[PATCH v2] KVM: VMX: Add array_index_nospec() to evmcs_field_offset()

[Amirmohammad Eftekhar <amirmohammad.eftekhar@cispa.de>]

evmcs_field_offset() bounds-checks the guest-supplied field index against
nr_evmcs_1_fields and then indexes vmcs_field_to_evmcs_1[] without an
array_index_nospec() clamp, unlike its vmcs12 counterpart
get_vmcs12_field_offset().

A regular VMREAD instruction is allowed while an enlightened VMCS is active
since 6cbbaab60ff3 ("KVM: nVMX: Allow VMREAD when Enlightened VMCS is in
use"), and it reaches evmcs_field_offset() with a guest-supplied field
encoding. Because the index is not clamped, the guest may be able to
speculatively access past the bound check and read out of bounds of
vmcs_field_to_evmcs_1[] (Spectre v1). The leaked value is only the u16
'offset', so the exposure is limited.

The other callers of evmcs_field_offset() are not under direct guest
control, but the array_index_nospec() clamp does not hurt them.

Clamp the index with array_index_nospec(), mirroring
get_vmcs12_field_offset().

Fixes: 6cbbaab60ff3 ("KVM: nVMX: Allow VMREAD when Enlightened VMCS is in use")
Signed-off-by: Amirmohammad Eftekhar <amirmohammad.eftekhar@cispa.de>
---
v2:
 - Reword the changelog per Vitaly Kuznetsov: the reachable path is a
   regular VMREAD (allowed since 6cbbaab60ff3), not an "enlightened
   VMREAD/VMWRITE"; note the limited exposure and the other callers.
 - Add the Fixes: tag.

v1: https://lore.kernel.org/all/20261017093016.32348-3-amirmohammad.eftekhar@cispa.de/

 arch/x86/kvm/vmx/hyperv_evmcs.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/x86/kvm/vmx/hyperv_evmcs.h b/arch/x86/kvm/vmx/hyperv_evmcs.h
index bc08fe405..052b62b8e 100644
--- a/arch/x86/kvm/vmx/hyperv_evmcs.h
+++ b/arch/x86/kvm/vmx/hyperv_evmcs.h
@@ -136,6 +136,8 @@ static __always_inline int evmcs_field_offset(unsigned long field,
 	if (unlikely(index >= nr_evmcs_1_fields))
 		return -ENOENT;

+	index = array_index_nospec(index, nr_evmcs_1_fields);
+
 	evmcs_field = &vmcs_field_to_evmcs_1[index];

 	/*
--
2.54.0 (Apple Git-157)