Code:  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;
 	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;
	}
 	return rc;
 }

Vulnerability Type: +Info 
CWE ID: CWE-200
Summary: 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.
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>