Re: [PATCH v10 1/9] mm: Introduce memfd_restricted system call to create restricted user memory

Nikunj A. Dadhania
2023-02-16T09:52:00Z

> +static struct file *restrictedmem_file_create(struct file *memfd)
> +{
> + struct restrictedmem_data *data;
> + struct address_space *mapping;
> + struct inode *inode;
> + struct file *file;
> +
> + data = kzalloc(sizeof(*data), GFP_KERNEL);
> + if (!data)
> + return ERR_PTR(-ENOMEM);
> +
> + data->memfd = memfd;
> + mutex_init(&data->lock);
> + INIT_LIST_HEAD(&data->notifiers);
> +
> + inode = alloc_anon_inode(restrictedmem_mnt->mnt_sb);
> + if (IS_ERR(inode)) {
> + kfree(data);
> + return ERR_CAST(inode);
> + }

alloc_anon_inode() uses new_pseudo_inode() to get the inode. As per the comment, new inode
is not added to the superblock s_inodes list.

/**
* new_inode_pseudo - obtain an inode
* @sb: superblock
*
* Allocates a new inode for given superblock.
* Inode wont be chained in superblock s_inodes list
* This means :
* - fs can't be unmount
* - quotas, fsnotify, writeback can't work
*/

So the restrictedmem_error_page will not find the inode as it was never added to the s_inodes list.

We might need to add the inode after allocating.

inode_sb_list_add(inode);

> +void restrictedmem_error_page(struct page *page, struct address_space *mapping)
> +{
> + struct super_block *sb = restrictedmem_mnt->mnt_sb;
> + struct inode *inode, *next;
> +
> + if (!shmem_mapping(mapping))
> + return;
> +
> + spin_lock(&sb->s_inode_list_lock);
> + list_for_each_entry_safe(inode, next, &sb->s_inodes, i_sb_list) {
> + struct restrictedmem_data *data = inode->i_mapping->private_data;
> + struct file *memfd = data->memfd;
> +
> + if (memfd->f_mapping == mapping) {
> + pgoff_t start, end;
> +
> + spin_unlock(&sb->s_inode_list_lock);
> +
> + start = page->index;
> + end = start + thp_nr_pages(page);
> + restrictedmem_notifier_error(data, start, end);
> + return;
> + }
> + }
> + spin_unlock(&sb->s_inode_list_lock);
> +}

Regards
Nikunj