Re: [Xen-devel] [PATCH V6 2/4] x86/altp2m: Add hypercall to set a range of sve bits

[Alexandru Stefan ISAILA <aisaila@xxxxxxxxxxxxxxx>]

>>>> +/*
>>>> + * Set/clear the #VE suppress bit for multiple pages.  Only available on 
>>>> VMX.
>>>> + */
>>>> +int p2m_set_suppress_ve_multi(struct domain *d,
>>>> +                              struct xen_hvm_altp2m_suppress_ve_multi 
>>>> *sve)
>>>> +{
>>>> +    struct p2m_domain *host_p2m = p2m_get_hostp2m(d);
>>>> +    struct p2m_domain *ap2m = NULL;
>>>> +    struct p2m_domain *p2m = host_p2m;
>>>> +    uint64_t start = sve->first_gfn;
>>>> +    int rc = 0;
>>>> +
>>>> +    if ( sve->view > 0 )
>>>> +    {
>>>> +        if ( sve->view >= MAX_ALTP2M ||
>>>> +             d->arch.altp2m_eptp[array_index_nospec(sve->view, 
>>>> MAX_ALTP2M)] ==
>>>> +             mfn_x(INVALID_MFN) )
>>>> +            return -EINVAL;
>>>> +
>>>> +        p2m = ap2m = d->arch.altp2m_p2m[array_index_nospec(sve->view,
>>>> +                                                           MAX_ALTP2M)];
>>>> +    }
>>>> +
>>>> +    p2m_lock(host_p2m);
>>>> +
>>>> +    if ( ap2m )
>>>> +        p2m_lock(ap2m);
>>>> +
>>>> +    while ( sve->last_gfn >= start )
>>>> +    {
>>>> +        p2m_access_t a;
>>>> +        p2m_type_t t;
>>>> +        mfn_t mfn;
>>>> +        int err = 0;
>>>> +
>>>> +        if ( altp2m_get_effective_entry(p2m, _gfn(start), &mfn, &t, &a, 
>>>> AP2MGET_query) )
>>>> +            a = p2m->default_access;
>>>
>>> So in the single-entry version, if altp2m_get_effective_entry() returns
>>> an error, you pass that error up the stack; but in the multiple-entry
>>> version, you ignore the error and simply set the access to
>>> default_access?  I don't think that can be right.  If it is right, then
>>> it definitely needs a comment.
>>>
>>
>> The idea behind this was to have a best effort try and signal the first
>> error. If the get_entry fails then the best way to go is with
>> default_access but this is open for debate.
> 
> I don't see how it's a good idea at all. If get_effective_entry fails,
> then mfn and t may both be uninitialized.  If an attacker can arrange
> for those to have the values she wants, she could use this to take over
> the system.
> 
>> Another way to solve this is to update the first_error_gfn/first_error
>> and then continue. I think this ca be used to make p2m_set_suppress_ve()
>> call p2m_set_suppress_ve_multi.
> 
> Isn't that exactly the semantics you want -- try gfn N, if that fails,
> record it and move on to the next one?  Why would "write an entry with
> random values for mfn and type, but with the default access" be a better
> response?
> 

That is right, I'll go with this for the next version. Should I have the 
single version call the _multi version after this change?

Alex
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