Re: [PATCH 3.2 091/102] KVM: x86: Emulator fixes for eip canonical checks on near branches

[Nadav Amit]

Sorry, but this patch is incomplete due to a bug.
The following patch - http://www.spinics.net/lists/kvm/msg109664.html - is needed as well (on top of the previous one).

Nadav

> On Nov 2, 2014, at 00:28, Ben Hutchings <ben@xxxxxxxxxxxxxxx> wrote:
> 
> 3.2.64-rc1 review patch.  If anyone has any objections, please let me know.
> 
> ------------------
> 
> From: Nadav Amit <namit@xxxxxxxxxxxxxxxxx>
> 
> commit 234f3ce485d54017f15cf5e0699cff4100121601 upstream.
> 
> 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.
> 
> Signed-off-by: Nadav Amit <namit@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> [bwh: Backported to 3.2:
> - Adjust context
> - Use ctxt->regs[] instead of reg_read(), reg_write(), reg_rmw()]
> Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
> ---
> arch/x86/kvm/emulate.c | 78 ++++++++++++++++++++++++++++++++++----------------
> 1 file changed, 54 insertions(+), 24 deletions(-)
> 
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -529,7 +529,8 @@ static int emulate_nm(struct x86_emulate
> 	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:
> @@ -539,16 +540,25 @@ static inline void assign_eip_near(struc
> 		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)
> @@ -1787,13 +1797,15 @@ static int em_grp45(struct x86_emulate_c
> 	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);
> @@ -1825,10 +1837,14 @@ static int em_grp9(struct x86_emulate_ct
> 
> 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)
> @@ -2060,7 +2076,7 @@ static int em_sysexit(struct x86_emulate
> {
> 	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;
> 
> @@ -2076,6 +2092,9 @@ static int em_sysexit(struct x86_emulate
> 	else
> 		usermode = X86EMUL_MODE_PROT32;
> 
> +	rcx = ctxt->regs[VCPU_REGS_RCX];
> +	rdx = ctxt->regs[VCPU_REGS_RDX];
> +
> 	cs.dpl = 3;
> 	ss.dpl = 3;
> 	ops->get_msr(ctxt, MSR_IA32_SYSENTER_CS, &msr_data);
> @@ -2093,6 +2112,9 @@ static int em_sysexit(struct x86_emulate
> 		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;
> @@ -2101,8 +2123,8 @@ static int em_sysexit(struct x86_emulate
> 	ops->set_segment(ctxt, cs_sel, &cs, 0, VCPU_SREG_CS);
> 	ops->set_segment(ctxt, ss_sel, &ss, 0, VCPU_SREG_SS);
> 
> -	ctxt->_eip = ctxt->regs[VCPU_REGS_RDX];
> -	ctxt->regs[VCPU_REGS_RSP] = ctxt->regs[VCPU_REGS_RCX];
> +	ctxt->_eip = rdx;
> +	ctxt->regs[VCPU_REGS_RSP] = rcx;
> 
> 	return X86EMUL_CONTINUE;
> }
> @@ -2555,10 +2577,13 @@ static int em_das(struct x86_emulate_ctx
> 
> 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);
> }
> 
> @@ -2590,11 +2615,12 @@ static int em_call_far(struct x86_emulat
> 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;
> 	register_address_increment(ctxt, &ctxt->regs[VCPU_REGS_RSP], ctxt->src.val);
> @@ -2840,20 +2866,24 @@ static int em_lmsw(struct x86_emulate_ct
> 
> static int em_loop(struct x86_emulate_ctxt *ctxt)
> {
> +	int rc = X86EMUL_CONTINUE;
> +
> 	register_address_increment(ctxt, &ctxt->regs[VCPU_REGS_RCX], -1);
> 	if ((address_mask(ctxt, ctxt->regs[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, ctxt->regs[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_cli(struct x86_emulate_ctxt *ctxt)
> @@ -3946,7 +3976,7 @@ special_insn:
> 		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;
> @@ -3994,7 +4024,7 @@ special_insn:
> 		goto do_io_out;
> 	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 0xec: /* in al,dx */
> @@ -4160,7 +4190,7 @@ twobyte_insn:
> 		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);
> 

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