[PATCH v2 1/3] 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 before returning the final value.

By not returning as soon as it checks that the register should be RAZ, the
function creates the opportunity for bugs, if, for example, 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, but readable in others.

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

Reviewed-by: Andrew Jones <drjones at redhat.com>
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