On 1/18/22 14:21, Chao Peng wrote:
> From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
>
> Introduce a new seal F_SEAL_INACCESSIBLE indicating the content of
> the file 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 seal 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.
>
> At this time only shmem implements this seal.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Signed-off-by: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
> ---
> include/uapi/linux/fcntl.h | 1 +
> mm/shmem.c | 40 ++++++++++++++++++++++++++++++++++++--
> 2 files changed, 39 insertions(+), 2 deletions(-)
>
> diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
> index 2f86b2ad6d7e..09ef34754dfa 100644
> --- a/include/uapi/linux/fcntl.h
> +++ b/include/uapi/linux/fcntl.h
> @@ -43,6 +43,7 @@
> #define F_SEAL_GROW 0x0004 /* prevent file from growing */
> #define F_SEAL_WRITE 0x0008 /* prevent writes */
> #define F_SEAL_FUTURE_WRITE 0x0010 /* prevent future writes while mapped */
> +#define F_SEAL_INACCESSIBLE 0x0020 /* prevent ordinary MMU access (e.g. read/write/mmap) to file content */
> /* (1U << 31) is reserved for signed error codes */
>
> /*
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 18f93c2d68f1..72185630e7c4 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1098,6 +1098,13 @@ static int shmem_setattr(struct user_namespace *mnt_userns,
> (newsize > oldsize && (info->seals & F_SEAL_GROW)))
> return -EPERM;
>
> + if (info->seals & F_SEAL_INACCESSIBLE) {
> + if(i_size_read(inode))
Is this needed? The rest of the function seems to trust oldsize obtained by
plain reading inode->i_size well enough, so why be suddenly paranoid here?
> + return -EPERM;
> + if (newsize & ~PAGE_MASK)
> + return -EINVAL;
> + }
> +
> if (newsize != oldsize) {
> error = shmem_reacct_size(SHMEM_I(inode)->flags,
> oldsize, newsize);
> @@ -1364,6 +1371,8 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
> goto redirty;
> if (!total_swap_pages)
> goto redirty;
> + if (info->seals & F_SEAL_INACCESSIBLE)
> + goto redirty;
>
> /*
> * Our capabilities prevent regular writeback or sync from ever calling
> @@ -2262,6 +2271,9 @@ static int shmem_mmap(struct file *file, struct vm_area_struct *vma)
> if (ret)
> return ret;
>
> + if (info->seals & F_SEAL_INACCESSIBLE)
> + return -EPERM;
> +
> /* arm64 - allow memory tagging on RAM-based files */
> vma->vm_flags |= VM_MTE_ALLOWED;
>
> @@ -2459,12 +2471,15 @@ shmem_write_begin(struct file *file, struct address_space *mapping,
> pgoff_t index = pos >> PAGE_SHIFT;
>
> /* i_rwsem is held by caller */
> - if (unlikely(info->seals & (F_SEAL_GROW |
> - F_SEAL_WRITE | F_SEAL_FUTURE_WRITE))) {
> + if (unlikely(info->seals & (F_SEAL_GROW | F_SEAL_WRITE |
> + F_SEAL_FUTURE_WRITE |
> + F_SEAL_INACCESSIBLE))) {
> if (info->seals & (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE))
> return -EPERM;
> if ((info->seals & F_SEAL_GROW) && pos + len > inode->i_size)
> return -EPERM;
> + if (info->seals & F_SEAL_INACCESSIBLE)
> + return -EPERM;
> }
>
> return shmem_getpage(inode, index, pagep, SGP_WRITE);
> @@ -2538,6 +2553,21 @@ 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 F_SEAL_INACCESSIBLE only allowed with
> + * i_size == 0.
> + *
> + * Check F_SEAL_INACCESSIBLE after i_size. It effectively
> + * serialize read vs. setting F_SEAL_INACCESSIBLE without
> + * taking inode_lock in read path.
> + */
> + if (SHMEM_I(inode)->seals & F_SEAL_INACCESSIBLE) {
> + error = -EPERM;
> + break;
> + }
> +
> if (index == end_index) {
> nr = i_size & ~PAGE_MASK;
> if (nr <= offset)
> @@ -2663,6 +2693,12 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset,
> goto out;
> }
>
> + if ((info->seals & F_SEAL_INACCESSIBLE) &&
> + (offset & ~PAGE_MASK || len & ~PAGE_MASK)) {
Could we use PAGE_ALIGNED()?
> + 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;
