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KVM: x86: Emulator fixes for eip canonical checks on near branches
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Before changing rip (during jmp, call, ret, etc.) the target should be asserted
to be canonical one, as real CPUs do.  During sysret, both target rsp and rip
should be canonical. If any of these values is noncanonical, a #GP exception
should occur.  The exception to this rule are syscall and sysenter instructions
in which the assigned rip is checked during the assignment to the relevant
MSRs.

This patch fixes the emulator to behave as real CPUs do for near branches.
Far branches are handled by the next patch.

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 05c83ec commit 234f3ce
Showing 1 changed file with 54 additions and 24 deletions.
78 changes: 54 additions & 24 deletions arch/x86/kvm/emulate.c
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Original file line number Diff line number Diff line change
Expand Up @@ -564,7 +564,8 @@ static int emulate_nm(struct x86_emulate_ctxt *ctxt)
return emulate_exception(ctxt, NM_VECTOR, 0, false);
}

static inline void assign_eip_near(struct x86_emulate_ctxt *ctxt, ulong dst)
static inline int assign_eip_far(struct x86_emulate_ctxt *ctxt, ulong dst,
int cs_l)
{
switch (ctxt->op_bytes) {
case 2:
Expand All @@ -574,16 +575,25 @@ static inline void assign_eip_near(struct x86_emulate_ctxt *ctxt, ulong dst)
ctxt->_eip = (u32)dst;
break;
case 8:
if ((cs_l && is_noncanonical_address(dst)) ||
(!cs_l && (dst & ~(u32)-1)))
return emulate_gp(ctxt, 0);
ctxt->_eip = dst;
break;
default:
WARN(1, "unsupported eip assignment size\n");
}
return X86EMUL_CONTINUE;
}

static inline int assign_eip_near(struct x86_emulate_ctxt *ctxt, ulong dst)
{
return assign_eip_far(ctxt, dst, ctxt->mode == X86EMUL_MODE_PROT64);
}

static inline void jmp_rel(struct x86_emulate_ctxt *ctxt, int rel)
static inline int jmp_rel(struct x86_emulate_ctxt *ctxt, int rel)
{
assign_eip_near(ctxt, ctxt->_eip + rel);
return assign_eip_near(ctxt, ctxt->_eip + rel);
}

static u16 get_segment_selector(struct x86_emulate_ctxt *ctxt, unsigned seg)
Expand Down Expand Up @@ -1998,13 +2008,15 @@ static int em_grp45(struct x86_emulate_ctxt *ctxt)
case 2: /* call near abs */ {
long int old_eip;
old_eip = ctxt->_eip;
ctxt->_eip = ctxt->src.val;
rc = assign_eip_near(ctxt, ctxt->src.val);
if (rc != X86EMUL_CONTINUE)
break;
ctxt->src.val = old_eip;
rc = em_push(ctxt);
break;
}
case 4: /* jmp abs */
ctxt->_eip = ctxt->src.val;
rc = assign_eip_near(ctxt, ctxt->src.val);
break;
case 5: /* jmp far */
rc = em_jmp_far(ctxt);
Expand Down Expand Up @@ -2039,10 +2051,14 @@ static int em_cmpxchg8b(struct x86_emulate_ctxt *ctxt)

static int em_ret(struct x86_emulate_ctxt *ctxt)
{
ctxt->dst.type = OP_REG;
ctxt->dst.addr.reg = &ctxt->_eip;
ctxt->dst.bytes = ctxt->op_bytes;
return em_pop(ctxt);
int rc;
unsigned long eip;

rc = emulate_pop(ctxt, &eip, ctxt->op_bytes);
if (rc != X86EMUL_CONTINUE)
return rc;

return assign_eip_near(ctxt, eip);
}

static int em_ret_far(struct x86_emulate_ctxt *ctxt)
Expand Down Expand Up @@ -2323,7 +2339,7 @@ static int em_sysexit(struct x86_emulate_ctxt *ctxt)
{
const struct x86_emulate_ops *ops = ctxt->ops;
struct desc_struct cs, ss;
u64 msr_data;
u64 msr_data, rcx, rdx;
int usermode;
u16 cs_sel = 0, ss_sel = 0;

Expand All @@ -2339,6 +2355,9 @@ static int em_sysexit(struct x86_emulate_ctxt *ctxt)
else
usermode = X86EMUL_MODE_PROT32;

rcx = reg_read(ctxt, VCPU_REGS_RCX);
rdx = reg_read(ctxt, VCPU_REGS_RDX);

cs.dpl = 3;
ss.dpl = 3;
ops->get_msr(ctxt, MSR_IA32_SYSENTER_CS, &msr_data);
Expand All @@ -2356,6 +2375,9 @@ static int em_sysexit(struct x86_emulate_ctxt *ctxt)
ss_sel = cs_sel + 8;
cs.d = 0;
cs.l = 1;
if (is_noncanonical_address(rcx) ||
is_noncanonical_address(rdx))
return emulate_gp(ctxt, 0);
break;
}
cs_sel |= SELECTOR_RPL_MASK;
Expand All @@ -2364,8 +2386,8 @@ static int em_sysexit(struct x86_emulate_ctxt *ctxt)
ops->set_segment(ctxt, cs_sel, &cs, 0, VCPU_SREG_CS);
ops->set_segment(ctxt, ss_sel, &ss, 0, VCPU_SREG_SS);

ctxt->_eip = reg_read(ctxt, VCPU_REGS_RDX);
*reg_write(ctxt, VCPU_REGS_RSP) = reg_read(ctxt, VCPU_REGS_RCX);
ctxt->_eip = rdx;
*reg_write(ctxt, VCPU_REGS_RSP) = rcx;

return X86EMUL_CONTINUE;
}
Expand Down Expand Up @@ -2905,10 +2927,13 @@ static int em_aad(struct x86_emulate_ctxt *ctxt)

static int em_call(struct x86_emulate_ctxt *ctxt)
{
int rc;
long rel = ctxt->src.val;

ctxt->src.val = (unsigned long)ctxt->_eip;
jmp_rel(ctxt, rel);
rc = jmp_rel(ctxt, rel);
if (rc != X86EMUL_CONTINUE)
return rc;
return em_push(ctxt);
}

Expand Down Expand Up @@ -2940,11 +2965,12 @@ static int em_call_far(struct x86_emulate_ctxt *ctxt)
static int em_ret_near_imm(struct x86_emulate_ctxt *ctxt)
{
int rc;
unsigned long eip;

ctxt->dst.type = OP_REG;
ctxt->dst.addr.reg = &ctxt->_eip;
ctxt->dst.bytes = ctxt->op_bytes;
rc = emulate_pop(ctxt, &ctxt->dst.val, ctxt->op_bytes);
rc = emulate_pop(ctxt, &eip, ctxt->op_bytes);
if (rc != X86EMUL_CONTINUE)
return rc;
rc = assign_eip_near(ctxt, eip);
if (rc != X86EMUL_CONTINUE)
return rc;
rsp_increment(ctxt, ctxt->src.val);
Expand Down Expand Up @@ -3271,20 +3297,24 @@ static int em_lmsw(struct x86_emulate_ctxt *ctxt)

static int em_loop(struct x86_emulate_ctxt *ctxt)
{
int rc = X86EMUL_CONTINUE;

register_address_increment(ctxt, reg_rmw(ctxt, VCPU_REGS_RCX), -1);
if ((address_mask(ctxt, reg_read(ctxt, VCPU_REGS_RCX)) != 0) &&
(ctxt->b == 0xe2 || test_cc(ctxt->b ^ 0x5, ctxt->eflags)))
jmp_rel(ctxt, ctxt->src.val);
rc = jmp_rel(ctxt, ctxt->src.val);

return X86EMUL_CONTINUE;
return rc;
}

static int em_jcxz(struct x86_emulate_ctxt *ctxt)
{
int rc = X86EMUL_CONTINUE;

if (address_mask(ctxt, reg_read(ctxt, VCPU_REGS_RCX)) == 0)
jmp_rel(ctxt, ctxt->src.val);
rc = jmp_rel(ctxt, ctxt->src.val);

return X86EMUL_CONTINUE;
return rc;
}

static int em_in(struct x86_emulate_ctxt *ctxt)
Expand Down Expand Up @@ -4743,7 +4773,7 @@ int x86_emulate_insn(struct x86_emulate_ctxt *ctxt)
break;
case 0x70 ... 0x7f: /* jcc (short) */
if (test_cc(ctxt->b, ctxt->eflags))
jmp_rel(ctxt, ctxt->src.val);
rc = jmp_rel(ctxt, ctxt->src.val);
break;
case 0x8d: /* lea r16/r32, m */
ctxt->dst.val = ctxt->src.addr.mem.ea;
Expand Down Expand Up @@ -4773,7 +4803,7 @@ int x86_emulate_insn(struct x86_emulate_ctxt *ctxt)
break;
case 0xe9: /* jmp rel */
case 0xeb: /* jmp rel short */
jmp_rel(ctxt, ctxt->src.val);
rc = jmp_rel(ctxt, ctxt->src.val);
ctxt->dst.type = OP_NONE; /* Disable writeback. */
break;
case 0xf4: /* hlt */
Expand Down Expand Up @@ -4898,7 +4928,7 @@ int x86_emulate_insn(struct x86_emulate_ctxt *ctxt)
break;
case 0x80 ... 0x8f: /* jnz rel, etc*/
if (test_cc(ctxt->b, ctxt->eflags))
jmp_rel(ctxt, ctxt->src.val);
rc = jmp_rel(ctxt, ctxt->src.val);
break;
case 0x90 ... 0x9f: /* setcc r/m8 */
ctxt->dst.val = test_cc(ctxt->b, ctxt->eflags);
Expand Down

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