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

[Dmitry Vyukov]

On Thu, Aug 10, 2017 at 7:33 AM, Wanpeng Li <kernellwp@xxxxxxxxx> 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.


Fixes the warning for me. Thanks for the quick fix!

Tested-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>


> 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;
>                 }
> --
> 2.7.4
>