KVM: MIPS: Improve kvm_get_inst() error return

Currently kvm_get_inst() returns KVM_INVALID_INST in the event of a
fault reading the guest instruction. This has the rather arbitrary magic
value 0xdeadbeef. This API isn't very robust, and in fact 0xdeadbeef is
a valid MIPS64 instruction encoding, namely "ld t1,-16657(s5)".

Therefore change the kvm_get_inst() API to return 0 or -EFAULT, and to
return the instruction via a u32 *out argument. We can then drop the
KVM_INVALID_INST definition entirely.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Radim Krčmář" <rkrcmar@redhat.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Cc: kvm@vger.kernel.org

4 files changed, 56 insertions, 53 deletions

diff --git a/arch/mips/include/asm/kvm_host.h b/arch/mips/include/asm/kvm_host.h
index 6f68f7545b66..f296ebeda9e3 100644
--- a/arch/mips/include/asm/kvm_host.h
+++ b/arch/mips/include/asm/kvm_host.h
@@ -104,7 +104,6 @@
 #define KVM_GUEST_KSEG23ADDR(a)         (KVM_GUEST_CPHYSADDR(a) | KVM_GUEST_KSEG23)
 
 #define KVM_INVALID_PAGE                0xdeadbeef
-#define KVM_INVALID_INST                0xdeadbeef
 #define KVM_INVALID_ADDR                0xdeadbeef
 
 /*
@@ -640,7 +639,7 @@ void kvm_trap_emul_invalidate_gva(struct kvm_vcpu *vcpu, unsigned long addr,
                                   bool user);
 
 /* Emulation */
-u32 kvm_get_inst(u32 *opc, struct kvm_vcpu *vcpu);
+int kvm_get_inst(u32 *opc, struct kvm_vcpu *vcpu, u32 *out);
 enum emulation_result update_pc(struct kvm_vcpu *vcpu, u32 cause);
 
 /**
diff --git a/arch/mips/kvm/emulate.c b/arch/mips/kvm/emulate.c
index 67ea39973b96..b906fc0589f3 100644
--- a/arch/mips/kvm/emulate.c
+++ b/arch/mips/kvm/emulate.c
@@ -38,23 +38,25 @@
  * Compute the return address and do emulate branch simulation, if required.
  * This function should be called only in branch delay slot active.
  */
-unsigned long kvm_compute_return_epc(struct kvm_vcpu *vcpu,
-        unsigned long instpc)
+static int kvm_compute_return_epc(struct kvm_vcpu *vcpu, unsigned long instpc,
+                                  unsigned long *out)
 {
         unsigned int dspcontrol;
         union mips_instruction insn;
         struct kvm_vcpu_arch *arch = &vcpu->arch;
         long epc = instpc;
-        long nextpc = KVM_INVALID_INST;
+        long nextpc;
+        int err;
 
-        if (epc & 3)
-                goto unaligned;
+        if (epc & 3) {
+                kvm_err("%s: unaligned epc\n", __func__);
+                return -EINVAL;
+        }
 
         /* Read the instruction */
-        insn.word = kvm_get_inst((u32 *) epc, vcpu);
-
-        if (insn.word == KVM_INVALID_INST)
-                return KVM_INVALID_INST;
+        err = kvm_get_inst((u32 *)epc, vcpu, &insn.word);
+        if (err)
+                return err;
 
         switch (insn.i_format.opcode) {
                 /* jr and jalr are in r_format format. */
@@ -66,6 +68,8 @@ unsigned long kvm_compute_return_epc(struct kvm_vcpu *vcpu,
                 case jr_op:
                         nextpc = arch->gprs[insn.r_format.rs];
                         break;
+                default:
+                        return -EINVAL;
                 }
                 break;
 
@@ -114,8 +118,11 @@ unsigned long kvm_compute_return_epc(struct kvm_vcpu *vcpu,
                         nextpc = epc;
                         break;
                 case bposge32_op:
-                        if (!cpu_has_dsp)
-                                goto sigill;
+                        if (!cpu_has_dsp) {
+                                kvm_err("%s: DSP branch but not DSP ASE\n",
+                                        __func__);
+                                return -EINVAL;
+                        }
 
                         dspcontrol = rddsp(0x01);
 
@@ -125,6 +132,8 @@ unsigned long kvm_compute_return_epc(struct kvm_vcpu *vcpu,
                                 epc += 8;
                         nextpc = epc;
                         break;
+                default:
+                        return -EINVAL;
                 }
                 break;
 
@@ -189,7 +198,7 @@ unsigned long kvm_compute_return_epc(struct kvm_vcpu *vcpu,
                 /* And now the FPA/cp1 branch instructions. */
         case cop1_op:
                 kvm_err("%s: unsupported cop1_op\n", __func__);
-                break;
+                return -EINVAL;
 
 #ifdef CONFIG_CPU_MIPSR6
         /* R6 added the following compact branches with forbidden slots */
@@ -198,19 +207,19 @@ unsigned long kvm_compute_return_epc(struct kvm_vcpu *vcpu,
                 /* only rt == 0 isn't compact branch */
                 if (insn.i_format.rt != 0)
                         goto compact_branch;
-                break;
+                return -EINVAL;
         case pop10_op:
         case pop30_op:
                 /* only rs == rt == 0 is reserved, rest are compact branches */
                 if (insn.i_format.rs != 0 || insn.i_format.rt != 0)
                         goto compact_branch;
-                break;
+                return -EINVAL;
         case pop66_op:
         case pop76_op:
                 /* only rs == 0 isn't compact branch */
                 if (insn.i_format.rs != 0)
                         goto compact_branch;
-                break;
+                return -EINVAL;
 compact_branch:
                 /*
                  * If we've hit an exception on the forbidden slot, then
@@ -221,42 +230,32 @@ compact_branch:
                 break;
 #else
 compact_branch:
-                /* Compact branches not supported before R6 */
-                break;
+                /* Fall through - Compact branches not supported before R6 */
 #endif
+        default:
+                return -EINVAL;
         }
 
-        return nextpc;
-
-unaligned:
-        kvm_err("%s: unaligned epc\n", __func__);
-        return nextpc;
-
-sigill:
-        kvm_err("%s: DSP branch but not DSP ASE\n", __func__);
-        return nextpc;
+        *out = nextpc;
+        return 0;
 }
 
 enum emulation_result update_pc(struct kvm_vcpu *vcpu, u32 cause)
 {
-        unsigned long branch_pc;
-        enum emulation_result er = EMULATE_DONE;
+        int err;
 
         if (cause & CAUSEF_BD) {
-                branch_pc = kvm_compute_return_epc(vcpu, vcpu->arch.pc);
-                if (branch_pc == KVM_INVALID_INST) {
-                        er = EMULATE_FAIL;
-                } else {
-                        vcpu->arch.pc = branch_pc;
-                        kvm_debug("BD update_pc(): New PC: %#lx\n",
-                                  vcpu->arch.pc);
-                }
-        } else
+                err = kvm_compute_return_epc(vcpu, vcpu->arch.pc,
+                                             &vcpu->arch.pc);
+                if (err)
+                        return EMULATE_FAIL;
+        } else {
                 vcpu->arch.pc += 4;
+        }
 
         kvm_debug("update_pc(): New PC: %#lx\n", vcpu->arch.pc);
 
-        return er;
+        return EMULATE_DONE;
 }
 
 /**
@@ -1835,12 +1834,14 @@ enum emulation_result kvm_mips_emulate_inst(u32 cause, u32 *opc,
 {
         union mips_instruction inst;
         enum emulation_result er = EMULATE_DONE;
+        int err;
 
         /* Fetch the instruction. */
         if (cause & CAUSEF_BD)
                 opc += 1;
-
-        inst.word = kvm_get_inst(opc, vcpu);
+        err = kvm_get_inst(opc, vcpu, &inst.word);
+        if (err)
+                return EMULATE_FAIL;
 
         switch (inst.r_format.opcode) {
         case cop0_op:
@@ -2419,6 +2420,7 @@ enum emulation_result kvm_mips_handle_ri(u32 cause, u32 *opc,
         enum emulation_result er = EMULATE_DONE;
         unsigned long curr_pc;
         union mips_instruction inst;
+        int err;
 
         /*
          * Update PC and hold onto current PC in case there is
@@ -2432,11 +2434,9 @@ enum emulation_result kvm_mips_handle_ri(u32 cause, u32 *opc,
         /* Fetch the instruction. */
         if (cause & CAUSEF_BD)
                 opc += 1;
-
-        inst.word = kvm_get_inst(opc, vcpu);
-
-        if (inst.word == KVM_INVALID_INST) {
-                kvm_err("%s: Cannot get inst @ %p\n", __func__, opc);
+        err = kvm_get_inst(opc, vcpu, &inst.word);
+        if (err) {
+                kvm_err("%s: Cannot get inst @ %p (%d)\n", __func__, opc, err);
                 return EMULATE_FAIL;
         }
 
diff --git a/arch/mips/kvm/mips.c b/arch/mips/kvm/mips.c
index 07ce10e3627a..29afd96069ef 100644
--- a/arch/mips/kvm/mips.c
+++ b/arch/mips/kvm/mips.c
@@ -1343,6 +1343,7 @@ int kvm_mips_handle_exit(struct kvm_run *run, struct kvm_vcpu *vcpu)
         u32 __user *opc = (u32 __user *) vcpu->arch.pc;
         unsigned long badvaddr = vcpu->arch.host_cp0_badvaddr;
         enum emulation_result er = EMULATE_DONE;
+        u32 inst;
         int ret = RESUME_GUEST;
 
         /* re-enable HTW before enabling interrupts */
@@ -1467,8 +1468,12 @@ int kvm_mips_handle_exit(struct kvm_run *run, struct kvm_vcpu *vcpu)
                 break;
 
         default:
+                if (cause & CAUSEF_BD)
+                        opc += 1;
+                inst = 0;
+                kvm_get_inst(opc, vcpu, &inst);
                 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,
+                        exccode, opc, inst, badvaddr,
                         kvm_read_c0_guest_status(vcpu->arch.cop0));
                 kvm_arch_vcpu_dump_regs(vcpu);
                 run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
diff --git a/arch/mips/kvm/mmu.c b/arch/mips/kvm/mmu.c
index aab604e75d3b..6379ac1bc7b9 100644
--- a/arch/mips/kvm/mmu.c
+++ b/arch/mips/kvm/mmu.c
@@ -503,16 +503,15 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
         local_irq_restore(flags);
 }
 
-u32 kvm_get_inst(u32 *opc, struct kvm_vcpu *vcpu)
+int kvm_get_inst(u32 *opc, struct kvm_vcpu *vcpu, u32 *out)
 {
-        u32 inst;
         int err;
 
-        err = get_user(inst, opc);
+        err = get_user(*out, opc);
         if (unlikely(err)) {
                 kvm_err("%s: illegal address: %p\n", __func__, opc);
-                return KVM_INVALID_INST;
+                return -EFAULT;
         }
 
-        return inst;
+        return 0;
 }