Re: [PATCH v4 03/14] digest_cache: Add securityfs interface

[Jarkko Sakkinen]

On Mon Apr 15, 2024 at 5:24 PM EEST, Roberto Sassu wrote:
> From: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
>
> Add the digest_cache_path file in securityfs, to let root change/read the
> default path (file or directory) from where digest lists are looked up.
>
> An RW semaphore prevents the default path from changing while
> digest_list_new() and read_default_path() are executed, so that those read
> a stable value. Multiple digest_list_new() and read_default_path() calls,
> instead, can be done in parallel, since they are the readers.
>
> Changing the default path does not affect digest caches created with the
> old path.
>
> Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> ---
>  security/digest_cache/Kconfig    |  4 ++
>  security/digest_cache/Makefile   |  2 +-
>  security/digest_cache/internal.h |  1 +
>  security/digest_cache/main.c     | 10 +++-
>  security/digest_cache/secfs.c    | 87 ++++++++++++++++++++++++++++++++
>  5 files changed, 102 insertions(+), 2 deletions(-)
>  create mode 100644 security/digest_cache/secfs.c
>
> diff --git a/security/digest_cache/Kconfig b/security/digest_cache/Kconfig
> index e53fbf0779d6..dfabe5d6e3ca 100644
> --- a/security/digest_cache/Kconfig
> +++ b/security/digest_cache/Kconfig
> @@ -14,3 +14,7 @@ config DIGEST_LIST_DEFAULT_PATH
>  	default "/etc/digest_lists"
>  	help
>  	  Default directory where digest_cache LSM expects to find digest lists.
> +
> +	  It can be changed at run-time, by writing the new path to the
> +	  securityfs interface. Digest caches created with the old path are
> +	  not affected by the change.
> diff --git a/security/digest_cache/Makefile b/security/digest_cache/Makefile
> index 48848c41253e..1330655e33b1 100644
> --- a/security/digest_cache/Makefile
> +++ b/security/digest_cache/Makefile
> @@ -4,4 +4,4 @@
>  
>  obj-$(CONFIG_SECURITY_DIGEST_CACHE) += digest_cache.o
>  
> -digest_cache-y := main.o
> +digest_cache-y := main.o secfs.o
> diff --git a/security/digest_cache/internal.h b/security/digest_cache/internal.h
> index 5f04844af3a5..bbf5eefe5c82 100644
> --- a/security/digest_cache/internal.h
> +++ b/security/digest_cache/internal.h
> @@ -49,6 +49,7 @@ struct digest_cache_security {
>  
>  extern struct lsm_blob_sizes digest_cache_blob_sizes;
>  extern char *default_path_str;
> +extern struct rw_semaphore default_path_sem;
>  
>  static inline struct digest_cache_security *
>  digest_cache_get_security(const struct inode *inode)
> diff --git a/security/digest_cache/main.c b/security/digest_cache/main.c
> index 14dba8915e99..661c8d106791 100644
> --- a/security/digest_cache/main.c
> +++ b/security/digest_cache/main.c
> @@ -18,6 +18,9 @@ static struct kmem_cache *digest_cache_cache __read_mostly;
>  
>  char *default_path_str = CONFIG_DIGEST_LIST_DEFAULT_PATH;
>  
> +/* Protects default_path_str. */
> +struct rw_semaphore default_path_sem;
> +
>  /**
>   * digest_cache_alloc_init - Allocate and initialize a new digest cache
>   * @path_str: Path string of the digest list
> @@ -274,9 +277,12 @@ struct digest_cache *digest_cache_get(struct dentry *dentry)
>  
>  	/* Serialize accesses to inode for which the digest cache is used. */
>  	mutex_lock(&dig_sec->dig_user_mutex);
> -	if (!dig_sec->dig_user)
> +	if (!dig_sec->dig_user) {
> +		down_read(&default_path_sem);
>  		/* Consume extra reference from digest_cache_create(). */
>  		dig_sec->dig_user = digest_cache_new(dentry);
> +		up_read(&default_path_sem);
> +	}
>  
>  	if (dig_sec->dig_user)
>  		/* Increment ref. count for reference returned to the caller. */
> @@ -386,6 +392,8 @@ static const struct lsm_id digest_cache_lsmid = {
>   */
>  static int __init digest_cache_init(void)
>  {
> +	init_rwsem(&default_path_sem);
> +
>  	digest_cache_cache = kmem_cache_create("digest_cache_cache",
>  					       sizeof(struct digest_cache),
>  					       0, SLAB_PANIC,
> diff --git a/security/digest_cache/secfs.c b/security/digest_cache/secfs.c
> new file mode 100644
> index 000000000000..d3a37bf3588e
> --- /dev/null
> +++ b/security/digest_cache/secfs.c
> @@ -0,0 +1,87 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright (C) 2023-2024 Huawei Technologies Duesseldorf GmbH
> + *
> + * Author: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> + *
> + * Implement the securityfs interface of the digest_cache LSM.
> + */
> +
> +#define pr_fmt(fmt) "DIGEST CACHE: "fmt
> +#include <linux/security.h>
> +
> +#include "internal.h"
> +
> +static struct dentry *default_path_dentry;
> +
> +/**
> + * write_default_path - Write default path
> + * @file: File descriptor of the securityfs file
> + * @buf: User space buffer
> + * @datalen: Amount of data to write
> + * @ppos: Current position in the file
> + *
> + * This function sets the new default path where digest lists can be found.
> + * Can be either a regular file or a directory.
> + *
> + * Return: Length of path written on success, a POSIX error code otherwise.
> + */
> +static ssize_t write_default_path(struct file *file, const char __user *buf,
> +				  size_t datalen, loff_t *ppos)
> +{
> +	char *new_default_path_str;
> +
> +	new_default_path_str = memdup_user_nul(buf, datalen);
> +	if (IS_ERR(new_default_path_str))
> +		return PTR_ERR(new_default_path_str);
> +
> +	down_write(&default_path_sem);
> +	kfree_const(default_path_str);
> +	default_path_str = new_default_path_str;
> +	up_write(&default_path_sem);
> +	return datalen;
> +}
> +
> +/**
> + * read_default_path - Read default path
> + * @file: File descriptor of the securityfs file
> + * @buf: User space buffer
> + * @datalen: Amount of data to read
> + * @ppos: Current position in the file
> + *
> + * This function returns the current default path where digest lists can be
> + * found. Can be either a regular file or a directory.
> + *
> + * Return: Length of path read on success, a POSIX error code otherwise.
> + */
> +static ssize_t read_default_path(struct file *file, char __user *buf,
> +				 size_t datalen, loff_t *ppos)
> +{
> +	int ret;
> +
> +	down_read(&default_path_sem);
> +	ret = simple_read_from_buffer(buf, datalen, ppos, default_path_str,
> +				      strlen(default_path_str) + 1);
> +	up_read(&default_path_sem);
> +	return ret;
> +}
> +
> +static const struct file_operations default_path_ops = {
> +	.open = generic_file_open,
> +	.write = write_default_path,
> +	.read = read_default_path,
> +	.llseek = generic_file_llseek,
> +};
> +
> +static int __init digest_cache_path_init(void)
> +{
> +	default_path_dentry = securityfs_create_file("digest_cache_path", 0660,
> +						     NULL, NULL,
> +						     &default_path_ops);
> +	if (IS_ERR(default_path_dentry))
> +		return -EFAULT;

Nit: when overwriting error value with another error value it would be
best to document it with an inline comment. Otherwise, it is fine.

> +
> +	return 0;
> +}
> +
> +late_initcall(digest_cache_path_init);


BR, Jarkko