Re: [PATCH v5 01/13] mm/memfd: Introduce MFD_INACCESSIBLE flag

[Vishal Annapurve <vannapurve@google.com>]

On Thu, Mar 10, 2022 at 6:09 AM Chao Peng <chao.p.peng@linux.intel.com> wrote:
>
> From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
>
> 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@linux.intel.com>
> Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
> ---
>  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
>