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

["Jarkko Sakkinen" <jarkko@kernel.org>]

On Tue Apr 16, 2024 at 1:15 PM EEST, Roberto Sassu wrote:
> On 4/15/2024 9:32 PM, Jarkko Sakkinen wrote:
> > On Mon Apr 15, 2024 at 5:24 PM EEST, Roberto Sassu wrote:
> >> From: Roberto Sassu <roberto.sassu@huawei.com>
> >>
> >> 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@huawei.com>
> >> ---
> >>   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@huawei.com>
> >> + *
> >> + * 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.
>
> Seems to make sense to return the right error. Will check why this one 
> (I probably took from somewhere).

Yeah, I mean often when I read legacy code from kernel and find
places like these I spend even few hours finding the root for
doing something like this so it really has value to do it when
the code is still fresh :-) Nothing wrong in the action itself
when it makes sense given the circumstances.

BR, Jarkko