On Thu, Mar 10, 2022 at 6:09 AM Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx> 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;
> };
>
> @@ -61,6 +65,9 @@ extern struct file *shmem_file_setup(const char *name,
> loff_t size, unsigned long flags);
> extern struct file *shmem_kernel_file_setup(const char *name, loff_t size,
> unsigned long flags);
> +extern struct file *shmem_file_setup_xflags(const char *name, loff_t size,
> + unsigned long flags,
> + unsigned int xflags);
> extern struct file *shmem_file_setup_with_mnt(struct vfsmount *mnt,
> const char *name, loff_t size, unsigned long flags);
> extern int shmem_zero_setup(struct vm_area_struct *);
> diff --git a/include/uapi/linux/memfd.h b/include/uapi/linux/memfd.h
> index 7a8a26751c23..48750474b904 100644
> --- a/include/uapi/linux/memfd.h
> +++ b/include/uapi/linux/memfd.h
> @@ -8,6 +8,7 @@
> #define MFD_CLOEXEC 0x0001U
> #define MFD_ALLOW_SEALING 0x0002U
> #define MFD_HUGETLB 0x0004U
> +#define MFD_INACCESSIBLE 0x0008U
>
> /*
> * Huge page size encoding when MFD_HUGETLB is specified, and a huge page
> diff --git a/mm/memfd.c b/mm/memfd.c
> index 9f80f162791a..74d45a26cf5d 100644
> --- a/mm/memfd.c
> +++ b/mm/memfd.c
> @@ -245,16 +245,20 @@ long memfd_fcntl(struct file *file, unsigned int cmd, unsigned long arg)
> #define MFD_NAME_PREFIX_LEN (sizeof(MFD_NAME_PREFIX) - 1)
> #define MFD_NAME_MAX_LEN (NAME_MAX - MFD_NAME_PREFIX_LEN)
>
> -#define MFD_ALL_FLAGS (MFD_CLOEXEC | MFD_ALLOW_SEALING | MFD_HUGETLB)
> +#define MFD_ALL_FLAGS (MFD_CLOEXEC | MFD_ALLOW_SEALING | MFD_HUGETLB | \
> + MFD_INACCESSIBLE)
>
> SYSCALL_DEFINE2(memfd_create,
> const char __user *, uname,
> unsigned int, flags)
> {
> + struct address_space *mapping;
> unsigned int *file_seals;
> + unsigned int xflags;
> struct file *file;
> int fd, error;
> char *name;
> + gfp_t gfp;
> long len;
>
> if (!(flags & MFD_HUGETLB)) {
> @@ -267,6 +271,10 @@ SYSCALL_DEFINE2(memfd_create,
> return -EINVAL;
> }
>
> + /* Disallow sealing when MFD_INACCESSIBLE is set. */
> + if (flags & MFD_INACCESSIBLE && flags & MFD_ALLOW_SEALING)
> + return -EINVAL;
> +
> /* length includes terminating zero */
> len = strnlen_user(uname, MFD_NAME_MAX_LEN + 1);
> if (len <= 0)
> @@ -301,8 +309,11 @@ SYSCALL_DEFINE2(memfd_create,
> HUGETLB_ANONHUGE_INODE,
> (flags >> MFD_HUGE_SHIFT) &
> MFD_HUGE_MASK);
Should hugetlbfs also be modified to be a backing store for private
memory like shmem when hugepages are to be used?
As of now, this series doesn't seem to support using private memfds
with backing hugepages.
> - } else
> - file = shmem_file_setup(name, 0, VM_NORESERVE);
> + } else {
> + xflags = flags & MFD_INACCESSIBLE ? SHM_F_INACCESSIBLE : 0;
> + file = shmem_file_setup_xflags(name, 0, VM_NORESERVE, xflags);
> + }
> +
> if (IS_ERR(file)) {
> error = PTR_ERR(file);
> goto err_fd;
> @@ -313,6 +324,15 @@ SYSCALL_DEFINE2(memfd_create,
> if (flags & MFD_ALLOW_SEALING) {
> file_seals = memfd_file_seals_ptr(file);
> *file_seals &= ~F_SEAL_SEAL;
> + } else if (flags & MFD_INACCESSIBLE) {
> + mapping = file_inode(file)->i_mapping;
> + gfp = mapping_gfp_mask(mapping);
> + gfp &= ~__GFP_MOVABLE;
> + mapping_set_gfp_mask(mapping, gfp);
> + mapping_set_unevictable(mapping);
> +
> + file_seals = memfd_file_seals_ptr(file);
> + *file_seals = F_SEAL_SEAL;
> }
>
> fd_install(fd, file);
> diff --git a/mm/shmem.c b/mm/shmem.c
> index a09b29ec2b45..9b31a7056009 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1084,6 +1084,13 @@ static int shmem_setattr(struct user_namespace *mnt_userns,
> (newsize > oldsize && (info->seals & F_SEAL_GROW)))
> return -EPERM;
>
> + if (info->xflags & SHM_F_INACCESSIBLE) {
> + if(oldsize)
> + return -EPERM;
> + if (!PAGE_ALIGNED(newsize))
> + return -EINVAL;
> + }
> +
> if (newsize != oldsize) {
> error = shmem_reacct_size(SHMEM_I(inode)->flags,
> oldsize, newsize);
> @@ -1331,6 +1338,8 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
> goto redirty;
> if (!total_swap_pages)
> goto redirty;
> + if (info->xflags & SHM_F_INACCESSIBLE)
> + goto redirty;
>
> /*
> * Our capabilities prevent regular writeback or sync from ever calling
> @@ -2228,6 +2237,9 @@ static int shmem_mmap(struct file *file, struct vm_area_struct *vma)
> if (ret)
> return ret;
>
> + if (info->xflags & SHM_F_INACCESSIBLE)
> + return -EPERM;
> +
> /* arm64 - allow memory tagging on RAM-based files */
> vma->vm_flags |= VM_MTE_ALLOWED;
>
> @@ -2433,6 +2445,8 @@ shmem_write_begin(struct file *file, struct address_space *mapping,
> if ((info->seals & F_SEAL_GROW) && pos + len > inode->i_size)
> return -EPERM;
> }
> + if (unlikely(info->xflags & SHM_F_INACCESSIBLE))
> + return -EPERM;
>
> ret = shmem_getpage(inode, index, pagep, SGP_WRITE);
>
> @@ -2517,6 +2531,21 @@ static ssize_t shmem_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
> end_index = i_size >> PAGE_SHIFT;
> if (index > end_index)
> break;
> +
> + /*
> + * inode_lock protects setting up seals as well as write to
> + * i_size. Setting SHM_F_INACCESSIBLE only allowed with
> + * i_size == 0.
> + *
> + * Check SHM_F_INACCESSIBLE after i_size. It effectively
> + * serialize read vs. setting SHM_F_INACCESSIBLE without
> + * taking inode_lock in read path.
> + */
> + if (SHMEM_I(inode)->xflags & SHM_F_INACCESSIBLE) {
> + error = -EPERM;
> + break;
> + }
> +
> if (index == end_index) {
> nr = i_size & ~PAGE_MASK;
> if (nr <= offset)
> @@ -2648,6 +2677,12 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset,
> goto out;
> }
>
> + if ((info->xflags & SHM_F_INACCESSIBLE) &&
> + (!PAGE_ALIGNED(offset) || !PAGE_ALIGNED(len))) {
> + error = -EINVAL;
> + goto out;
> + }
> +
> shmem_falloc.waitq = &shmem_falloc_waitq;
> shmem_falloc.start = (u64)unmap_start >> PAGE_SHIFT;
> shmem_falloc.next = (unmap_end + 1) >> PAGE_SHIFT;
> @@ -4082,6 +4117,28 @@ struct file *shmem_kernel_file_setup(const char *name, loff_t size, unsigned lon
> return __shmem_file_setup(shm_mnt, name, size, flags, S_PRIVATE);
> }
>
> +/**
> + * shmem_file_setup_xflags - get an unlinked file living in tmpfs with
> + * additional xflags.
> + * @name: name for dentry (to be seen in /proc/<pid>/maps
> + * @size: size to be set for the file
> + * @flags: VM_NORESERVE suppresses pre-accounting of the entire object size
> + * @xflags: SHM_F_INACCESSIBLE prevents ordinary MMU access to the file content
> + */
> +
> +struct file *shmem_file_setup_xflags(const char *name, loff_t size,
> + unsigned long flags, unsigned int xflags)
> +{
> + struct shmem_inode_info *info;
> + struct file *res = __shmem_file_setup(shm_mnt, name, size, flags, 0);
> +
> + if(!IS_ERR(res)) {
> + info = SHMEM_I(file_inode(res));
> + info->xflags = xflags & SHM_F_INACCESSIBLE;
> + }
> + return res;
> +}
> +
> /**
> * shmem_file_setup - get an unlinked file living in tmpfs
> * @name: name for dentry (to be seen in /proc/<pid>/maps
> --
> 2.17.1
>
