Re: [PATCH 1/3] KVM: arm64: Fix AArch32 register narrowing on userspace write

[Oliver Upton <oliver.upton@linux.dev>]

On Fri, May 24, 2024 at 03:19:54PM +0100, Marc Zyngier wrote:
> When userspace writes to once of the core registers, we make

s/once/one/

> sure to narrow the corresponding GPRs if PSTATE indicates
> an AArch32 context.
> 
> The code tries to check whether the context is EL0 or EL1 so
> that it narrows the correct registers. But it does so by checking
> the full PSTATE instead of PSTATE.M.
> 
> As a consequence, and if we are restoring an AArch32 EL0 context
> in a 64bit guest, and that PSTATE has *any* bit set outside of
> PSTATE.M, we narrow *all* registers instead of only the first 15,
> destroying the 64bit state.
> 
> Obviously, this is not something the guest is likely to enjoy.
> 
> Correctly masking PSTATE to only evaluate PSTATE.M fixes it.
> 
> Fixes: 90c1f934ed71 ("KVM: arm64: Get rid of the AArch32 register mapping code")
> Reported-by: Nina Schoetterl-Glausch <nsg@linux.ibm.com>
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> Cc: stable@vger.kernel.org
> ---
>  arch/arm64/kvm/guest.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
> index e2f762d959bb..d9617b11f7a8 100644
> --- a/arch/arm64/kvm/guest.c
> +++ b/arch/arm64/kvm/guest.c
> @@ -276,7 +276,7 @@ static int set_core_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg)
>  	if (*vcpu_cpsr(vcpu) & PSR_MODE32_BIT) {
>  		int i, nr_reg;
>  
> -		switch (*vcpu_cpsr(vcpu)) {
> +		switch (*vcpu_cpsr(vcpu) & PSR_AA32_MODE_MASK) {
>  		/*
>  		 * Either we are dealing with user mode, and only the
>  		 * first 15 registers (+ PC) must be narrowed to 32bit.
> -- 
> 2.39.2
>