arm64: KVM: Tighten guest core register access from userspace

We currently allow userspace to access the core register file
in about any possible way, including straddling multiple
registers and doing unaligned accesses.

This is not the expected use of the ABI, and nobody is actually
using it that way. Let's tighten it by explicitly checking
the size and alignment for each field of the register file.

Cc: <stable@vger.kernel.org>
Fixes: 2f4a07c ("arm64: KVM: guest one-reg interface")
Reviewed-by: Christoffer Dall <christoffer.dall@arm.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
[maz: rewrote Dave's initial patch to be more easily backported]
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/kvm/guest.c

@@ -57,6 +57,45 @@ static u64 core_reg_offset_from_id(u64 id)
 	return id & ~(KVM_REG_ARCH_MASK | KVM_REG_SIZE_MASK | KVM_REG_ARM_CORE);
 }
 
+static int validate_core_offset(const struct kvm_one_reg *reg)
+{
+	u64 off = core_reg_offset_from_id(reg->id);
+	int size;
+
+	switch (off) {
+	case KVM_REG_ARM_CORE_REG(regs.regs[0]) ...
+	     KVM_REG_ARM_CORE_REG(regs.regs[30]):
+	case KVM_REG_ARM_CORE_REG(regs.sp):
+	case KVM_REG_ARM_CORE_REG(regs.pc):
+	case KVM_REG_ARM_CORE_REG(regs.pstate):
+	case KVM_REG_ARM_CORE_REG(sp_el1):
+	case KVM_REG_ARM_CORE_REG(elr_el1):
+	case KVM_REG_ARM_CORE_REG(spsr[0]) ...
+	     KVM_REG_ARM_CORE_REG(spsr[KVM_NR_SPSR - 1]):
+		size = sizeof(__u64);
+		break;
+
+	case KVM_REG_ARM_CORE_REG(fp_regs.vregs[0]) ...
+	     KVM_REG_ARM_CORE_REG(fp_regs.vregs[31]):
+		size = sizeof(__uint128_t);
+		break;
+
+	case KVM_REG_ARM_CORE_REG(fp_regs.fpsr):
+	case KVM_REG_ARM_CORE_REG(fp_regs.fpcr):
+		size = sizeof(__u32);
+		break;
+
+	default:
+		return -EINVAL;
+	}
+
+	if (KVM_REG_SIZE(reg->id) == size &&
+	    IS_ALIGNED(off, size / sizeof(__u32)))
+		return 0;
+
+	return -EINVAL;
+}
+
 static int get_core_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg)
 {
 	/*
@@ -76,6 +115,9 @@ static int get_core_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg)
 	    (off + (KVM_REG_SIZE(reg->id) / sizeof(__u32))) >= nr_regs)
 		return -ENOENT;
 
+	if (validate_core_offset(reg))
+		return -EINVAL;
+
 	if (copy_to_user(uaddr, ((u32 *)regs) + off, KVM_REG_SIZE(reg->id)))
 		return -EFAULT;
 
@@ -98,6 +140,9 @@ static int set_core_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg)
 	    (off + (KVM_REG_SIZE(reg->id) / sizeof(__u32))) >= nr_regs)
 		return -ENOENT;
 
+	if (validate_core_offset(reg))
+		return -EINVAL;
+
 	if (KVM_REG_SIZE(reg->id) > sizeof(tmp))
 		return -EINVAL;