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

["Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>]

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.

> > +	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,
> > +};
> > +
> 

-- 
  Kiryl Shutsemau / Kirill A. Shutemov