Re: [RFC PATCH 7/8] mm: secretmem: use AS_INACCESSIBLE to prohibit GUP

David Hildenbrand
2024-07-10T21:14:09Z

On 10.07.24 11:50, Patrick Roy wrote:
>
>
> On 7/10/24 08:32, Mike Rapoport wrote:
>> CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe.
>>
>>
>>
>> On Tue, Jul 09, 2024 at 11:09:29PM +0200, David Hildenbrand wrote:
>>> On 09.07.24 15:20, Patrick Roy wrote:
>>>> Inside of vma_is_secretmem and secretmem_mapping, instead of checking
>>>> whether a vm_area_struct/address_space has the secretmem ops structure
>>>> attached to it, check whether the address_space has the AS_INACCESSIBLE
>>>> bit set. Then set the AS_INACCESSIBLE flag for secretmem's
>>>> address_space.
>>>>
>>>> This means that get_user_pages and friends are disables for all
>>>> adress_spaces that set AS_INACCESIBLE. The AS_INACCESSIBLE flag was
>>>> introduced in commit c72ceafbd12c ("mm: Introduce AS_INACCESSIBLE for
>>>> encrypted/confidential memory") specifically for guest_memfd to indicate
>>>> that no reads and writes should ever be done to guest_memfd
>>>> address_spaces. Disallowing gup seems like a reasonable semantic
>>>> extension, and means that potential future mmaps of guest_memfd cannot
>>>> be GUP'd.
>>>>
>>>> Signed-off-by: Patrick Roy <roypat@amazon.co.uk>
>>>> ---
>>>> include/linux/secretmem.h | 13 +++++++++++--
>>>> mm/secretmem.c | 6 +-----
>>>> 2 files changed, 12 insertions(+), 7 deletions(-)
>>>>
>>>> diff --git a/include/linux/secretmem.h b/include/linux/secretmem.h
>>>> index e918f96881f5..886c8f7eb63e 100644
>>>> --- a/include/linux/secretmem.h
>>>> +++ b/include/linux/secretmem.h
>>>> @@ -8,10 +8,19 @@ extern const struct address_space_operations secretmem_aops;
>>>> static inline bool secretmem_mapping(struct address_space *mapping)
>>>> {
>>>> - return mapping->a_ops == &secretmem_aops;
>>>> + return mapping->flags & AS_INACCESSIBLE;
>>>> +}
>>>> +
>>>> +static inline bool vma_is_secretmem(struct vm_area_struct *vma)
>>>> +{
>>>> + struct file *file = vma->vm_file;
>>>> +
>>>> + if (!file)
>>>> + return false;
>>>> +
>>>> + return secretmem_mapping(file->f_inode->i_mapping);
>>>> }
>>>
>>> That sounds wrong. You should leave *secretmem alone and instead have
>>> something like inaccessible_mapping that is used where appropriate.
>>>
>>> vma_is_secretmem() should not suddenly succeed on something that is not
>>> mm/secretmem.c
>>
>> I'm with David here.
>>
>
> Right, that makes sense. My thinking here was that if memfd_secret and
> potential mappings of guest_memfd have the same behavior wrt GUP, then
> it makes sense to just have them rely on the same checks. But I guess I
> didn't follow that thought to its logical conclusion of renaming the
> "secretmem" checks into "inaccessible" checks and moving them out of
> secretmem.h.
>
> Or do you mean to just leave secretmem untouched and add separate
> "inaccessible" checks? But then we'd have two different ways of
> disabling GUP for specific VMAs that both rely on checks in exactly the
> same places :/

You can just replace the vma_is_secretmem in relevant places by checks
if inaccessible address spaces. No need for the additional
vma_is_secretmem check then.

BUT, as raised in my other reply, I wonder if adding support for
secretmem in KVM (I assume) would be simpler+cleaner.

>
>>> --
>>> Cheers,
>>>
>>> David / dhildenb
>>>
>>
>> --
>> Sincerely yours,
>> Mike.
>

--
Cheers,

David / dhildenb