[PATCH] KVM: X86: Fix warning in handle_desc

[Wanpeng Li <kernellwp@gmail.com>]

From: Wanpeng Li <wanpengli@tencent.com>

Reported by syzkaller:

	WARNING: CPU: 0 PID: 6544 at /home/kernel/data/kvm/arch/x86/kvm//vmx/vmx.c:4689 handle_desc+0x37/0x40 [kvm_intel]
	CPU: 0 PID: 6544 Comm: a.out Tainted: G           OE     5.3.0-rc4+ #4
	RIP: 0010:handle_desc+0x37/0x40 [kvm_intel]
	Call Trace:
	 vmx_handle_exit+0xbe/0x6b0 [kvm_intel]
	 vcpu_enter_guest+0x4dc/0x18d0 [kvm]
	 kvm_arch_vcpu_ioctl_run+0x407/0x660 [kvm]
	 kvm_vcpu_ioctl+0x3ad/0x690 [kvm]
	 do_vfs_ioctl+0xa2/0x690
	 ksys_ioctl+0x6d/0x80
	 __x64_sys_ioctl+0x1a/0x20
	 do_syscall_64+0x74/0x720
	 entry_SYSCALL_64_after_hwframe+0x49/0xbe

When CR4.UMIP is set, guest should have UMIP cpuid flag. Current 
kvm set_sregs function doesn't have such check when userspace inputs 
sregs values. SECONDARY_EXEC_DESC is enabled on writes to CR4.UMIP in 
vmx_set_cr4 though guest doesn't have UMIP cpuid flag. The testcast 
triggers handle_desc warning when executing ltr instruction since guest 
architectural CR4 doesn't set UMIP. This patch fixes it by adding check 
for guest UMIP cpuid flag when get sreg inputs from userspace.

Reported-by: syzbot+0f1819555fbdce992df9@syzkaller.appspotmail.com
Fixes: 0367f205a3b7 ("KVM: vmx: add support for emulating UMIP")
Cc: stable@vger.kernel.org
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
---
Note: syzbot report link https://lkml.org/lkml/2019/9/11/799

 arch/x86/kvm/x86.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index f7cfd8e..83288ba 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8645,6 +8645,10 @@ static int kvm_valid_sregs(struct kvm_vcpu *vcpu, struct kvm_sregs *sregs)
 			(sregs->cr4 & X86_CR4_OSXSAVE))
 		return  -EINVAL;
 
+	if (!guest_cpuid_has(vcpu, X86_FEATURE_UMIP) &&
+			(sregs->cr4 & X86_CR4_UMIP))
+		return -EINVAL;
+
 	if ((sregs->efer & EFER_LME) && (sregs->cr0 & X86_CR0_PG)) {
 		/*
 		 * When EFER.LME and CR0.PG are set, the processor is in
-- 
2.7.4