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

[Andrew Jones]

On Mon, Sep 27, 2021 at 01:49:10PM +0100, Alexandru Elisei wrote:
> 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
> 
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Reviewed-by: Andrew Jones <drjones at redhat.com>