Re: [PATCH v2 1/3] Add a new LSM-supporting anonymous inode interface

[Stephen Smalley <sds@tycho.nsa.gov>]

On 3/25/20 7:02 PM, Daniel Colascione wrote:
> This change adds two new functions, anon_inode_getfile_secure and
> anon_inode_getfd_secure, that create anonymous-node files with
> individual non-S_PRIVATE inodes to which security modules can apply
> policy. Existing callers continue using the original singleton-inode
> kind of anonymous-inode file. We can transition anonymous inode users
> to the new kind of anonymous inode in individual patches for the sake
> of bisection and review.
> 
> The new functions accept an optional context_inode parameter that
> callers can use to provide additional contextual information to
> security modules, e.g., indicating that one anonymous struct file is a
> logical child of another, allowing a security model to propagate
> security information from one to the other.
> 
> Signed-off-by: Daniel Colascione <dancol@google.com>
> ---

> diff --git a/fs/anon_inodes.c b/fs/anon_inodes.c
> index 89714308c25b..114a04fc1db4 100644
> --- a/fs/anon_inodes.c
> +++ b/fs/anon_inodes.c
> @@ -55,75 +55,135 @@ static struct file_system_type anon_inode_fs_type = {
>   	.kill_sb	= kill_anon_super,
>   };
>   
> -/**
> - * anon_inode_getfile - creates a new file instance by hooking it up to an
> - *                      anonymous inode, and a dentry that describe the "class"
> - *                      of the file
> - *
> - * @name:    [in]    name of the "class" of the new file
> - * @fops:    [in]    file operations for the new file
> - * @priv:    [in]    private data for the new file (will be file's private_data)
> - * @flags:   [in]    flags
> - *
> - * Creates a new file by hooking it on a single inode. This is useful for files
> - * that do not need to have a full-fledged inode in order to operate correctly.
> - * All the files created with anon_inode_getfile() will share a single inode,
> - * hence saving memory and avoiding code duplication for the file/inode/dentry
> - * setup.  Returns the newly created file* or an error pointer.
> - */
> -struct file *anon_inode_getfile(const char *name,
> -				const struct file_operations *fops,
> -				void *priv, int flags)
> +static struct inode *anon_inode_make_secure_inode(
> +	const char *name,
> +	const struct inode *context_inode,
> +	const struct file_operations *fops)
> +{
> +	struct inode *inode;
> +	const struct qstr qname = QSTR_INIT(name, strlen(name));
> +	int error;
> +
> +	inode = alloc_anon_inode(anon_inode_mnt->mnt_sb);
> +	if (IS_ERR(inode))
> +		return ERR_PTR(PTR_ERR(inode));

Just return inode here?

> +	inode->i_flags &= ~S_PRIVATE;
> +	error =	security_inode_init_security_anon(
> +		inode, &qname, fops, context_inode);

Would drop fops argument to the security hook until we have an actual 
user of it; one can always add it later.


> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index 37df7c9eedb1..07b0f6e03849 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -1014,8 +1014,6 @@ static __poll_t userfaultfd_poll(struct file *file, poll_table *wait)
>   	}
>   }
>   
> -static const struct file_operations userfaultfd_fops;
> -
>   static int resolve_userfault_fork(struct userfaultfd_ctx *ctx,
>   				  struct userfaultfd_ctx *new,
>   				  struct uffd_msg *msg)
> @@ -1920,7 +1918,7 @@ static void userfaultfd_show_fdinfo(struct seq_file *m, struct file *f)
>   }
>   #endif
>   
> -static const struct file_operations userfaultfd_fops = {
> +const struct file_operations userfaultfd_fops = {
>   #ifdef CONFIG_PROC_FS
>   	.show_fdinfo	= userfaultfd_show_fdinfo,
>   #endif

These changes can be dropped entirely AFAICT.

> diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
> index 20d8cf194fb7..de5d37e388df 100644
> --- a/include/linux/lsm_hooks.h
> +++ b/include/linux/lsm_hooks.h
> @@ -215,6 +215,10 @@
>    *	Returns 0 if @name and @value have been successfully set,
>    *	-EOPNOTSUPP if no security attribute is needed, or
>    *	-ENOMEM on memory allocation failure.
> + * @inode_init_security_anon:
> + *      Set up a secure anonymous inode.
> + *	Returns 0 on success. Returns -EPERM if	the security module denies
> + *	the creation of this inode.

I'd favor describing the arguments (@name, @context_inode) too.

>    * @inode_create:
>    *	Check permission to create a regular file.
>    *	@dir contains inode structure of the parent of the new file.
> @@ -1552,6 +1556,10 @@ union security_list_options {
>   					const struct qstr *qstr,
>   					const char **name, void **value,
>   					size_t *len);
> +	int (*inode_init_security_anon)(struct inode *inode,
> +					const struct qstr *name,
> +					const struct file_operations *fops,
> +					const struct inode *context_inode);
>   	int (*inode_create)(struct inode *dir, struct dentry *dentry,

Can drop the fops argument.

> diff --git a/include/linux/security.h b/include/linux/security.h
> index 64b19f050343..8ea76af0be7a 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -320,6 +320,10 @@ void security_inode_free(struct inode *inode);
>   int security_inode_init_security(struct inode *inode, struct inode *dir,
>   				 const struct qstr *qstr,
>   				 initxattrs initxattrs, void *fs_data);
> +int security_inode_init_security_anon(struct inode *inode,
> +				      const struct qstr *name,
> +				      const struct file_operations *fops,
> +				      const struct inode *context_inode);

Ditto

> diff --git a/security/security.c b/security/security.c
> index 565bc9b67276..d06f3969c030 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -1033,6 +1033,16 @@ int security_inode_init_security(struct inode *inode, struct inode *dir,
>   }
>   EXPORT_SYMBOL(security_inode_init_security);
>   
> +int
> +security_inode_init_security_anon(struct inode *inode,
> +				  const struct qstr *name,
> +				  const struct file_operations *fops,
> +				  const struct inode *context_inode)
> +{
> +	return call_int_hook(inode_init_security_anon, 0, inode, name,
> +			     fops, context_inode);
> +}
> +

And again.