KVM: x86: Protect ioapic_write_indirect() from Spectre-v1/L1TF attacks

This fixes a Spectre-v1/L1TF vulnerability in ioapic_write_indirect().
This function contains index computations based on the
(attacker-controlled) IOREGSEL register.

This patch depends on patch
"KVM: x86: Protect ioapic_read_indirect() from Spectre-v1/L1TF attacks".

Fixes: 70f93dae ("KVM: Use temporary variable to shorten lines.")
Signed-off-by: Nick Finco <nifi@google.com>
Signed-off-by: Marios Pomonis <pomonis@google.com>
Reviewed-by: Andrew Honig <ahonig@google.com>
Cc: stable@vger.kernel.org
Reviewed-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/ioapic.c

@@ -297,6 +297,7 @@ static void ioapic_write_indirect(struct kvm_ioapic *ioapic, u32 val)
 
 		if (index >= IOAPIC_NUM_PINS)
 			return;
+		index = array_index_nospec(index, IOAPIC_NUM_PINS);
 		e = &ioapic->redirtbl[index];
 		mask_before = e->fields.mask;
 		/* Preserve read-only fields */