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

Patrick Roy
2024-07-10T09:50:43Z

On 7/10/24 08:32, Mike Rapoport wrote:
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> 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 :/

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