Re: [PATCH v2 1/3] LSM: Add new hook for generic node initialization

[Casey Schaufler <casey@schaufler-ca.com>]

On 1/9/2019 8:28 AM, Ondrej Mosnacek wrote:
> This patch introduces a new security hook that is intended for
> initializing the security data for newly created pseudo filesystem
> objects (such as kernfs nodes) that provide a way of storing a
> non-default security context, but need to operate independently from
> mounts.
>
> The main motivation is to allow kernfs nodes to inherit the context of
> the parent under SELinux, similar to the behavior of
> security_inode_init_security(). Other LSMs may implement their own logic
> for handling the creation of new nodes.
>
> Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
> ---
>  include/linux/lsm_hooks.h | 30 ++++++++++++++++++++++++++++++
>  include/linux/security.h  | 14 ++++++++++++++
>  security/security.c       | 10 ++++++++++
>  3 files changed, 54 insertions(+)
>
> diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
> index aaeb7fa24dc4..3a2399d7721f 100644
> --- a/include/linux/lsm_hooks.h
> +++ b/include/linux/lsm_hooks.h
> @@ -429,6 +429,31 @@
>   *	to abort the copy up. Note that the caller is responsible for reading
>   *	and writing the xattrs as this hook is merely a filter.
>   *
> + * Security hooks for special file-like objects
> + *
> + * @object_init_security:

I don't like the name. There are too many things that are "objects"
for this to be meaningful. I also dislike seeing names like
security_object_init_security. How about init_from_parent? If there's
never a chance that it will be used anywhere but with kernfs, it could
be kernfs_node_init. The existing set of hook names are sufficiently
confusing without adding to the mystery.

> + *	Obtain the security context for a newly created filesystem object
> + *	based on the security context of the parent node.  The purpose is
> + *	similar to @inode_init_security, but this hook is intended for
> + *	non-inode objects that need to behave like a directory tree (e.g.
> + *	kernfs nodes).  In this case it is assumed that the LSM assigns some
> + *	default context to the node by default and the object internally stores
> + *	a copy of the security context if (and only if) it has been set to a
> + *	non-default value explicitly (e.g. via *setxattr(2)).
> + *
> + *	@parent_ctx contains the security context of the parent directory
> + *	(must not be NULL -- if the parent has no explicit context set,
> + *	the child should also keep the default context and the hook should
> + *	not be called).
> + *	@parent_ctxlen contains the length of @parent_ctx data.
> + *	@qstr contains the last path component of the new object.
> + *	@mode contanis the file mode of the object.

s/contanis/contains/

> + *	@ctx is a pointer in which to place the allocated security context.
> + *	@ctxlen points to the place to put the length of @ctx.
> + *
> + *	Returns 0 if @ctx and @ctxlen have been successfully set or
> + *	-ENOMEM on memory allocation failure.
> + *
>   * Security hooks for file operations
>   *
>   * @file_permission:
> @@ -1556,6 +1581,10 @@ union security_list_options {
>  	int (*inode_copy_up)(struct dentry *src, struct cred **new);
>  	int (*inode_copy_up_xattr)(const char *name);
>  
> +	int (*object_init_security)(void *parent_ctx, u32 parent_ctxlen,
> +				    const struct qstr *qstr, u16 mode,
> +				    void **ctx, u32 *ctxlen);
> +
>  	int (*file_permission)(struct file *file, int mask);
>  	int (*file_alloc_security)(struct file *file);
>  	void (*file_free_security)(struct file *file);
> @@ -1855,6 +1884,7 @@ struct security_hook_heads {
>  	struct hlist_head inode_getsecid;
>  	struct hlist_head inode_copy_up;
>  	struct hlist_head inode_copy_up_xattr;
> +	struct hlist_head object_init_security;
>  	struct hlist_head file_permission;
>  	struct hlist_head file_alloc_security;
>  	struct hlist_head file_free_security;
> diff --git a/include/linux/security.h b/include/linux/security.h
> index d170a5b031f3..1e7971d10fe6 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -315,6 +315,9 @@ int security_inode_listsecurity(struct inode *inode, char *buffer, size_t buffer
>  void security_inode_getsecid(struct inode *inode, u32 *secid);
>  int security_inode_copy_up(struct dentry *src, struct cred **new);
>  int security_inode_copy_up_xattr(const char *name);
> +int security_object_init_security(void *parent_ctx, u32 parent_ctxlen,
> +				  const struct qstr *qstr, u16 mode,
> +				  void **ctx, u32 *ctxlen);
>  int security_file_permission(struct file *file, int mask);
>  int security_file_alloc(struct file *file);
>  void security_file_free(struct file *file);
> @@ -815,6 +818,17 @@ static inline int security_inode_copy_up_xattr(const char *name)
>  	return -EOPNOTSUPP;
>  }
>  
> +static inline int security_object_init_security(void *parent_ctx,
> +						u32 parent_ctxlen,
> +						const struct qstr *qstr,
> +						u16 mode, void **ctx,
> +						u32 *ctxlen)
> +{
> +	*ctx = NULL;
> +	*ctxlen = 0;
> +	return 0;
> +}
> +
>  static inline int security_file_permission(struct file *file, int mask)
>  {
>  	return 0;
> diff --git a/security/security.c b/security/security.c
> index 04d173eb93f6..a010bfbe3fc6 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -879,6 +879,16 @@ int security_inode_copy_up_xattr(const char *name)
>  }
>  EXPORT_SYMBOL(security_inode_copy_up_xattr);
>  
> +int security_object_init_security(void *parent_ctx, u32 parent_ctxlen,
> +				  const struct qstr *qstr, u16 mode,
> +				  void **ctx, u32 *ctxlen)
> +{
> +	*ctx = NULL;
> +	*ctxlen = 0;
> +	return call_int_hook(object_init_security, 0, parent_ctx, parent_ctxlen,
> +			     qstr, mode, ctx, ctxlen);
> +}
> +
>  int security_file_permission(struct file *file, int mask)
>  {
>  	int ret;