KVM: VMX: Fix rflags cache during vCPU reset

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]
    do_vfs_ioctl+0x142/0x9a0
    SyS_ioctl+0x74/0x80
    entry_SYSCALL_64_fastpath+0x23/0x9a

The failed vmentry is triggered by the following beautified testcase:

    #include <unistd.h>
    #include <sys/syscall.h>
    #include <string.h>
    #include <stdint.h>
    #include <linux/kvm.h>
    #include <fcntl.h>
    #include <sys/ioctl.h>

    long r[5];
    int main()
    {
        struct kvm_debugregs dr = { 0 };

        r[2] = open("/dev/kvm", O_RDONLY);
        r[3] = ioctl(r[2], KVM_CREATE_VM, 0);
        r[4] = ioctl(r[3], KVM_CREATE_VCPU, 7);
        struct kvm_guest_debug debug = {
                .control = 0xf0403,
                .arch = {
                        .debugreg[6] = 0x2,
                        .debugreg[7] = 0x2
                }
        };
        ioctl(r[4], KVM_SET_GUEST_DEBUG, &debug);
        ioctl(r[4], KVM_RUN, 0);
    }

which 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 <[email protected]>
Tested-by: Dmitry Vyukov <[email protected]>
Reviewed-by: David Hildenbrand <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Cc: Radim Krčmář <[email protected]>
Cc: Nadav Amit <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Signed-off-by: Wanpeng Li <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>

arch/x86/kvm/vmx.c

@@ -5602,7 +5602,7 @@ static void vmx_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
 		vmcs_write64(GUEST_IA32_DEBUGCTL, 0);
 	}
 
-	vmcs_writel(GUEST_RFLAGS, 0x02);
+	kvm_set_rflags(vcpu, X86_EFLAGS_FIXED);
 	kvm_rip_write(vcpu, 0xfff0);
 
 	vmcs_writel(GUEST_GDTR_BASE, 0);