Re: [PATCH] KVM: X86: Fix residual mmio emulation request to userspace

[Paolo Bonzini]

On 10/08/2017 16:09, Dmitry Vyukov wrote:
> On Thu, Aug 10, 2017 at 3:44 PM, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>> On 10/08/2017 07:33, Wanpeng Li wrote:
>>> Reported by syzkaller:
>>>
>>> The kvm-intel.unrestricted_guest=0
>>>
>>>    WARNING: CPU: 5 PID: 1014 at /home/kernel/data/kvm/arch/x86/kvm//x86.c:7227 kvm_arch_vcpu_ioctl_run+0x38b/0x1be0 [kvm]
>>>    CPU: 5 PID: 1014 Comm: warn_test Tainted: G        W  OE   4.13.0-rc3+ #8
>>>    RIP: 0010:kvm_arch_vcpu_ioctl_run+0x38b/0x1be0 [kvm]
>>>    Call Trace:
>>>     ? put_pid+0x3a/0x50
>>>     ? rcu_read_lock_sched_held+0x79/0x80
>>>     ? kmem_cache_free+0x2f2/0x350
>>>     kvm_vcpu_ioctl+0x340/0x700 [kvm]
>>>     ? kvm_vcpu_ioctl+0x340/0x700 [kvm]
>>>     ? __fget+0xfc/0x210
>>>     do_vfs_ioctl+0xa4/0x6a0
>>>     ? __fget+0x11d/0x210
>>>     SyS_ioctl+0x79/0x90
>>>     entry_SYSCALL_64_fastpath+0x23/0xc2
>>>     ? __this_cpu_preempt_check+0x13/0x20
>>>
>>> The syszkaller folks reported a residual mmio emulation request to userspace
>>> due to vm86 fails to emulate inject real mode interrupt(fails to read CS) and
>>> incurs a triple fault. The vCPU returns to userspace with vcpu->mmio_needed == true
>>> and KVM_EXIT_SHUTDOWN exit reason. However, the syszkaller testcase constructs
>>> several threads to launch the same vCPU, the thread which lauch this vCPU after
>>> the thread whichs get the vcpu->mmio_needed == true and KVM_EXIT_SHUTDOWN will
>>> trigger the warning.
>>>
>>>    #define _GNU_SOURCE
>>>    #include <pthread.h>
>>>    #include <stdio.h>
>>>    #include <stdlib.h>
>>>    #include <string.h>
>>>    #include <sys/wait.h>
>>>    #include <sys/types.h>
>>>    #include <sys/stat.h>
>>>    #include <sys/mman.h>
>>>    #include <fcntl.h>
>>>    #include <unistd.h>
>>>    #include <linux/kvm.h>
>>>    #include <stdio.h>
>>>
>>>    int kvmcpu;
>>>    struct kvm_run *run;
>>>
>>>    void* thr(void* arg)
>>>    {
>>>      int res;
>>>      res = ioctl(kvmcpu, KVM_RUN, 0);
>>>      printf("ret1=%d exit_reason=%d suberror=%d\n",
>>>          res, run->exit_reason, run->internal.suberror);
>>>      return 0;
>>>    }
>>>
>>>    void test()
>>>    {
>>>      int i, kvm, kvmvm;
>>>      pthread_t th[4];
>>>
>>>      kvm = open("/dev/kvm", O_RDWR);
>>>      kvmvm = ioctl(kvm, KVM_CREATE_VM, 0);
>>>      kvmcpu = ioctl(kvmvm, KVM_CREATE_VCPU, 0);
>>>      run = (struct kvm_run*)mmap(0, 4096, PROT_READ|PROT_WRITE, MAP_SHARED, kvmcpu, 0);
>>>      srand(getpid());
>>>      for (i = 0; i < 4; i++) {
>>>        pthread_create(&th[i], 0, thr, 0);
>>>        usleep(rand() % 10000);
>>>      }
>>>      for (i = 0; i < 4; i++)
>>>        pthread_join(th[i], 0);
>>>    }
>>>
>>>    int main()
>>>    {
>>>      for (;;) {
>>>        int pid = fork();
>>>        if (pid < 0)
>>>          exit(1);
>>>        if (pid == 0) {
>>>          test();
>>>          exit(0);
>>>        }
>>>        int status;
>>>        while (waitpid(pid, &status, __WALL) != pid) {}
>>>      }
>>>      return 0;
>>>    }
>>>
>>> This patch fixes it by resetting the vcpu->mmio_needed once we receive
>>> the triple fault to avoid the residue.
>>>
>>> Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
>>> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
>>> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
>>> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
>>> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>>> ---
>>>  arch/x86/kvm/vmx.c | 1 +
>>>  arch/x86/kvm/x86.c | 1 +
>>>  2 files changed, 2 insertions(+)
>>>
>>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>>> index 8e4a2dc..77ab10b 100644
>>> --- a/arch/x86/kvm/vmx.c
>>> +++ b/arch/x86/kvm/vmx.c
>>> @@ -5864,6 +5864,7 @@ static int handle_external_interrupt(struct kvm_vcpu *vcpu)
>>>  static int handle_triple_fault(struct kvm_vcpu *vcpu)
>>>  {
>>>       vcpu->run->exit_reason = KVM_EXIT_SHUTDOWN;
>>> +     vcpu->mmio_needed = 0;
>>>       return 0;
>>>  }
>>>
>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>>> index 72d82ab..1e143f7 100644
>>> --- a/arch/x86/kvm/x86.c
>>> +++ b/arch/x86/kvm/x86.c
>>> @@ -6776,6 +6776,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>>>               }
>>>               if (kvm_check_request(KVM_REQ_TRIPLE_FAULT, vcpu)) {
>>>                       vcpu->run->exit_reason = KVM_EXIT_SHUTDOWN;
>>> +                     vcpu->mmio_needed = 0;
>>>                       r = 0;
>>>                       goto out;
>>>               }
>>>
>>
>>
>> Queued, thanks.
> 
> Hi Paolo,
> 
> Where is it queued? I've checked
> git://git.kernel.org/pub/scm/virt/kvm/kvm.git 
> {next,master,fixes,queue} and can't find it.

I do run some tests before pushing. :)  Sometimes I don't have time the
same day, so I just run kvm-unit-tests and push to the queue branch,
which is rebased.  Sometimes I do, and it takes a few hours before it
ends up with a definitive SHA1 commit hash on the master and next branches.

Today it's the latter, so you'll find it in a couple hours if everything
goes according to the plan.

Paolo