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KVM: nSVM: Always intercept VMMCALL when L2 is active

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Always intercept VMMCALL now that KVM properly synthesizes a #UD as
appropriate, i.e. when L1 doesn't want to intercept VMMCALL, to avoid
putting L2 into an infinite #UD loop if KVM_X86_QUIRK_FIX_HYPERCALL_INSN
is enabled.

By letting L2 execute VMMCALL natively and thus #UD, for all intents and
purposes KVM morphs the VMMCALL intercept into a #UD intercept (KVM always
intercepts #UD).  When the hypercall quirk is enabled, KVM "emulates"
VMMCALL in response to the #UD by trying to fixup the opcode to the "right"
vendor, then restarts the guest, without skipping the VMMCALL.  As a
result, the guest sees an endless stream of #UDs since it's already
executing the correct vendor hypercall instruction, i.e. the emulator
doesn't anticipate that the #UD could be due to lack of interception, as
opposed to a truly undefined opcode.

Fixes: 0d945bd935 ("KVM: SVM: Don't allow nested guest to VMMCALL into host")
Cc: stable@vger.kernel.org
Reviewed-by: Yosry Ahmed <yosry@kernel.org>
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Link: https://patch.msgid.link/20260304002223.1105129-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
This commit is contained in:
Sean Christopherson 2026-03-03 16:22:23 -08:00
parent c36991c6f8
commit 33d3617a52
2 changed files with 0 additions and 11 deletions
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4
arch/x86/kvm/svm/hyperv.h
View File

@ -51,10 +51,6 @@ static inline bool nested_svm_is_l2_tlb_flush_hcall(struct kvm_vcpu *vcpu)
void svm_hv_inject_synthetic_vmexit_post_tlb_flush(struct kvm_vcpu *vcpu);
#else /* CONFIG_KVM_HYPERV */
static inline void nested_svm_hv_update_vm_vp_ids(struct kvm_vcpu *vcpu) {}
static inline bool nested_svm_l2_tlb_flush_enabled(struct kvm_vcpu *vcpu)
{
return false;
}
static inline bool nested_svm_is_l2_tlb_flush_hcall(struct kvm_vcpu *vcpu)
{
return false;

7
arch/x86/kvm/svm/nested.c
View File

@ -158,13 +158,6 @@ void recalc_intercepts(struct vcpu_svm *svm)
vmcb_clr_intercept(c, INTERCEPT_VINTR);
}
/*
* We want to see VMMCALLs from a nested guest only when Hyper-V L2 TLB
* flush feature is enabled.
*/
if (!nested_svm_l2_tlb_flush_enabled(&svm->vcpu))
vmcb_clr_intercept(c, INTERCEPT_VMMCALL);
for (i = 0; i < MAX_INTERCEPT; i++)
c->intercepts[i] |= g->intercepts[i];