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

[Ben Hutchings <ben@decadent.org.uk>]

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On Sun, 2014-11-02 at 10:44 +0200, Nadav Amit wrote:
> 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).

Thanks, I've added that (commit 7e46dddd6f6c).

Ben.

> Nadav
> 
> > On Nov 2, 2014, at 00:28, Ben Hutchings <ben@decadent.org.uk> wrote:
> > 
> > 3.2.64-rc1 review patch.  If anyone has any objections, please let me know.
> > 
> > ------------------
> > 
> > From: Nadav Amit <namit@cs.technion.ac.il>
> > 
> > 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@cs.technion.ac.il>
> > Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> > [bwh: Backported to 3.2:
> > - Adjust context
> > - Use ctxt->regs[] instead of reg_read(), reg_write(), reg_rmw()]
> > Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
> > ---
> > 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);
> > 
> 

-- 
Ben Hutchings
Power corrupts.  Absolute power is kind of neat.
                           - John Lehman, Secretary of the US Navy 1981-1987

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