Re: [PATCH RFC 4/4] mm: guest_memfd: Add ability for mmap'ing pages

[David Hildenbrand <david@redhat.com>]

On 15.08.24 09:24, Fuad Tabba wrote:
> Hi David,

Hi!

> 
> On Tue, 6 Aug 2024 at 14:51, David Hildenbrand <david@redhat.com> wrote:
>>
>>>
>>> -     if (gmem_flags & GUEST_MEMFD_FLAG_NO_DIRECT_MAP) {
>>> +     if (!ops->accessible && (gmem_flags & GUEST_MEMFD_FLAG_NO_DIRECT_MAP)) {
>>>                r = guest_memfd_folio_private(folio);
>>>                if (r)
>>>                        goto out_err;
>>> @@ -107,6 +109,82 @@ struct folio *guest_memfd_grab_folio(struct file *file, pgoff_t index, u32 flags
>>>    }
>>>    EXPORT_SYMBOL_GPL(guest_memfd_grab_folio);
>>>
>>> +int guest_memfd_make_inaccessible(struct file *file, struct folio *folio)
>>> +{
>>> +     unsigned long gmem_flags = (unsigned long)file->private_data;
>>> +     unsigned long i;
>>> +     int r;
>>> +
>>> +     unmap_mapping_folio(folio);
>>> +
>>> +     /**
>>> +      * We can't use the refcount. It might be elevated due to
>>> +      * guest/vcpu trying to access same folio as another vcpu
>>> +      * or because userspace is trying to access folio for same reason
>>
>> As discussed, that's insufficient. We really have to drive the refcount
>> to 1 -- the single reference we expect.
>>
>> What is the exact problem you are running into here? Who can just grab a
>> reference and maybe do nasty things with it?
> 
> I was wondering, why do we need to check the refcount? Isn't it enough
> to check for page_mapped() || page_maybe_dma_pinned(), while holding
> the folio lock?

(folio_mapped() + folio_maybe_dma_pinned())

Not everything goes trough FOLL_PIN. vmsplice() is an example, or just 
some very simple read/write through /proc/pid/mem. Further, some 
O_DIRECT implementations still don't use FOLL_PIN.

So if you see an additional folio reference, as soon as you mapped that 
thing to user space, you have to assume that it could be someone 
reading/writing that memory in possibly sane context. (vmsplice() should 
be using FOLL_PIN|FOLL_LONGTERM, but that's a longer discussion)

(noting that also folio_maybe_dma_pinned() can have false positives in 
some cases due to speculative references or *many* references).

-- 
Cheers,

David / dhildenb