Re: [Xen-devel] [PATCH V5 1/4] x86/mm: Add array_index_nospec to guest provided index values

[Alexandru Stefan ISAILA <aisaila@xxxxxxxxxxxxxxx>]

On 20.12.2019 11:39, Jan Beulich wrote:
> On 20.12.2019 10:09, Alexandru Stefan ISAILA wrote:
>>
>>
>> On 19.12.2019 12:43, Jan Beulich wrote:
>>> On 19.12.2019 10:42, Alexandru Stefan ISAILA wrote:
>>>> This patch aims to sanitize indexes, potentially guest provided
>>>> values, for altp2m_eptp[] and altp2m_p2m[] arrays.
>>>>
>>>> Requested-by: Jan Beulich <jbeulich@xxxxxxxx>
>>>> Signed-off-by: Alexandru Isaila <aisaila@xxxxxxxxxxxxxxx>
>>>> ---
>>>> CC: Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>
>>>> CC: Tamas K Lengyel <tamas@xxxxxxxxxxxxx>
>>>> CC: Petre Pircalabu <ppircalabu@xxxxxxxxxxxxxxx>
>>>> CC: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
>>>> CC: Jan Beulich <jbeulich@xxxxxxxx>
>>>> CC: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>>>> CC: Wei Liu <wl@xxxxxxx>
>>>> CC: "Roger Pau Monné" <roger.pau@xxxxxxxxxx>
>>>> CC: Jun Nakajima <jun.nakajima@xxxxxxxxx>
>>>> CC: Kevin Tian <kevin.tian@xxxxxxxxx>
>>>> ---
>>>> Changes since V4:
>>>>    - Change bounds check from MAX_EPTP to MAX_ALTP2M
>>>>    - Move array_index_nospec() closer to the bounds check.
>>>> ---
>>>>    xen/arch/x86/mm/mem_access.c | 15 +++++++++------
>>>>    xen/arch/x86/mm/p2m.c        | 20 ++++++++++++++------
>>>>    2 files changed, 23 insertions(+), 12 deletions(-)
>>>>
>>>> diff --git a/xen/arch/x86/mm/mem_access.c b/xen/arch/x86/mm/mem_access.c
>>>> index 320b9fe621..33e379db8f 100644
>>>> --- 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 ||
>>
>> Ok, so have if ( altp2m_idx >=  min(ARRAY_SIZE(d->arch.altp2m_eptp),
>> MAX_EPTP) ||
>> here and then...
>>
>>>> -             d->arch.altp2m_eptp[altp2m_idx] == mfn_x(INVALID_MFN) )
>>>> +             d->arch.altp2m_eptp[array_index_nospec(altp2m_idx, 
>>>> MAX_ALTP2M)] ==
>>
>> have MAX_EPTP here and ...
>>
>>>
>>> As implied by a reply to v4, this is still latently buggy: There's
>>> no guarantee anyone will notice the issue here when bumping
>>> MAX_ALTP2M past MAX_EPTP. The only future proof thing to do here
>>> is, as suggested, using some form of min(MAX_ALTP2M, MAX_EPTP) in
>>> the actual bounds check. Then each array access itself can be made
>>> use the correct bound. In fact you'd probably have noticed this if
>>> you had made use of array_access_nospec() where possible (which
>>> also would help readability) - apparently not here, but ... >
>>>> +             mfn_x(INVALID_MFN) )
>>>>                return -EINVAL;
>>>>    
>>>> -        ap2m = d->arch.altp2m_p2m[altp2m_idx];
>>>> +        ap2m = d->arch.altp2m_p2m[array_index_nospec(altp2m_idx, 
>>>> MAX_ALTP2M)];
>>
>> MAX_ALTP2M here ?
> 
> Yes, that's how I think it ought to be. Give others a chance to
> disagree with me, though.
> 

There is a slight problem with using (ARRAY_SIZE(..)) it will give 
"error: static assertion failed:" on  __must_be_array(x) because 
d->arch.altp2m_eptp is not static.

Alex
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