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[PATCH 2/3] KVM: MIPS: Improve kvm_get_inst() error return

To: <linux-mips@linux-mips.org>
Subject: [PATCH 2/3] KVM: MIPS: Improve kvm_get_inst() error return
From: James Hogan <james.hogan@imgtec.com>
Date: Fri, 6 Jan 2017 14:44:42 +0000
Cc: James Hogan <james.hogan@imgtec.com>, Paolo Bonzini <pbonzini@redhat.com>, Radim Krčmář <rkrcmar@redhat.com>, Ralf Baechle <ralf@linux-mips.org>, <kvm@vger.kernel.org>
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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
---
 arch/mips/include/asm/kvm_host.h |  3 +-
 arch/mips/kvm/emulate.c          | 90 ++++++++++++++++-----------------
 arch/mips/kvm/mips.c             |  7 ++-
 arch/mips/kvm/mmu.c              |  9 +--
 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 3c39ccd8856e..b0936a4d931a 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
 
 /*
@@ -637,7 +636,7 @@ 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 @@ unsigned long kvm_compute_return_epc(struct kvm_vcpu 
*vcpu,
                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 001349124bad..69775fb9c408 100644
--- a/arch/mips/kvm/mips.c
+++ b/arch/mips/kvm/mips.c
@@ -1348,6 +1348,7 @@ 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 */
@@ -1472,8 +1473,12 @@ 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;
 }
-- 
git-series 0.8.10

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