Re: [Xen-devel] [PATCH V4 1/4] x86/mm: Add array_index_nospec to guest provided index values

[Alexandru Stefan ISAILA <aisaila@xxxxxxxxxxxxxxx>]

On 18.12.2019 10:06, Alexandru Stefan ISAILA wrote:
> 
> 
> On 17.12.2019 18:50, Jan Beulich wrote:
>> On 17.12.2019 16:12, Alexandru Stefan ISAILA wrote:
>>> --- a/xen/arch/x86/mm/mem_access.c
>>> +++ b/xen/arch/x86/mm/mem_access.c
>>> @@ -367,10 +367,11 @@ long p2m_set_mem_access(struct domain *d, gfn_t gfn, 
>>> uint32_t nr,
>>>        if ( altp2m_idx )
>>>        {
>>>            if ( altp2m_idx >= MAX_ALTP2M ||
>>> -             d->arch.altp2m_eptp[altp2m_idx] == mfn_x(INVALID_MFN) )
>>> +             d->arch.altp2m_eptp[array_index_nospec(altp2m_idx, MAX_EPTP)] 
>>> ==
>>
>> The bounds check is against MAX_ALTP2M. Both MAX_ values look to be
>> independent, which means bounds check and value passed to the
>> helper need to match up (not just here).
> 
> I will have both checks against MAX_ALTP2M.
> 
>>
>>> --- a/xen/arch/x86/mm/p2m-ept.c
>>> +++ b/xen/arch/x86/mm/p2m-ept.c
>>> @@ -1353,7 +1353,8 @@ void setup_ept_dump(void)
>>>    
>>>    void p2m_init_altp2m_ept(struct domain *d, unsigned int i)
>>>    {
>>> -    struct p2m_domain *p2m = d->arch.altp2m_p2m[i];
>>> +    struct p2m_domain *p2m =
>>> +           d->arch.altp2m_p2m[array_index_nospec(i, MAX_ALTP2M)];
>>>        struct p2m_domain *hostp2m = p2m_get_hostp2m(d);
>>>        struct ept_data *ept;
>>>    
>>> @@ -1366,7 +1367,7 @@ void p2m_init_altp2m_ept(struct domain *d, unsigned 
>>> int i)
>>>        p2m->max_mapped_pfn = p2m->max_remapped_gfn = 0;
>>>        ept = &p2m->ept;
>>>        ept->mfn = pagetable_get_pfn(p2m_get_pagetable(p2m));
>>> -    d->arch.altp2m_eptp[i] = ept->eptp;
>>> +    d->arch.altp2m_eptp[array_index_nospec(i, MAX_EPTP)] = ept->eptp;
>>>    }
>>>    
>>>    unsigned int p2m_find_altp2m_by_eptp(struct domain *d, uint64_t eptp)
>>> --- a/xen/arch/x86/mm/p2m.c
>>> +++ b/xen/arch/x86/mm/p2m.c
>>> @@ -2499,7 +2499,7 @@ static void p2m_reset_altp2m(struct domain *d, 
>>> unsigned int idx,
>>>        struct p2m_domain *p2m;
>>>    
>>>        ASSERT(idx < MAX_ALTP2M);
>>> -    p2m = d->arch.altp2m_p2m[idx];
>>> +    p2m = d->arch.altp2m_p2m[array_index_nospec(idx, MAX_ALTP2M)];
>>>    
>>>        p2m_lock(p2m);
>>>    
>>> @@ -2540,7 +2540,7 @@ static int p2m_activate_altp2m(struct domain *d, 
>>> unsigned int idx)
>>>    
>>>        ASSERT(idx < MAX_ALTP2M);
>>>    
>>> -    p2m = d->arch.altp2m_p2m[idx];
>>> +    p2m = d->arch.altp2m_p2m[array_index_nospec(idx, MAX_ALTP2M)];
>>
>> All of the above have a more or less significant disconnect between
>> the bounds check and the use as array index. I think it would be
>> quite helpful if these could live close to one another, so one can
>> (see further up) easily prove that both specified bounds actually
>> match up.
>>
> 
> Sure, I can move the array use closer together.
> 

Sorry to come back on this but I was looking in the code and I am not 
sure I follow where is the disconnect. If you are talking about 
p2m_init_altp2m_ept() the eptp code will move up in patch 3/4.

Can you please clarify?

Thanks,
Alex
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