[PATCH 1/2] KVM: arm64: Return early from read_id_reg() if register is RAZ

[Alexandru Elisei]

If read_id_reg() is called for an ID register which is Read-As-Zero
(RAZ), it initializes the return value to zero, then goes through a list
of registers which require special handling.

By not returning as soon as it tests if the register is RAZ, it creates
the opportunity for bugs, if a patch changes a register to RAZ (like has
happened with PMSWINC_EL0 in commit 11663111cd49), but doesn't remove the
special handling from read_id_reg(); or if a register is RAZ in certain
situations, and readable in others.

Return early as to make it impossible for a RAZ register to be anything
other than zero.

Signed-off-by: Alexandru Elisei <alexandru.elisei at arm.com>
---
 arch/arm64/kvm/sys_regs.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index 1d46e185f31e..4adda8bf3168 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -1064,7 +1064,12 @@ static u64 read_id_reg(const struct kvm_vcpu *vcpu,
 		struct sys_reg_desc const *r, bool raz)
 {
 	u32 id = reg_to_encoding(r);
-	u64 val = raz ? 0 : read_sanitised_ftr_reg(id);
+	u64 val;
+
+	if (raz)
+		return 0;
+
+	val = read_sanitised_ftr_reg(id);
 
 	switch (id) {
 	case SYS_ID_AA64PFR0_EL1:
-- 
2.33.0