CVE-2016-9756: Exposure of Sensitive Information to an Unauthorized Actor (CWE-200)
value: arch/x86/kvm/emulate.c in the Linux kernel before 4.8.12 does not properly initialize Code Segment (CS) in certain error cases, which allows local users to obtain sensitive information from kernel stack memory via a crafted application.
CWE description: The product exposes sensitive information to an actor that is not explicitly authorized to have access to that information.
CVSS v3 base score: 5.5
Published: 2016-12-28T07:59Z
Repository: https://github.com/torvalds/linux
Language: C
Files changed: arch/x86/kvm/emulate.c
Commit date: 2016-11-24 18:36:54 +0100
Commit message: KVM: x86: drop error recovery in em_jmp_far and em_ret_far

em_jmp_far and em_ret_far assumed that setting IP can only fail in 64
bit mode, but syzkaller proved otherwise (and SDM agrees).
Code segment was restored upon failure, but it was left uninitialized
outside of long mode, which could lead to a leak of host kernel stack.
We could have fixed that by always saving and restoring the CS, but we
take a simpler approach and just break any guest that manages to fail
as the error recovery is error-prone and modern CPUs don't need emulator
for this.

Found by syzkaller:

  WARNING: CPU: 2 PID: 3668 at arch/x86/kvm/emulate.c:2217 em_ret_far+0x428/0x480
  Kernel panic - not syncing: panic_on_warn set ...

  CPU: 2 PID: 3668 Comm: syz-executor Not tainted 4.9.0-rc4+ #49
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
   [...]
  Call Trace:
   [...] __dump_stack lib/dump_stack.c:15
   [...] dump_stack+0xb3/0x118 lib/dump_stack.c:51
   [...] panic+0x1b7/0x3a3 kernel/panic.c:179
   [...] __warn+0x1c4/0x1e0 kernel/panic.c:542
   [...] warn_slowpath_null+0x2c/0x40 kernel/panic.c:585
   [...] em_ret_far+0x428/0x480 arch/x86/kvm/emulate.c:2217
   [...] em_ret_far_imm+0x17/0x70 arch/x86/kvm/emulate.c:2227
   [...] x86_emulate_insn+0x87a/0x3730 arch/x86/kvm/emulate.c:5294
   [...] x86_emulate_instruction+0x520/0x1ba0 arch/x86/kvm/x86.c:5545
   [...] emulate_instruction arch/x86/include/asm/kvm_host.h:1116
   [...] complete_emulated_io arch/x86/kvm/x86.c:6870
   [...] complete_emulated_mmio+0x4e9/0x710 arch/x86/kvm/x86.c:6934
   [...] kvm_arch_vcpu_ioctl_run+0x3b7a/0x5a90 arch/x86/kvm/x86.c:6978
   [...] kvm_vcpu_ioctl+0x61e/0xdd0 arch/x86/kvm/../../../virt/kvm/kvm_main.c:2557
   [...] vfs_ioctl fs/ioctl.c:43
   [...] do_vfs_ioctl+0x18c/0x1040 fs/ioctl.c:679
   [...] SYSC_ioctl fs/ioctl.c:694
   [...] SyS_ioctl+0x8f/0xc0 fs/ioctl.c:685
   [...] entry_SYSCALL_64_fastpath+0x1f/0xc2

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Cc: stable@vger.kernel.org
Fixes: d1442d85cc30 ("KVM: x86: Handle errors when RIP is set during far jumps")
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

Diff: diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index cbd7b92585bbba..a3ce9d260d6875 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -2103,35 +2103,27 @@ static int em_iret(struct x86_emulate_ctxt *ctxt)
 }
 
 static int em_jmp_far(struct x86_emulate_ctxt *ctxt)
 {
 	int rc;
-	unsigned short sel, old_sel;
-	struct desc_struct old_desc, new_desc;
-	const struct x86_emulate_ops *ops = ctxt->ops;
+	unsigned short sel;
+	struct desc_struct new_desc;
 	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, cpl,
 				       X86_TRANSFER_CALL_JMP,
 				       &new_desc);
 	if (rc != X86EMUL_CONTINUE)
 		return rc;
 
 	rc = assign_eip_far(ctxt, ctxt->src.val, &new_desc);
-	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;
-	}
+	/* Error handling is not implemented. */
+	if (rc != X86EMUL_CONTINUE)
+		return X86EMUL_UNHANDLEABLE;
+
 	return rc;
 }
 
 static int em_jmp_abs(struct x86_emulate_ctxt *ctxt)
 {
@@ -2187,18 +2179,12 @@ static int em_ret(struct x86_emulate_ctxt *ctxt)
 
 static int em_ret_far(struct x86_emulate_ctxt *ctxt)
 {
 	int rc;
 	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);
+	struct desc_struct new_desc;
 
 	rc = emulate_pop(ctxt, &eip, ctxt->op_bytes);
 	if (rc != X86EMUL_CONTINUE)
 		return rc;
 	rc = emulate_pop(ctxt, &cs, ctxt->op_bytes);
@@ -2211,14 +2197,14 @@ static int em_ret_far(struct x86_emulate_ctxt *ctxt)
 				       X86_TRANSFER_RET,
 				       &new_desc);
 	if (rc != X86EMUL_CONTINUE)
 		return rc;
 	rc = assign_eip_far(ctxt, eip, &new_desc);
-	if (rc != X86EMUL_CONTINUE) {
-		WARN_ON(ctxt->mode != X86EMUL_MODE_PROT64);
-		ops->set_segment(ctxt, old_cs, &old_desc, 0, VCPU_SREG_CS);
-	}
+	/* Error handling is not implemented. */
+	if (rc != X86EMUL_CONTINUE)
+		return X86EMUL_UNHANDLEABLE;
+
 	return rc;
 }
 
 static int em_ret_far_imm(struct x86_emulate_ctxt *ctxt)
 {

Vulnerable code: 	unsigned short sel, old_sel;
	struct desc_struct old_desc, new_desc;
	const struct x86_emulate_ops *ops = ctxt->ops;
	/* 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);

	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;
	}
	u16 old_cs;
	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);
	if (rc != X86EMUL_CONTINUE) {
		WARN_ON(ctxt->mode != X86EMUL_MODE_PROT64);
		ops->set_segment(ctxt, old_cs, &old_desc, 0, VCPU_SREG_CS);
	}
Fixed code: 	unsigned short sel;
	struct desc_struct new_desc;
	/* Error handling is not implemented. */
	if (rc != X86EMUL_CONTINUE)
		return X86EMUL_UNHANDLEABLE;

	struct desc_struct new_desc;
	/* Error handling is not implemented. */
	if (rc != X86EMUL_CONTINUE)
		return X86EMUL_UNHANDLEABLE;