CVE-2014-3647 (published 2014-11-10T11:55Z)

arch/x86/kvm/emulate.c in the KVM subsystem in the Linux kernel through 3.17.2 does not properly perform RIP changes, which allows guest OS users to cause a denial of service (guest OS crash) via a crafted application.

Severity: LOW
CVSS v2 base score: 1.9 (AV:L/AC:M/Au:N/C:N/I:N/A:P)
CVSS v3 base score: 5.5, MEDIUM (CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H)

Weakness information:
cwe_id: NVD-CWE-noinfo
cwe_name: Insufficient Information
description: There is insufficient information about the issue to classify it; details are unkown or unspecified.
extended_description: Insufficient Information

Fix information:
repo_url: https://github.com/torvalds/linux
commit_details: repo_url: https://github.com/torvalds/linux
author: Nadav Amit
author_date: 2014-09-18 22:39:39+03:00
msg: 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>
repository: repo_url: https://github.com/torvalds/linux
repo_name: torvalds/linux
description: Linux kernel source tree
repo_language: C
owner: torvalds
file_changes: filename: emulate.c
change_type: ModificationType.MODIFY
diff: @@ -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)

programming_language: C++
repo_url: https://github.com/torvalds/linux
commit_details: repo_url: https://github.com/torvalds/linux
author: Nadav Amit
author_date: 2014-09-18 22:39:38+03:00
msg: KVM: x86: Emulator fixes for eip canonical checks on near branches

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>
repository: repo_url: https://github.com/torvalds/linux
repo_name: torvalds/linux
description: Linux kernel source tree
repo_language: C
owner: torvalds
file_changes: filename: emulate.c
change_type: ModificationType.MODIFY
diff: @@ -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:
@@ -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)
@@ -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);
@@ -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)
@@ -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;
 
@@ -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);
@@ -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;
@@ -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;
 }
@@ -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);
 }
 
@@ -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);
@@ -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)
@@ -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;
@@ -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 */
@@ -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);

programming_language: C++