KVM: arm64: Fix AArch32 register narrowing on userspace write

When userspace writes to one of the core registers, we make
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: 90c1f93 ("KVM: arm64: Get rid of the AArch32 register mapping code")
Reported-by: Nina Schoetterl-Glausch <[email protected]>
Cc: [email protected]
Reviewed-by: Nina Schoetterl-Glausch <[email protected]>
Acked-by: Oliver Upton <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Marc Zyngier <[email protected]>

Author: Marc Zyngier

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.