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KVM: x86: Handle errors when RIP is set during far jumps
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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>
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@anadav @bonzini
anadav authored and bonzini committed Oct 24, 2014
1 parent 234f3ce commit d1442d8
Showing 1 changed file with 88 additions and 30 deletions.
118 changes: 88 additions & 30 deletions arch/x86/kvm/emulate.c
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Original file line number Diff line number Diff line change
Expand Up @@ -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;
Expand Down Expand Up @@ -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);
Expand All @@ -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)
Expand Down Expand Up @@ -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)
Expand Down Expand Up @@ -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;
}

Expand Down Expand Up @@ -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;

Expand Down Expand Up @@ -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;

Expand Down Expand Up @@ -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)
Expand Down

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