[PATCH v2 01/13] KVM: arm64: Harden __ctxt_sys_reg() against out-of-range values

[Marc Zyngier <maz@kernel.org>]

The unsuspecting kernel tinkerer can be easily confused into
writing something that looks like this:

	ikey.lo = __vcpu_sys_reg(vcpu, SYS_APIAKEYLO_EL1);

which seems vaguely sensible, until you realise that the second
parameter is the encoding of a sysreg, and not the index into
the vcpu sysreg file... Debugging what happens in this case is
an interesting exercise in head<->wall interactions.

As they often say: "Any resemblance to actual persons, living
or dead, or actual events is purely coincidental".

In order to save people's time, add some compile-time hardening
that will at least weed out the "stupidly out of range" values.
This will *not* catch anything that isn't a compile-time constant.

Reviewed-by: Joey Gouly <joey.gouly@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
---
 arch/arm64/include/asm/kvm_host.h | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index 181fef12e8e8..a5ec4c7d3966 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -895,7 +895,7 @@ struct kvm_vcpu_arch {
  * Don't bother with VNCR-based accesses in the nVHE code, it has no
  * business dealing with NV.
  */
-static inline u64 *__ctxt_sys_reg(const struct kvm_cpu_context *ctxt, int r)
+static inline u64 *___ctxt_sys_reg(const struct kvm_cpu_context *ctxt, int r)
 {
 #if !defined (__KVM_NVHE_HYPERVISOR__)
 	if (unlikely(cpus_have_final_cap(ARM64_HAS_NESTED_VIRT) &&
@@ -905,6 +905,13 @@ static inline u64 *__ctxt_sys_reg(const struct kvm_cpu_context *ctxt, int r)
 	return (u64 *)&ctxt->sys_regs[r];
 }
 
+#define __ctxt_sys_reg(c,r)						\
+	({								\
+	    	BUILD_BUG_ON(__builtin_constant_p(r) &&			\
+			     (r) >= NR_SYS_REGS);			\
+		___ctxt_sys_reg(c, r);					\
+	})
+
 #define ctxt_sys_reg(c,r)	(*__ctxt_sys_reg(c,r))
 
 u64 kvm_vcpu_sanitise_vncr_reg(const struct kvm_vcpu *, enum vcpu_sysreg);
-- 
2.39.2