[PATCH v2] KVM: VMX: Fix rflags cache during vCPU reset

From: Wanpeng Li
Date: Mon Nov 20 2017 - 17:53:52 EST

From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>

Reported by syzkaller:

*** Guest State ***
CR0: actual=0x0000000080010031, shadow=0x0000000060000010, gh_mask=fffffffffffffff7
CR4: actual=0x0000000000002061, shadow=0x0000000000000000, gh_mask=ffffffffffffe8f1
CR3 = 0x000000002081e000
RSP = 0x000000000000fffa RIP = 0x0000000000000000
RFLAGS=0x00023000 DR7 = 0x00000000000000
------------[ cut here ]------------
WARNING: CPU: 6 PID: 24431 at /home/kernel/linux/arch/x86/kvm//x86.c:7302 kvm_arch_vcpu_ioctl_run+0x651/0x2ea0 [kvm]
CPU: 6 PID: 24431 Comm: reprotest Tainted: G W OE 4.14.0+ #26
RIP: 0010:kvm_arch_vcpu_ioctl_run+0x651/0x2ea0 [kvm]
RSP: 0018:ffff880291d179e0 EFLAGS: 00010202
Call Trace:
kvm_vcpu_ioctl+0x479/0x880 [kvm]

The BUG is triggered while doing:

*(uint32_t*)0x20afd000 = (uint32_t)0xf0403;
*(uint32_t*)0x20afd004 = (uint32_t)0x0;
*(uint64_t*)0x20afd008 = (uint64_t)0x65;
*(uint64_t*)0x20afd010 = (uint64_t)0x5;
*(uint64_t*)0x20afd018 = (uint64_t)0xfffffffffffffffe;
*(uint64_t*)0x20afd020 = (uint64_t)0xfff;
*(uint64_t*)0x20afd038 = (uint64_t)0x2;
*(uint64_t*)0x20afd040 = (uint64_t)0x2;
r[15] = syscall(__NR_ioctl, r[4], 0x4048ae9bul, 0x20afd000ul);

i.e. KVM_SET_GUEST_DEBUG ioctl with

struct kvm_guest_debug {
.control = 0xf0403,
.pad = 0,
.arch = {
.debugreg[0] = 0x65,
.debugreg[1] = 0x5,
.debugreg[2] = 0xfffffffffffffffe,
.debugreg[3] = 0xfff,
.debugreg[6] = 0x2,
.debugreg[7] = 0x2

The syzkaller testcase tries to setup the processor specific debug registers
and configure vCPU for handling guest debug events through KVM_SET_GUEST_DEBUG.
The KVM_SET_GUEST_DEBUG ioctl will get and set rflags in order to set TF bit
if single step is needed. All regs' caches are reset to avail and GUEST_RFLAGS
vmcs field is reset to 0x2 during vCPU reset. However, the cache of rflags is
not reset during vCPU reset. The function vmx_get_rflags() returns an unreset
rflags cache value since the cache is marked avail, it is 0 after boot. Vmentry
fails if the rflags reserved bit 1 is 0.

This patch fixes it by resetting both the GUEST_RFLAGS vmcs field and its cache
to 0x2 during vCPU reset.

Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Tested-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
Cc: Nadav Amit <nadav.amit@xxxxxxxxx>
Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
arch/x86/kvm/vmx.c | 1 +
1 file changed, 1 insertion(+)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index b348920..131fa1c 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -5590,6 +5590,7 @@ static void vmx_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
vmcs_write64(GUEST_IA32_DEBUGCTL, 0);

+ kvm_set_rflags(vcpu, 2);
vmcs_writel(GUEST_RFLAGS, 0x02);
kvm_rip_write(vcpu, 0xfff0);