On Thu, 2018-10-04 at 13:30 -0700, Matthew Garrett wrote:
> IMA wants to know what the hash of a file is, and currently does so by
> reading the entire file and generating the hash. Some filesystems may
> have the ability to store the hash in a secure manner resistant to
> offline attacks (eg, filesystem-level file signing), and in that case
> it's a performance win for IMA to be able to use that rather than having
> to re-hash everything. This patch simply adds VFS-level support for
> calling down to filesystems.
This patch description starts out saying that IMA needs the file hash
without explaining why. Without that explanation, simply extracting
the file hash included in the file signature might sound plausible,
but kind of defeats the purpose of IMA.
Mimi
>
> Signed-off-by: Matthew Garrett <mjg59@xxxxxxxxxx>
> ---
> fs/read_write.c | 24 ++++++++++++++++++++++++
> include/linux/fs.h | 6 +++++-
> 2 files changed, 29 insertions(+), 1 deletion(-)
>
> diff --git a/fs/read_write.c b/fs/read_write.c
> index 39b4a21dd933..9ba3ce4bb838 100644
> --- a/fs/read_write.c
> +++ b/fs/read_write.c
> @@ -2081,3 +2081,27 @@ int vfs_dedupe_file_range(struct file *file, struct file_dedupe_range *same)
> return ret;
> }
> EXPORT_SYMBOL(vfs_dedupe_file_range);
> +
> +/**
> + * vfs_gethash - obtain a file's hash
> + * @file: file structure in question
> + * @hash_algo: the hash algorithm requested
> + * @buf: buffer to return the hash in
> + * @size: size allocated for the buffer by the caller
> + *
> + * This function allows filesystems that support securely storing the hash
> + * of a file to return it rather than forcing the kernel to recalculate it.
> + * Filesystems that cannot provide guarantees about the hash being resistant
> + * to offline attack should not implement this functionality.
> + *
> + * Returns 0 on success, -EOPNOTSUPP if the filesystem doesn't support it.
> + */
> +int vfs_get_hash(struct file *file, enum hash_algo hash, uint8_t *buf,
> + size_t size)
> +{
> + if (!file->f_op->get_hash)
> + return -EOPNOTSUPP;
> +
> + return file->f_op->get_hash(file, hash, buf, size);
> +}
> +EXPORT_SYMBOL(vfs_get_hash);
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 6c0b4a1c22ff..540316cfd461 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -40,6 +40,7 @@
>
> #include <asm/byteorder.h>
> #include <uapi/linux/fs.h>
> +#include <uapi/linux/hash_info.h>
>
> struct backing_dev_info;
> struct bdi_writeback;
> @@ -1764,6 +1765,8 @@ struct file_operations {
> int (*dedupe_file_range)(struct file *, loff_t, struct file *, loff_t,
> u64);
> int (*fadvise)(struct file *, loff_t, loff_t, int);
> + int (*get_hash)(struct file *, enum hash_algo hash, uint8_t *buf,
> + size_t size);
> } __randomize_layout;
>
> struct inode_operations {
> @@ -1838,7 +1841,8 @@ extern int vfs_dedupe_file_range(struct file *file,
> extern int vfs_dedupe_file_range_one(struct file *src_file, loff_t src_pos,
> struct file *dst_file, loff_t dst_pos,
> u64 len);
> -
> +extern int vfs_get_hash(struct file *file, enum hash_algo hash, uint8_t *buf,
> + size_t size);
>
> struct super_operations {
> struct inode *(*alloc_inode)(struct super_block *sb);
