MIPS: KVM: Handle MSA Disabled exceptions from guest

Guest user mode can generate a guest MSA Disabled exception on an MSA
capable core by simply trying to execute an MSA instruction. Since this
exception is unknown to KVM it will be passed on to the guest kernel.
However guest Linux kernels prior to v3.15 do not set up an exception
handler for the MSA Disabled exception as they don't support any MSA
capable cores. This results in a guest OS panic.

Since an older processor ID may be being emulated, and MSA support is
not advertised to the guest, the correct behaviour is to generate a
Reserved Instruction exception in the guest kernel so it can send the
guest process an illegal instruction signal (SIGILL), as would happen
with a non-MSA-capable core.

Fix this as minimally as reasonably possible by preventing
kvm_mips_check_privilege() from relaying MSA Disabled exceptions from
guest user mode to the guest kernel, and handling the MSA Disabled
exception by emulating a Reserved Instruction exception in the guest,
via a new handle_msa_disabled() KVM callback.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Paul Burton <paul.burton@imgtec.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Gleb Natapov <gleb@kernel.org>
Cc: linux-mips@linux-mips.org
Cc: kvm@vger.kernel.org
Cc: <stable@vger.kernel.org> # v3.15+

3 files changed, 33 insertions, 0 deletions

diff --git a/arch/mips/kvm/emulate.c b/arch/mips/kvm/emulate.c
index fb3e8dfd1ff6..838d3a6a5b7d 100644
--- a/arch/mips/kvm/emulate.c
+++ b/arch/mips/kvm/emulate.c
@@ -2176,6 +2176,7 @@ enum emulation_result kvm_mips_check_privilege(unsigned long cause,
 		case T_SYSCALL:
 		case T_BREAK:
 		case T_RES_INST:
+		case T_MSADIS:
 			break;
 
 		case T_COP_UNUSABLE:
diff --git a/arch/mips/kvm/mips.c b/arch/mips/kvm/mips.c
index c9eccf5df912..f5e7ddab02f7 100644
--- a/arch/mips/kvm/mips.c
+++ b/arch/mips/kvm/mips.c
@@ -1119,6 +1119,10 @@ int kvm_mips_handle_exit(struct kvm_run *run, struct kvm_vcpu *vcpu)
 		ret = kvm_mips_callbacks->handle_break(vcpu);
 		break;
 
+	case T_MSADIS:
+		ret = kvm_mips_callbacks->handle_msa_disabled(vcpu);
+		break;
+
 	default:
 		kvm_err("Exception Code: %d, not yet handled, @ PC: %p, inst: 0x%08x  BadVaddr: %#lx Status: %#lx\n",
 			exccode, opc, kvm_get_inst(opc, vcpu), badvaddr,
diff --git a/arch/mips/kvm/trap_emul.c b/arch/mips/kvm/trap_emul.c
index fd7257b70e65..4372cc86650c 100644
--- a/arch/mips/kvm/trap_emul.c
+++ b/arch/mips/kvm/trap_emul.c
@@ -330,6 +330,33 @@ static int kvm_trap_emul_handle_break(struct kvm_vcpu *vcpu)
 	return ret;
 }
 
+static int kvm_trap_emul_handle_msa_disabled(struct kvm_vcpu *vcpu)
+{
+	struct kvm_run *run = vcpu->run;
+	uint32_t __user *opc = (uint32_t __user *) vcpu->arch.pc;
+	unsigned long cause = vcpu->arch.host_cp0_cause;
+	enum emulation_result er = EMULATE_DONE;
+	int ret = RESUME_GUEST;
+
+	/* No MSA supported in guest, guest reserved instruction exception */
+	er = kvm_mips_emulate_ri_exc(cause, opc, run, vcpu);
+
+	switch (er) {
+	case EMULATE_DONE:
+		ret = RESUME_GUEST;
+		break;
+
+	case EMULATE_FAIL:
+		run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
+		ret = RESUME_HOST;
+		break;
+
+	default:
+		BUG();
+	}
+	return ret;
+}
+
 static int kvm_trap_emul_vm_init(struct kvm *kvm)
 {
 	return 0;
@@ -470,6 +497,7 @@ static struct kvm_mips_callbacks kvm_trap_emul_callbacks = {
 	.handle_syscall = kvm_trap_emul_handle_syscall,
 	.handle_res_inst = kvm_trap_emul_handle_res_inst,
 	.handle_break = kvm_trap_emul_handle_break,
+	.handle_msa_disabled = kvm_trap_emul_handle_msa_disabled,
 
 	.vm_init = kvm_trap_emul_vm_init,
 	.vcpu_init = kvm_trap_emul_vcpu_init,