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

[Dmitry Vyukov]

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.