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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 18.12.2019 10:57, Alexandru Stefan ISAILA wrote:
> 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/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.

My remark was about all four hunks left in context (and then still
possibly extending to other ones). Let's take the last one above:
p2m_activate_altp2m() has two callers, one of which loops over
altp2m-s (and hence doesn't need the guard). The other one is
p2m_init_altp2m_by_id() which does the range check I'm talking
about (ASSERT() doesn't count), and which therefore is the place
to use array_index_nospec(). Once you look there you'll notice
that the function also has an array access itself which you've
left untouched.

Jan

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