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

[Vlastimil Babka]

On 9/15/22 16:29, Chao Peng wrote:
> From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> 
> 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@xxxxxxxxxxxxxxx>
> Signed-off-by: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
> ---

...

> +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?

> +	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]"?

> +	.init_fs_context = inaccessible_init_fs_context,
> +	.kill_sb	= kill_anon_super,
> +};
> +