signed-off-by: Paolo Bonzini --- arch/x86/kvm/mmu.h | 16 ++++++++++++---- arch/x86/kvm/mmu/paging_tmpl.h | 3 +-- 2 files changed, 13 insertions(+), 6 deletions(-) diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h index 2ae7f9ed4cf8..b2a295a9f084 100644 --- a/arch/x86/kvm/mmu.h +++ b/arch/x86/kvm/mmu.h @@ -275,8 +275,18 @@ static inline u8 permission_fault(struct kvm_vcpu *vcpu, struct kvm_pagewalk *w, unsigned pte_access, unsigned pte_pkey, u64 access) { - /* strip nested paging fault error codes */ - unsigned int pfec = access; + /* + * Of the error codes that do not contribute to the index in + * fmt->permissions, PK and SS are not supported by shadow paging + * and also not provided to EPT violations, but SS *is* included + * in the NPT exit code. RSVD faults are handled elsewhere. + */ + unsigned int pfec = access & ~PFERR_SS_MASK; + if (WARN_ON_ONCE(pfec & ~(PFERR_PRESENT_MASK | PFERR_WRITE_MASK | + PFERR_USER_MASK | PFERR_FETCH_MASK))) + pfec &= (PFERR_PRESENT_MASK | PFERR_WRITE_MASK | + PFERR_USER_MASK | PFERR_FETCH_MASK); + unsigned long rflags = kvm_x86_call(get_rflags)(vcpu); /* @@ -301,8 +311,6 @@ static inline u8 permission_fault(struct kvm_vcpu *vcpu, struct kvm_pagewalk *w, kvm_mmu_refresh_passthrough_bits(vcpu, w); fault = (fmt->permissions[index] >> pte_access) & 1; - - WARN_ON_ONCE(pfec & (PFERR_PK_MASK | PFERR_SS_MASK | PFERR_RSVD_MASK)); if (unlikely(fmt->pkru_mask)) { u32 pkru_bits, offset; diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h index 27427e7f22fa..e5450faa669e 100644 --- a/arch/x86/kvm/mmu/paging_tmpl.h +++ b/arch/x86/kvm/mmu/paging_tmpl.h @@ -328,7 +328,6 @@ static int FNAME(walk_addr_generic)(struct guest_walker *walker, int offset; u64 walk_nx_mask = 0; const int write_fault = access & PFERR_WRITE_MASK; - const int user_fault = access & PFERR_USER_MASK; const int fetch_fault = access & PFERR_FETCH_MASK; /* * Note! Track the error_code that's common to legacy shadow paging @@ -486,7 +485,7 @@ retry_walk: return 1; error: - errcode |= write_fault | user_fault; + errcode |= access & (PFERR_WRITE_MASK | PFERR_USER_MASK | PFERR_SS_MASK); if (fetch_fault && has_pferr_fetch(w)) errcode |= PFERR_FETCH_MASK;