KVM: X86: Fix userspace set invalid CR4

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 valid CR4 and CPUID combination checking in __set_sregs.

syzkaller source: https://syzkaller.appspot.com/x/repro.c?x=138efb99600000

Reported-by: syzbot+0f1819555fbdce992df9@syzkaller.appspotmail.com
Cc: stable@vger.kernel.org
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
Reviewed-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -885,34 +885,42 @@ int kvm_set_xcr(struct kvm_vcpu *vcpu, u32 index, u64 xcr)
 }
 EXPORT_SYMBOL_GPL(kvm_set_xcr);
 
-int kvm_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
+static int kvm_valid_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
 {
-	unsigned long old_cr4 = kvm_read_cr4(vcpu);
-	unsigned long pdptr_bits = X86_CR4_PGE | X86_CR4_PSE | X86_CR4_PAE |
-				   X86_CR4_SMEP | X86_CR4_SMAP | X86_CR4_PKE;
-
 	if (cr4 & CR4_RESERVED_BITS)
-		return 1;
+		return -EINVAL;
 
 	if (!guest_cpuid_has(vcpu, X86_FEATURE_XSAVE) && (cr4 & X86_CR4_OSXSAVE))
-		return 1;
+		return -EINVAL;
 
 	if (!guest_cpuid_has(vcpu, X86_FEATURE_SMEP) && (cr4 & X86_CR4_SMEP))
-		return 1;
+		return -EINVAL;
 
 	if (!guest_cpuid_has(vcpu, X86_FEATURE_SMAP) && (cr4 & X86_CR4_SMAP))
-		return 1;
+		return -EINVAL;
 
 	if (!guest_cpuid_has(vcpu, X86_FEATURE_FSGSBASE) && (cr4 & X86_CR4_FSGSBASE))
-		return 1;
+		return -EINVAL;
 
 	if (!guest_cpuid_has(vcpu, X86_FEATURE_PKU) && (cr4 & X86_CR4_PKE))
-		return 1;
+		return -EINVAL;
 
 	if (!guest_cpuid_has(vcpu, X86_FEATURE_LA57) && (cr4 & X86_CR4_LA57))
-		return 1;
+		return -EINVAL;
 
 	if (!guest_cpuid_has(vcpu, X86_FEATURE_UMIP) && (cr4 & X86_CR4_UMIP))
+		return -EINVAL;
+
+	return 0;
+}
+
+int kvm_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
+{
+	unsigned long old_cr4 = kvm_read_cr4(vcpu);
+	unsigned long pdptr_bits = X86_CR4_PGE | X86_CR4_PSE | X86_CR4_PAE |
+				   X86_CR4_SMEP | X86_CR4_SMAP | X86_CR4_PKE;
+
+	if (kvm_valid_cr4(vcpu, cr4))
 		return 1;
 
 	if (is_long_mode(vcpu)) {
@@ -8714,10 +8722,6 @@ EXPORT_SYMBOL_GPL(kvm_task_switch);
 
 static int kvm_valid_sregs(struct kvm_vcpu *vcpu, struct kvm_sregs *sregs)
 {
-	if (!guest_cpuid_has(vcpu, X86_FEATURE_XSAVE) &&
-			(sregs->cr4 & X86_CR4_OSXSAVE))
-		return  -EINVAL;
-
 	if ((sregs->efer & EFER_LME) && (sregs->cr0 & X86_CR0_PG)) {
 		/*
 		 * When EFER.LME and CR0.PG are set, the processor is in
@@ -8736,7 +8740,7 @@ static int kvm_valid_sregs(struct kvm_vcpu *vcpu, struct kvm_sregs *sregs)
 			return -EINVAL;
 	}
 
-	return 0;
+	return kvm_valid_cr4(vcpu, sregs->cr4);
 }
 
 static int __set_sregs(struct kvm_vcpu *vcpu, struct kvm_sregs *sregs)