Re: kvm: WARNING in kvm_arch_vcpu_ioctl_run

[David Hildenbrand <david@redhat.com>]

On 09.08.2017 19:07, Dmitry Vyukov wrote:
> Hello,
> 
> syzkaller fuzzer has hit the following WARNING in kvm_arch_vcpu_ioctl_run.
> This is easily reproducible and reproducer is attached at the bottom.
> The report is on upstream commit
> 26c5cebfdb6ca799186f1e56be7d6f2480c5012c. This requires setting
> kvm-intel.unrestricted_guest=0 on the machine, with
> unrestricted_guest=1 the WARNING does not happen. Output of the
> program is:
> 
> ret1=0 exit_reason=17 suberror=1
> ret2=0 exit_reason=8 suberror=65530
> 
> 

I wonder if the following is possible:

1) we set vcpu->arch.pio.count != 0

2) we set vcpu->arch.complete_userspace_io = complete_emulated_pio;
(in x86_emulate_instruction)

3) in kvm_arch_vcpu_ioctl_run(), we execute
vcpu->arch.complete_userspace_io and set it to NULL.

4. complete_emulated_pio() calls complete_emulated_io(), which fails
(in some different way) and leaves vcpu->arch.pio.count set.

5. we now have vcpu->arch.pio.count set but
vcpu->arch.complete_userspace_io cleared.

Same should not apply to vcpu->mmio_needed, as we clear it before
calling complete_emulated_io() in complete_emulated_mmio().

AFAIK, we cannot simply clear vcpu->arch.pio.count before calling
complete_emulated_io().

Maybe something like this could help: (untested, wild guess)


diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 86242ee..60296fef 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -7227,10 +7227,14 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
                int (*cui)(struct kvm_vcpu *) = vcpu->arch.complete_userspace_io;
                vcpu->arch.complete_userspace_io = NULL;
                r = cui(vcpu);
+               if (!vcpu->arch.complete_userspace_io) {
+                       /* no new handler -> everything handled */
+                       vcpu->arch.pio.count = 0;
+                       vcpu->mmio_needed = 0;
+               }
                if (r <= 0)
                        goto out;
-       } else
-               WARN_ON(vcpu->arch.pio.count || vcpu->mmio_needed);
+       }
 
        if (kvm_run->immediate_exit)
                r = -EINTR;


Alternatively, something like this


diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 86242ee..58be388 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -7127,9 +7127,15 @@ static inline int complete_emulated_io(struct kvm_vcpu *vcpu)
 
 static int complete_emulated_pio(struct kvm_vcpu *vcpu)
 {
+       int ret;
+
        BUG_ON(!vcpu->arch.pio.count);
 
-       return complete_emulated_io(vcpu);
+       ret = complete_emulated_io(vcpu);
+       if (ret && !vcpu->arch.complete_userspace_io)
+               vcpu->arch.pio.count = 0;
+
+       return ret;
 }
 
 /*


Haven't tried to reproduce on upstream yet (at least on my Fedora 25
I can't trigger it), so only wild guesses.

-- 

Thanks,

David