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

[David Hildenbrand <david@redhat.com>]

On 15.09.22 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.

Initially my thaught was that this whole inaccessible thing is TDX 
specific and there is no need to force that on other mechanisms. That's 
why I suggested to not expose this to user space but handle the notifier 
requirements internally.

IIUC now, protected KVM has similar demands. Either access (read/write) 
of guest RAM would result in a fault and possibly crash the hypervisor 
(at least not the whole machine IIUC).

> 
> 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.

With secretmem we decided to not add such "concept switch" flags and 
instead use a dedicated syscall.

What about memfd_inaccessible()? Especially, sealing and hugetlb are not 
even supported and it might take a while to support either.


> 
> 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>
> ---
>   include/linux/memfd.h      |  24 ++++
>   include/uapi/linux/magic.h |   1 +
>   include/uapi/linux/memfd.h |   1 +
>   mm/Makefile                |   2 +-
>   mm/memfd.c                 |  25 ++++-
>   mm/memfd_inaccessible.c    | 219 +++++++++++++++++++++++++++++++++++++
>   6 files changed, 270 insertions(+), 2 deletions(-)
>   create mode 100644 mm/memfd_inaccessible.c
> 
> diff --git a/include/linux/memfd.h b/include/linux/memfd.h
> index 4f1600413f91..334ddff08377 100644
> --- a/include/linux/memfd.h
> +++ b/include/linux/memfd.h
> @@ -3,6 +3,7 @@
>   #define __LINUX_MEMFD_H
>   
>   #include <linux/file.h>
> +#include <linux/pfn_t.h>
>   
>   #ifdef CONFIG_MEMFD_CREATE
>   extern long memfd_fcntl(struct file *file, unsigned int cmd, unsigned long arg);
> @@ -13,4 +14,27 @@ static inline long memfd_fcntl(struct file *f, unsigned int c, unsigned long a)
>   }
>   #endif
>   
> +struct inaccessible_notifier;
> +
> +struct inaccessible_notifier_ops {
> +	void (*invalidate)(struct inaccessible_notifier *notifier,
> +			   pgoff_t start, pgoff_t end);
> +};
> +
> +struct inaccessible_notifier {
> +	struct list_head list;
> +	const struct inaccessible_notifier_ops *ops;
> +};
> +
> +void inaccessible_register_notifier(struct file *file,
> +				    struct inaccessible_notifier *notifier);
> +void inaccessible_unregister_notifier(struct file *file,
> +				      struct inaccessible_notifier *notifier);
> +
> +int inaccessible_get_pfn(struct file *file, pgoff_t offset, pfn_t *pfn,
> +			 int *order);
> +void inaccessible_put_pfn(struct file *file, pfn_t pfn);
> +
> +struct file *memfd_mkinaccessible(struct file *memfd);
> +
>   #endif /* __LINUX_MEMFD_H */
> diff --git a/include/uapi/linux/magic.h b/include/uapi/linux/magic.h
> index 6325d1d0e90f..9d066be3d7e8 100644
> --- a/include/uapi/linux/magic.h
> +++ b/include/uapi/linux/magic.h
> @@ -101,5 +101,6 @@
>   #define DMA_BUF_MAGIC		0x444d4142	/* "DMAB" */
>   #define DEVMEM_MAGIC		0x454d444d	/* "DMEM" */
>   #define SECRETMEM_MAGIC		0x5345434d	/* "SECM" */
> +#define INACCESSIBLE_MAGIC	0x494e4143	/* "INAC" */


[...]

> +
> +int inaccessible_get_pfn(struct file *file, pgoff_t offset, pfn_t *pfn,
> +			 int *order)
> +{
> +	struct inaccessible_data *data = file->f_mapping->private_data;
> +	struct file *memfd = data->memfd;
> +	struct page *page;
> +	int ret;
> +
> +	ret = shmem_getpage(file_inode(memfd), offset, &page, SGP_WRITE);
> +	if (ret)
> +		return ret;
> +
> +	*pfn = page_to_pfn_t(page);
> +	*order = thp_order(compound_head(page));
> +	SetPageUptodate(page);
> +	unlock_page(page);
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(inaccessible_get_pfn);
> +
> +void inaccessible_put_pfn(struct file *file, pfn_t pfn)
> +{
> +	struct page *page = pfn_t_to_page(pfn);
> +
> +	if (WARN_ON_ONCE(!page))
> +		return;
> +
> +	put_page(page);
> +}
> +EXPORT_SYMBOL_GPL(inaccessible_put_pfn);

Sorry, I missed your reply regarding get/put interface.

https://lore.kernel.org/linux-mm/20220810092532.GD862421@chaop.bj.intel.com/

"We have a design assumption that somedays this can even support 
non-page based backing stores."

As long as there is no such user in sight (especially how to get the 
memfd from even allocating such memory which will require bigger 
changes), I prefer to keep it simple here and work on pages/folios. No 
need to over-complicate it for now.


-- 
Thanks,

David / dhildenb