Re: [PATCH v8 1/8] mm/memfd: Introduce userspace inaccessible memfd

["Gupta, Pankaj" <pankaj.gupta@amd.com>]

On 10/17/2022 6:19 PM, Kirill A . Shutemov wrote:
> On Mon, Oct 17, 2022 at 03:00:21PM +0200, Vlastimil Babka wrote:
>> On 9/15/22 16:29, Chao Peng wrote:
>>> From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
>>>
>>> KVM can use memfd-provided memory for guest memory. For normal userspace
>>> accessible memory, KVM userspace (e.g. QEMU) mmaps the memfd into its
>>> virtual address space and then tells KVM to use the virtual address to
>>> setup the mapping in the secondary page table (e.g. EPT).
>>>
>>> With confidential computing technologies like Intel TDX, the
>>> memfd-provided memory may be encrypted with special key for special
>>> software domain (e.g. KVM guest) and is not expected to be directly
>>> accessed by userspace. Precisely, userspace access to such encrypted
>>> memory may lead to host crash so it should be prevented.
>>>
>>> This patch introduces userspace inaccessible memfd (created with
>>> MFD_INACCESSIBLE). Its memory is inaccessible from userspace through
>>> ordinary MMU access (e.g. read/write/mmap) but can be accessed via
>>> in-kernel interface so KVM can directly interact with core-mm without
>>> the need to map the memory into KVM userspace.
>>>
>>> It provides semantics required for KVM guest private(encrypted) memory
>>> support that a file descriptor with this flag set is going to be used as
>>> the source of guest memory in confidential computing environments such
>>> as Intel TDX/AMD SEV.
>>>
>>> KVM userspace is still in charge of the lifecycle of the memfd. It
>>> should pass the opened fd to KVM. KVM uses the kernel APIs newly added
>>> in this patch to obtain the physical memory address and then populate
>>> the secondary page table entries.
>>>
>>> The userspace inaccessible memfd can be fallocate-ed and hole-punched
>>> from userspace. When hole-punching happens, KVM can get notified through
>>> inaccessible_notifier it then gets chance to remove any mapped entries
>>> of the range in the secondary page tables.
>>>
>>> The userspace inaccessible memfd itself is implemented as a shim layer
>>> on top of real memory file systems like tmpfs/hugetlbfs but this patch
>>> only implemented tmpfs. The allocated memory is currently marked as
>>> unmovable and unevictable, this is required for current confidential
>>> usage. But in future this might be changed.
>>>
>>> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
>>> Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
>>> ---
>>
>> ...
>>
>>> +static long inaccessible_fallocate(struct file *file, int mode,
>>> +				   loff_t offset, loff_t len)
>>> +{
>>> +	struct inaccessible_data *data = file->f_mapping->private_data;
>>> +	struct file *memfd = data->memfd;
>>> +	int ret;
>>> +
>>> +	if (mode & FALLOC_FL_PUNCH_HOLE) {
>>> +		if (!PAGE_ALIGNED(offset) || !PAGE_ALIGNED(len))
>>> +			return -EINVAL;
>>> +	}
>>> +
>>> +	ret = memfd->f_op->fallocate(memfd, mode, offset, len);
>>> +	inaccessible_notifier_invalidate(data, offset, offset + len);
>>
>> Wonder if invalidate should precede the actual hole punch, otherwise we open
>> a window where the page tables point to memory no longer valid?
> 
> Yes, you are right. Thanks for catching this.

I also noticed this. But then thought the memory would be anyways zeroed 
(hole punched) before this call?

> 
>>> +	return ret;
>>> +}
>>> +
>>
>> ...
>>
>>> +
>>> +static struct file_system_type inaccessible_fs = {
>>> +	.owner		= THIS_MODULE,
>>> +	.name		= "[inaccessible]",
>>
>> Dunno where exactly is this name visible, but shouldn't it better be
>> "[memfd:inaccessible]"?
> 
> Maybe. And skip brackets.
> 
>>
>>> +	.init_fs_context = inaccessible_init_fs_context,
>>> +	.kill_sb	= kill_anon_super,
>>> +};
>>> +
>>
>