Re: [RFC PATCH 05/25] fs: add ->s_master_keys to struct super_block

[Michael Halcrow <mhalcrow@google.com>]

On Mon, Oct 23, 2017 at 02:40:38PM -0700, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.com>
> 
> Add an ->s_master_keys keyring to 'struct super_block' for holding
> encryption keys which have been added to the filesystem.  This keyring
> will be populated using a new fscrypt ioctl.
> 
> This is needed for several reasons, including:
> 
> - To solve the visibility problems of having filesystem encryption keys
>   stored in process-subscribed keyrings, while the VFS state of the
>   filesystem is actually global.
> 
> - To implement a proper API for removing keys, which among other things
>   will require maintaining the list of inodes that are using each master
>   key so that we can evict the inodes when the key is removed.
> 
> - To allow caching a crypto transform for each master key so that we
>   don't have to repeatedly allocate one over and over.
> 
> See later patches for full details, including why it wouldn't be enough
> to add the concept of a "global keyring" to the keyrings API instead.
> 
> ->s_master_keys will only be allocated when someone tries to add a key
> for the first time.  Otherwise it will stay NULL.
> 
> Note that this could go in the filesystem-specific superblocks instead.
> However, we already have three filesystems using fs/crypto/, so it's
> useful to have it in the VFS.
> 
> Signed-off-by: Eric Biggers <ebiggers@google.com>

Reviewed-by: Michael Halcrow <mhalcrow@google.com>

> ---
>  fs/super.c         | 3 +++
>  include/linux/fs.h | 4 ++++
>  2 files changed, 7 insertions(+)
> 
> diff --git a/fs/super.c b/fs/super.c
> index 166c4ee0d0ed..161a9d05aa9f 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -168,6 +168,9 @@ static void destroy_super(struct super_block *s)
>  	WARN_ON(!list_empty(&s->s_mounts));
>  	put_user_ns(s->s_user_ns);
>  	kfree(s->s_subtype);
> +#if IS_ENABLED(CONFIG_FS_ENCRYPTION)
> +	key_put(s->s_master_keys);
> +#endif
>  	call_rcu(&s->rcu, destroy_super_rcu);
>  }
>  
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 3efd5ded21c9..8cfb0877d32c 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1440,6 +1440,10 @@ struct super_block {
>  
>  	spinlock_t		s_inode_wblist_lock;
>  	struct list_head	s_inodes_wb;	/* writeback inodes */
> +
> +#if IS_ENABLED(CONFIG_FS_ENCRYPTION)
> +	struct key		*s_master_keys; /* master crypto keys in use */
> +#endif
>  } __randomize_layout;
>  
>  /* Helper functions so that in most cases filesystems will
> -- 
> 2.15.0.rc0.271.g36b669edcc-goog
>