[PATCH v3 1/2] KVM: x86: Fix array_index_nospec() protection in kvm_vcpu_ioctl_x86_set_mce()

["Carlos López" <clopez@suse.de>]

Commit aebc3ca19063 ("KVM: x86: Enable CMCI capability by default and
handle injected UCNA errors") introduced kvm_vcpu_x86_set_ucna(), which
accesses @vcpu->arch.mci_ctl2_banks[] using @mce->bank as the index. The
@mce struct is user-controlled, provided via the KVM_X86_SET_MCE ioctl.

The caller of this function, kvm_vcpu_ioctl_x86_set_mce(), bounds-checks
@mce->bank and applies array_index_nospec() to advance the @banks
pointer, but @mce->bank itself is passed through unclamped. On a
speculative path that bypasses the bounds check, the raw @mce->bank
value can index mci_ctl2_banks[] out-of-bounds.

In practice this is a very weak gadget, and would at most allow leaking
a single bit in a 64-bit integer, but prevent potential future issues by
clamping @mce->bank in place with array_index_nospec(), before passing
the struct to kvm_vcpu_x86_set_ucna().

Fixes: aebc3ca19063 ("KVM: x86: Enable CMCI capability by default and handle injected UCNA errors")
Signed-off-by: Carlos López <clopez@suse.de>
---
 arch/x86/kvm/x86.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index cf122b8c3210..77a780177c4e 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5472,7 +5472,8 @@ static int kvm_vcpu_ioctl_x86_set_mce(struct kvm_vcpu *vcpu,
 	if (mce->bank >= bank_num || !(mce->status & MCI_STATUS_VAL))
 		return -EINVAL;
 
-	banks += array_index_nospec(4 * mce->bank, 4 * bank_num);
+	mce->bank = array_index_nospec(mce->bank, bank_num);
+	banks += 4 * mce->bank;
 
 	if (is_ucna(mce))
 		return kvm_vcpu_x86_set_ucna(vcpu, mce, banks);
-- 
2.51.0