Re: [PATCH v7 1/3] Add a new LSM-supporting anonymous inode interface

[Eric Biggers <ebiggers@kernel.org>]

On Fri, Aug 21, 2020 at 11:56:43AM -0700, Lokesh Gidra wrote:
> From: Daniel Colascione <dancol@google.com>
> 
> This change adds a new function, anon_inode_getfd_secure, that creates
> anonymous-node file with individual non-S_PRIVATE inode 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 function accepts an optional context_inode parameter that
> callers can use to provide additional contextual information to
> security modules for granting/denying permission to create an anon inode
> of the same type.
> 
> For example, in case of userfaultfd, the created inode is a
> 'logical child' of the context_inode (userfaultfd inode of the
> parent process) in the sense that it provides the security context
> required during creation of the child process' userfaultfd inode.
> 
> Signed-off-by: Daniel Colascione <dancol@google.com>
> 
> [Fix comment documenting return values of inode_init_security_anon()]
> [Add context_inode description in comments to anon_inode_getfd_secure()]
> [Remove definition of anon_inode_getfile_secure() as there are no callers]
> [Make _anon_inode_getfile() static]
> [Use correct error cast in _anon_inode_getfile()]
> 
> Signed-off-by: Lokesh Gidra <lokeshgidra@google.com>
> ---
>  fs/anon_inodes.c              | 148 ++++++++++++++++++++++++----------
>  include/linux/anon_inodes.h   |  13 +++
>  include/linux/lsm_hook_defs.h |   2 +
>  include/linux/lsm_hooks.h     |   7 ++
>  include/linux/security.h      |   3 +
>  security/security.c           |   9 +++
>  6 files changed, 141 insertions(+), 41 deletions(-)
> 
> diff --git a/fs/anon_inodes.c b/fs/anon_inodes.c
> index 89714308c25b..2aa8b57be895 100644
> --- a/fs/anon_inodes.c
> +++ b/fs/anon_inodes.c
> @@ -55,61 +55,78 @@ 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)
> +{
> +	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 inode;
> +	inode->i_flags &= ~S_PRIVATE;
> +	error =	security_inode_init_security_anon(
> +		inode, &qname, context_inode);

Weird indentation here.  The call to security_inode_init_security_anon() fits on
one line.

> +	if (error) {
> +		iput(inode);
> +		return ERR_PTR(error);
> +	}
> +	return inode;
> +}
> +
> +static struct file *_anon_inode_getfile(const char *name,
> +					const struct file_operations *fops,
> +					void *priv, int flags,
> +					const struct inode *context_inode,
> +					bool secure)
>  {
> +	struct inode *inode;
>  	struct file *file;
>  
> -	if (IS_ERR(anon_inode_inode))
> -		return ERR_PTR(-ENODEV);
> +	if (secure) {
> +		inode =	anon_inode_make_secure_inode(
> +			name, context_inode);

Likewise here.  The call to anon_inode_make_secure_inode() fits on one line.

> +		if (IS_ERR(inode))
> +			return ERR_CAST(inode);
> +	} else {
> +		inode =	anon_inode_inode;
> +		if (IS_ERR(inode))
> +			return ERR_PTR(-ENODEV);
> +		/*
> +		 * We know the anon_inode inode count is always
> +		 * greater than zero, so ihold() is safe.
> +		 */
> +		ihold(inode);
> +	}
>  
> -	if (fops->owner && !try_module_get(fops->owner))
> -		return ERR_PTR(-ENOENT);
> +	if (fops->owner && !try_module_get(fops->owner)) {
> +		file = ERR_PTR(-ENOENT);
> +		goto err;
> +	}

The error path here does module_put(fops->owner), even though a reference wasn't
acquired.

> +
> +/**
> + * anon_inode_getfd - 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 use the same singleton inode, reducing

This should say anon_inode_getfd(), not anon_inode_getfile().

> +/**
> + * Like anon_inode_getfd(), but adds the @context_inode argument to
> + * allow security modules to control creation of the new file. Once the
> + * security module makes the decision, this inode is no longer needed
> + * and hence reference to it is not held.
> + */
> +int anon_inode_getfd_secure(const char *name, const struct file_operations *fops,
> +			    void *priv, int flags,
> +			    const struct inode *context_inode)
> +{
> +	return _anon_inode_getfd(name, fops, priv, flags,
> +				 context_inode, true);
> +}

Weird indentation here again.  The call to _anon_inode_getfd() fits on one line.

> @@ -162,4 +229,3 @@ static int __init anon_inode_init(void)
>  }
>  
>  fs_initcall(anon_inode_init);
> -

Unnecessary whitespace change.

> diff --git a/include/linux/anon_inodes.h b/include/linux/anon_inodes.h
> index d0d7d96261ad..67bd85d92dca 100644
> --- a/include/linux/anon_inodes.h
> +++ b/include/linux/anon_inodes.h
> @@ -10,12 +10,25 @@
>  #define _LINUX_ANON_INODES_H
>  
>  struct file_operations;
> +struct inode;
> +
> +struct file *anon_inode_getfile_secure(const char *name,
> +				       const struct file_operations *fops,
> +				       void *priv, int flags,
> +				       const struct inode *context_inode);

This function isn't defined anywhere.

> + * @inode_init_security_anon:
> + *      Set up a secure anonymous inode.
> + *      @inode contains the inode structure
> + *      @name name of the anonymous inode class
> + *      @context_inode optional related inode
> + *	Returns 0 on success, -EACCESS if the security module denies the
> + *	creation of this inode, or another -errno upon other errors.

Is there a better name for this than "secure anonymous inode"?
(What is meant by "secure"?)

> diff --git a/include/linux/security.h b/include/linux/security.h
> index 0a0a03b36a3b..95c133a8f8bb 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -322,6 +322,9 @@ 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 inode *context_inode);
>  int security_old_inode_init_security(struct inode *inode, struct inode *dir,
>  				     const struct qstr *qstr, const char **name,
>  				     void **value, size_t *len);

This patch doesn't compile when !CONFIG_SECURITY because this file is missing a
!CONFIG_SECURITY stub for security_inode_init_security_anon().

> diff --git a/security/security.c b/security/security.c
> index 70a7ad357bc6..149b3f024e2d 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -1057,6 +1057,15 @@ 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 inode *context_inode)
> +{
> +	return call_int_hook(inode_init_security_anon, 0, inode, name,
> +			     context_inode);
> +}

Nit: everything else in this file has 'int' on the same line as the function
name.

- Eric