Re: [PATCH RFC v2 6/6] KVM: introduce a new API for getting dirty bitmaps

Alexander Graf
2010-04-23T10:20:31Z

On 23.04.2010, at 12:17, Fernando Luis Vázquez Cao wrote:

> On 04/23/2010 08:29 AM, Alexander Graf wrote:
>>
>> On 22.04.2010, at 08:09, Fernando Luis Vázquez Cao wrote:
>>
>>> On 04/22/2010 11:45 AM, Fernando Luis Vázquez Cao wrote:
>>>> On 04/21/2010 06:41 PM, Alexander Graf wrote:
>>>>> On 21.04.2010, at 10:29, Fernando Luis Vázquez Cao wrote:
>>>>>
>>>>>> On 04/20/2010 08:03 PM, Takuya Yoshikawa wrote:
>>>>>>> @@ -318,7 +318,7 @@ struct kvm_dirty_log {
>>>>>>> __u32 padding1;
>>>>>>> union {
>>>>>>> void __user *dirty_bitmap; /* one bit per page */
>>>>>>> - __u64 padding2;
>>>>>>> + __u64 addr;
>>>>>>
>>>>>> This can break on x86_32 and x86_64-compat. addr is a long not a __u64.
>>>>>
>>>>> So the high 32 bits are zero. Where's the problem?
>>>>
>>>> If we are careful enough to cast the addr appropriately we should be fine,
>>>> even if we keep the padding field in the union. I am not saying that it
>>>> breaks 32 architectures but that it can potentially be problematic.
>>>>
>>>>>>> + case KVM_SWITCH_DIRTY_LOG: {
>>>>>>> + struct kvm_dirty_log log;
>>>>>>> +
>>>>>>> + r = -EFAULT;
>>>>>>> + if (copy_from_user(&log, argp, sizeof log))
>>>>>>> + goto out;
>>>>>>> + r = kvm_vm_ioctl_switch_dirty_log(kvm, &log);
>>>>>>> + if (r)
>>>>>>> + goto out;
>>>>>>> + r = -EFAULT;
>>>>>>> + if (copy_to_user(argp, &log, sizeof log))
>>>>>>> + goto out;
>>>>>>> + r = 0;
>>>>>>> + break;
>>>>>>> + }
>>>>>>
>>>>>> In x86_64-compat mode we are handling 32bit user-space addresses
>>>>>> so we need the compat counterpart of KVM_SWITCH_DIRTY_LOG too.
>>>>>
>>>>> The compat code just forwards everything to the generic ioctls.
>>>>
>>>> The compat code uses struct compat_kvm_dirty_log instead of
>>>> struct kvm_dirty_log to communicate with user space so
>>>> the necessary conversions needs to be done before invoking
>>>> the generic ioctl (see KVM_GET_DIRTY_LOG in kvm_vm_compat_ioctl).
>>>>
>>>> By the way we probable should move the definition of struct
>>>> compat_kvm_dirty_log to a header file.
>>>
>>> It seems that it was you and Arnd who added the kvm_vm compat ioctl :-).
>>> Are you considering a different approach to tackle the issues that we
>>> have with a big-endian userspace?
>>
>> IIRC the issue was a pointer inside of a nested structure, no?
>
> I would say the reason is that if we did not convert the user-space pointer to
> a "void *" kvm_get_dirty_log() would end up copying the dirty log to
>
> (log->dirty_bitmap << 32) | 0x00000000

Well yes, that was the problem. If we always set the __u64 value to the pointer we're safe though.

union {
void *p;
__u64 q;
}

void x(void *r)
{
// breaks:
p = r;

// works:
q = (ulong)r;
}

Alex