KVM: x86: Handle errors when RIP is set during far jumps

Far jmp/call/ret may fault while loading a new RIP.  Currently KVM does not
handle this case, and may result in failed vm-entry once the assignment is
done.  The tricky part of doing so is that loading the new CS affects the
VMCS/VMCB state, so if we fail during loading the new RIP, we are left in
unconsistent state.  Therefore, this patch saves on 64-bit the old CS
descriptor and restores it if loading RIP failed.

This fixes CVE-2014-3647.

Cc: stable@vger.kernel.org
Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 88 insertions, 30 deletions

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index a1b9139169f6..c0deaff8d9f0 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -1443,7 +1443,9 @@ static int write_segment_descriptor(struct x86_emulate_ctxt *ctxt,
 
 /* Does not support long mode */
 static int __load_segment_descriptor(struct x86_emulate_ctxt *ctxt,
-                                     u16 selector, int seg, u8 cpl, bool in_task_switch)
+                                     u16 selector, int seg, u8 cpl,
+                                     bool in_task_switch,
+                                     struct desc_struct *desc)
 {
         struct desc_struct seg_desc, old_desc;
         u8 dpl, rpl;
@@ -1584,6 +1586,8 @@ static int __load_segment_descriptor(struct x86_emulate_ctxt *ctxt,
         }
 load:
         ctxt->ops->set_segment(ctxt, selector, &seg_desc, base3, seg);
+        if (desc)
+                *desc = seg_desc;
         return X86EMUL_CONTINUE;
 exception:
         return emulate_exception(ctxt, err_vec, err_code, true);
@@ -1593,7 +1597,7 @@ static int load_segment_descriptor(struct x86_emulate_ctxt *ctxt,
                                    u16 selector, int seg)
 {
         u8 cpl = ctxt->ops->cpl(ctxt);
-        return __load_segment_descriptor(ctxt, selector, seg, cpl, false);
+        return __load_segment_descriptor(ctxt, selector, seg, cpl, false, NULL);
 }
 
 static void write_register_operand(struct operand *op)
@@ -1987,17 +1991,31 @@ static int em_iret(struct x86_emulate_ctxt *ctxt)
 static int em_jmp_far(struct x86_emulate_ctxt *ctxt)
 {
         int rc;
-        unsigned short sel;
+        unsigned short sel, old_sel;
+        struct desc_struct old_desc, new_desc;
+        const struct x86_emulate_ops *ops = ctxt->ops;
+        u8 cpl = ctxt->ops->cpl(ctxt);
+
+        /* Assignment of RIP may only fail in 64-bit mode */
+        if (ctxt->mode == X86EMUL_MODE_PROT64)
+                ops->get_segment(ctxt, &old_sel, &old_desc, NULL,
+                                 VCPU_SREG_CS);
 
         memcpy(&sel, ctxt->src.valptr + ctxt->op_bytes, 2);
 
-        rc = load_segment_descriptor(ctxt, sel, VCPU_SREG_CS);
+        rc = __load_segment_descriptor(ctxt, sel, VCPU_SREG_CS, cpl, false,
+                                       &new_desc);
         if (rc != X86EMUL_CONTINUE)
                 return rc;
 
-        ctxt->_eip = 0;
-        memcpy(&ctxt->_eip, ctxt->src.valptr, ctxt->op_bytes);
-        return X86EMUL_CONTINUE;
+        rc = assign_eip_far(ctxt, ctxt->src.val, new_desc.l);
+        if (rc != X86EMUL_CONTINUE) {
+                WARN_ON(!ctxt->mode != X86EMUL_MODE_PROT64);
+                /* assigning eip failed; restore the old cs */
+                ops->set_segment(ctxt, old_sel, &old_desc, 0, VCPU_SREG_CS);
+                return rc;
+        }
+        return rc;
 }
 
 static int em_grp45(struct x86_emulate_ctxt *ctxt)
@@ -2064,21 +2082,34 @@ static int em_ret(struct x86_emulate_ctxt *ctxt)
 static int em_ret_far(struct x86_emulate_ctxt *ctxt)
 {
         int rc;
-        unsigned long cs;
+        unsigned long eip, cs;
+        u16 old_cs;
         int cpl = ctxt->ops->cpl(ctxt);
+        struct desc_struct old_desc, new_desc;
+        const struct x86_emulate_ops *ops = ctxt->ops;
+
+        if (ctxt->mode == X86EMUL_MODE_PROT64)
+                ops->get_segment(ctxt, &old_cs, &old_desc, NULL,
+                                 VCPU_SREG_CS);
 
-        rc = emulate_pop(ctxt, &ctxt->_eip, ctxt->op_bytes);
+        rc = emulate_pop(ctxt, &eip, ctxt->op_bytes);
         if (rc != X86EMUL_CONTINUE)
                 return rc;
-        if (ctxt->op_bytes == 4)
-                ctxt->_eip = (u32)ctxt->_eip;
         rc = emulate_pop(ctxt, &cs, ctxt->op_bytes);
         if (rc != X86EMUL_CONTINUE)
                 return rc;
         /* Outer-privilege level return is not implemented */
         if (ctxt->mode >= X86EMUL_MODE_PROT16 && (cs & 3) > cpl)
                 return X86EMUL_UNHANDLEABLE;
-        rc = load_segment_descriptor(ctxt, (u16)cs, VCPU_SREG_CS);
+        rc = __load_segment_descriptor(ctxt, (u16)cs, VCPU_SREG_CS, 0, false,
+                                       &new_desc);
+        if (rc != X86EMUL_CONTINUE)
+                return rc;
+        rc = assign_eip_far(ctxt, eip, new_desc.l);
+        if (rc != X86EMUL_CONTINUE) {
+                WARN_ON(!ctxt->mode != X86EMUL_MODE_PROT64);
+                ops->set_segment(ctxt, old_cs, &old_desc, 0, VCPU_SREG_CS);
+        }
         return rc;
 }
 
@@ -2505,19 +2536,24 @@ static int load_state_from_tss16(struct x86_emulate_ctxt *ctxt,
          * Now load segment descriptors. If fault happens at this stage
          * it is handled in a context of new task
          */
-        ret = __load_segment_descriptor(ctxt, tss->ldt, VCPU_SREG_LDTR, cpl, true);
+        ret = __load_segment_descriptor(ctxt, tss->ldt, VCPU_SREG_LDTR, cpl,
+                                        true, NULL);
         if (ret != X86EMUL_CONTINUE)
                 return ret;
-        ret = __load_segment_descriptor(ctxt, tss->es, VCPU_SREG_ES, cpl, true);
+        ret = __load_segment_descriptor(ctxt, tss->es, VCPU_SREG_ES, cpl,
+                                        true, NULL);
         if (ret != X86EMUL_CONTINUE)
                 return ret;
-        ret = __load_segment_descriptor(ctxt, tss->cs, VCPU_SREG_CS, cpl, true);
+        ret = __load_segment_descriptor(ctxt, tss->cs, VCPU_SREG_CS, cpl,
+                                        true, NULL);
         if (ret != X86EMUL_CONTINUE)
                 return ret;
-        ret = __load_segment_descriptor(ctxt, tss->ss, VCPU_SREG_SS, cpl, true);
+        ret = __load_segment_descriptor(ctxt, tss->ss, VCPU_SREG_SS, cpl,
+                                        true, NULL);
         if (ret != X86EMUL_CONTINUE)
                 return ret;
-        ret = __load_segment_descriptor(ctxt, tss->ds, VCPU_SREG_DS, cpl, true);
+        ret = __load_segment_descriptor(ctxt, tss->ds, VCPU_SREG_DS, cpl,
+                                        true, NULL);
         if (ret != X86EMUL_CONTINUE)
                 return ret;
 
@@ -2642,25 +2678,32 @@ static int load_state_from_tss32(struct x86_emulate_ctxt *ctxt,
          * Now load segment descriptors. If fault happenes at this stage
          * it is handled in a context of new task
          */
-        ret = __load_segment_descriptor(ctxt, tss->ldt_selector, VCPU_SREG_LDTR, cpl, true);
+        ret = __load_segment_descriptor(ctxt, tss->ldt_selector, VCPU_SREG_LDTR,
+                                        cpl, true, NULL);
         if (ret != X86EMUL_CONTINUE)
                 return ret;
-        ret = __load_segment_descriptor(ctxt, tss->es, VCPU_SREG_ES, cpl, true);
+        ret = __load_segment_descriptor(ctxt, tss->es, VCPU_SREG_ES, cpl,
+                                        true, NULL);
         if (ret != X86EMUL_CONTINUE)
                 return ret;
-        ret = __load_segment_descriptor(ctxt, tss->cs, VCPU_SREG_CS, cpl, true);
+        ret = __load_segment_descriptor(ctxt, tss->cs, VCPU_SREG_CS, cpl,
+                                        true, NULL);
         if (ret != X86EMUL_CONTINUE)
                 return ret;
-        ret = __load_segment_descriptor(ctxt, tss->ss, VCPU_SREG_SS, cpl, true);
+        ret = __load_segment_descriptor(ctxt, tss->ss, VCPU_SREG_SS, cpl,
+                                        true, NULL);
         if (ret != X86EMUL_CONTINUE)
                 return ret;
-        ret = __load_segment_descriptor(ctxt, tss->ds, VCPU_SREG_DS, cpl, true);
+        ret = __load_segment_descriptor(ctxt, tss->ds, VCPU_SREG_DS, cpl,
+                                        true, NULL);
         if (ret != X86EMUL_CONTINUE)
                 return ret;
-        ret = __load_segment_descriptor(ctxt, tss->fs, VCPU_SREG_FS, cpl, true);
+        ret = __load_segment_descriptor(ctxt, tss->fs, VCPU_SREG_FS, cpl,
+                                        true, NULL);
         if (ret != X86EMUL_CONTINUE)
                 return ret;
-        ret = __load_segment_descriptor(ctxt, tss->gs, VCPU_SREG_GS, cpl, true);
+        ret = __load_segment_descriptor(ctxt, tss->gs, VCPU_SREG_GS, cpl,
+                                        true, NULL);
         if (ret != X86EMUL_CONTINUE)
                 return ret;
 
@@ -2942,24 +2985,39 @@ static int em_call_far(struct x86_emulate_ctxt *ctxt)
         u16 sel, old_cs;
         ulong old_eip;
         int rc;
+        struct desc_struct old_desc, new_desc;
+        const struct x86_emulate_ops *ops = ctxt->ops;
+        int cpl = ctxt->ops->cpl(ctxt);
 
-        old_cs = get_segment_selector(ctxt, VCPU_SREG_CS);
         old_eip = ctxt->_eip;
+        ops->get_segment(ctxt, &old_cs, &old_desc, NULL, VCPU_SREG_CS);
 
         memcpy(&sel, ctxt->src.valptr + ctxt->op_bytes, 2);
-        if (load_segment_descriptor(ctxt, sel, VCPU_SREG_CS))
+        rc = __load_segment_descriptor(ctxt, sel, VCPU_SREG_CS, cpl, false,
+                                       &new_desc);
+        if (rc != X86EMUL_CONTINUE)
                 return X86EMUL_CONTINUE;
 
-        ctxt->_eip = 0;
-        memcpy(&ctxt->_eip, ctxt->src.valptr, ctxt->op_bytes);
+        rc = assign_eip_far(ctxt, ctxt->src.val, new_desc.l);
+        if (rc != X86EMUL_CONTINUE)
+                goto fail;
 
         ctxt->src.val = old_cs;
         rc = em_push(ctxt);
         if (rc != X86EMUL_CONTINUE)
-                return rc;
+                goto fail;
 
         ctxt->src.val = old_eip;
-        return em_push(ctxt);
+        rc = em_push(ctxt);
+        /* If we failed, we tainted the memory, but the very least we should
+           restore cs */
+        if (rc != X86EMUL_CONTINUE)
+                goto fail;
+        return rc;
+fail:
+        ops->set_segment(ctxt, old_cs, &old_desc, 0, VCPU_SREG_CS);
+        return rc;
+
 }
 
 static int em_ret_near_imm(struct x86_emulate_ctxt *ctxt)