On Thu, Mar 10, 2022 at 10:08:59PM +0800, Chao Peng wrote:
> From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
>
> Introduce a new memfd_create() flag indicating the content of the
> created memfd is inaccessible from userspace through ordinary MMU
> access (e.g., read/write/mmap). However, the file content can be
> accessed via a different mechanism (e.g. KVM MMU) indirectly.
>
> It provides semantics required for KVM guest private 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 but may not be accessible from host userspace.
>
> Since page migration/swapping is not yet supported for such usages
> so these pages are currently marked as UNMOVABLE and UNEVICTABLE
> which makes them behave like long-term pinned pages.
>
> The flag can not coexist with MFD_ALLOW_SEALING, future sealing is
> also impossible for a memfd created with this flag.
>
> At this time only shmem implements this flag.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Signed-off-by: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
> ---
> include/linux/shmem_fs.h | 7 +++++
> include/uapi/linux/memfd.h | 1 +
> mm/memfd.c | 26 +++++++++++++++--
> mm/shmem.c | 57 ++++++++++++++++++++++++++++++++++++++
> 4 files changed, 88 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
> index e65b80ed09e7..2dde843f28ef 100644
> --- a/include/linux/shmem_fs.h
> +++ b/include/linux/shmem_fs.h
> @@ -12,6 +12,9 @@
>
> /* inode in-kernel data */
>
> +/* shmem extended flags */
> +#define SHM_F_INACCESSIBLE 0x0001 /* prevent ordinary MMU access (e.g. read/write/mmap) to file content */
> +
> struct shmem_inode_info {
> spinlock_t lock;
> unsigned int seals; /* shmem seals */
> @@ -24,6 +27,7 @@ struct shmem_inode_info {
> struct shared_policy policy; /* NUMA memory alloc policy */
> struct simple_xattrs xattrs; /* list of xattrs */
> atomic_t stop_eviction; /* hold when working on inode */
> + unsigned int xflags; /* shmem extended flags */
> struct inode vfs_inode;
> };
>
AFAICS, only two bits of 'flags' are used. And that's very strange that
VM_ flags are used for the purpose. My guess that someone was lazy to
introduce new constants for this.
I think we should fix this: introduce SHM_F_LOCKED and SHM_F_NORESERVE
alongside with SHM_F_INACCESSIBLE and stuff them all into info->flags.
It also makes shmem_file_setup_xflags() go away.
--
Kirill A. Shutemov
