[PATCH 1/1] KVM/x86: Check input paging mode when cs.l is set

[Lan Tianyu]

Reported by syzkaller:
    WARNING: CPU: 0 PID: 27962 at arch/x86/kvm/emulate.c:5631 x86_emulate_insn+0x557/0x15f0 [kvm]
    Modules linked in: kvm_intel kvm [last unloaded: kvm]
    CPU: 0 PID: 27962 Comm: syz-executor Tainted: G    B   W        4.15.0-rc2-next-20171208+ #32
    Hardware name: Intel Corporation S1200SP/S1200SP, BIOS S1200SP.86B.01.03.0006.040720161253 04/07/2016
    RIP: 0010:x86_emulate_insn+0x557/0x15f0 [kvm]
    RSP: 0018:ffff8807234476d0 EFLAGS: 00010282
    RAX: 0000000000000000 RBX: ffff88072d0237a0 RCX: ffffffffa0065c4d
    RDX: 1ffff100e5a046f9 RSI: 0000000000000003 RDI: ffff88072d0237c8
    RBP: ffff880723447728 R08: ffff88072d020000 R09: ffffffffa008d240
    R10: 0000000000000002 R11: ffffed00e7d87db3 R12: ffff88072d0237c8
    R13: ffff88072d023870 R14: ffff88072d0238c2 R15: ffffffffa008d080
    FS:  00007f8a68666700(0000) GS:ffff880802200000(0000) knlGS:0000000000000000
    CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
    CR2: 000000002009506c CR3: 000000071fec4005 CR4: 00000000003626f0
    Call Trace:
     x86_emulate_instruction+0x3bc/0xb70 [kvm]
     ? reexecute_instruction.part.162+0x130/0x130 [kvm]
     vmx_handle_exit+0x46d/0x14f0 [kvm_intel]
     ? trace_event_raw_event_kvm_entry+0xe7/0x150 [kvm]
     ? handle_vmfunc+0x2f0/0x2f0 [kvm_intel]
     ? wait_lapic_expire+0x25/0x270 [kvm]
     vcpu_enter_guest+0x720/0x1ef0 [kvm]
     ...

When cs.l is set, vcpu should run in the 64 bit paging mode.
Current kvm set_sregs function doesn't have such check when userspace
inputs sreg values. This will lead unexpected behavior. This patch
is to add such check.

Suggested-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Cc: Jim Mattson <jmattson@xxxxxxxxxx>
Signed-off-by: Tianyu Lan <tianyu.lan@xxxxxxxxx>
Signed-off-by: Lan Tianyu <tianyu.lan@xxxxxxxxx>
---
 arch/x86/kvm/x86.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 1e1e617..07d6d6c 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -7485,6 +7485,18 @@ int kvm_task_switch(struct kvm_vcpu *vcpu, u16 tss_selector, int idt_index,
 }
 EXPORT_SYMBOL_GPL(kvm_task_switch);
 
+int kvm_valid_sregs(struct kvm_vcpu *vcpu, struct kvm_sregs *sregs)
+{
+	/* When cs.l is set, vcpu should run in 64-bit mode */
+	if (sregs->cs.l)
+		if (!((sregs->cr0 & X86_CR0_PG_BIT) &&
+		      (sregs->cr4 & X86_CR4_PAE_BIT) &&
+		      (sregs->efer & EFER_LME)))
+			return -EINVAL;
+
+	return 0;
+}
+
 int kvm_arch_vcpu_ioctl_set_sregs(struct kvm_vcpu *vcpu,
 				  struct kvm_sregs *sregs)
 {
@@ -7497,6 +7509,9 @@ int kvm_arch_vcpu_ioctl_set_sregs(struct kvm_vcpu *vcpu,
 			(sregs->cr4 & X86_CR4_OSXSAVE))
 		return -EINVAL;
 
+	if (kvm_valid_sregs(vcpu, sregs))
+		return -EINVAL;
+
 	apic_base_msr.data = sregs->apic_base;
 	apic_base_msr.host_initiated = true;
 	if (kvm_set_apic_base(vcpu, &apic_base_msr))
-- 
1.8.3.1