Re: [PATCH v4 02/14] security: Introduce the digest_cache LSM

[Roberto Sassu]

On Mon, 2024-04-15 at 22:31 +0300, Jarkko Sakkinen wrote:
> On Mon Apr 15, 2024 at 5:24 PM EEST, Roberto Sassu wrote:
> > From: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> > 
> > Introduce the digest_cache LSM, to collect digests from various sources
> > (called digest lists), and to store them in kernel memory, in a set of hash
> > tables forming a digest cache. Extracted digests can be used as reference
> > values for integrity verification of file data or metadata.
> > 
> > A digest cache has three types of references: in the inode security blob of
> > the digest list the digest cache was created from (dig_owner field); in the
> > security blob of the inodes for which the digest cache is requested
> > (dig_user field); a reference returned by digest_cache_get().
> > 
> > References are released with digest_cache_put(), in the first two cases
> > when inodes are evicted from memory, in the last case when that function is
> > explicitly called. Obtaining a digest cache reference means that the digest
> > cache remains valid and cannot be freed until releasing it and until the
> > total number of references (stored in the digest cache) becomes zero.
> > 
> > When digest_cache_get() is called on an inode to compare its digest with
> > a reference value, the digest_cache LSM knows which digest cache to get
> > from the new security.digest_list xattr added to that inode, which contains
> > the file name of the desired digest list digests will be extracted from.
> > 
> > All digest lists are expected to be in the same directory, defined in the
> > kernel config, and modifiable (with a later patch) at run-time through
> > securityfs. When the digest_cache LSM reads the security.digest_list xattr,
> > it uses its value as last path component, appended to the default path
> > (unless the default path is a file). If an inode does not have that xattr,
> > the default path is considered as the final destination.
> > 
> > The default path can be either a file or a directory. If it is a file, the
> > digest_cache LSM always uses the same digest cache from that file to verify
> > all inodes (the xattr, if present, is ignored). If it is a directory, and
> > the inode to verify does not have the xattr, a subsequent patch will make
> > it possible to iterate and lookup on the digest caches created from each
> > directory entry.
> > 
> > Digest caches are created on demand, only when digest_cache_get() is
> > called. The first time a digest cache is requested, the digest_cache LSM
> > creates it and sets its reference in the dig_owner and dig_user fields of
> > the respective inode security blobs. On the next requests, the previously
> > set reference is returned, after incrementing the reference count.
> > 
> > Since there might be multiple digest_cache_get() calls for the same inode,
> > or for different inodes pointing to the same digest list, dig_owner_mutex
> > and dig_user_mutex have been introduced to protect the check and assignment
> > of the digest cache reference in the inode security blob.
> > 
> > Contenders that didn't get the lock also have to wait until the digest
> > cache is fully instantiated (when the bit INIT_IN_PROGRESS is cleared).
> > Dig_owner_mutex cannot be used for waiting on the instantiation to avoid
> > lock inversion with the inode lock for directories.
> > 
> > Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> > ---
> >  MAINTAINERS                                   |   6 +
> >  include/linux/digest_cache.h                  |  32 ++
> >  include/uapi/linux/lsm.h                      |   1 +
> >  include/uapi/linux/xattr.h                    |   3 +
> >  security/Kconfig                              |  11 +-
> >  security/Makefile                             |   1 +
> >  security/digest_cache/Kconfig                 |  16 +
> >  security/digest_cache/Makefile                |   7 +
> >  security/digest_cache/internal.h              |  86 ++++
> >  security/digest_cache/main.c                  | 404 ++++++++++++++++++
> >  security/security.c                           |   3 +-
> >  .../selftests/lsm/lsm_list_modules_test.c     |   3 +
> >  12 files changed, 567 insertions(+), 6 deletions(-)
> >  create mode 100644 include/linux/digest_cache.h
> >  create mode 100644 security/digest_cache/Kconfig
> >  create mode 100644 security/digest_cache/Makefile
> >  create mode 100644 security/digest_cache/internal.h
> >  create mode 100644 security/digest_cache/main.c
> > 
> > diff --git a/MAINTAINERS b/MAINTAINERS
> > index b1ca23ab8732..72801a88449c 100644
> > --- a/MAINTAINERS
> > +++ b/MAINTAINERS
> > @@ -6193,6 +6193,12 @@ L:	linux-gpio@xxxxxxxxxxxxxxx
> >  S:	Maintained
> >  F:	drivers/gpio/gpio-gpio-mm.c
> >  
> > +DIGEST_CACHE LSM
> > +M:	Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> > +L:	linux-security-module@xxxxxxxxxxxxxxx
> > +S:	Maintained
> > +F:	security/digest_cache/
> > +
> >  DIGITEQ AUTOMOTIVE MGB4 V4L2 DRIVER
> >  M:	Martin Tuma <martin.tuma@xxxxxxxxxxxxxxxxxxxxx>
> >  L:	linux-media@xxxxxxxxxxxxxxx
> 
> Nit: afaik, MAINTAINER updates should be split.

Ok, didn't know...

> > diff --git a/include/linux/digest_cache.h b/include/linux/digest_cache.h
> > new file mode 100644
> > index 000000000000..e79f94a60b0f
> > --- /dev/null
> > +++ b/include/linux/digest_cache.h
> > @@ -0,0 +1,32 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +/*
> > + * Copyright (C) 2023-2024 Huawei Technologies Duesseldorf GmbH
> > + *
> > + * Author: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> 
> Nit: Git has an author field in commit granularity so adding author
> fields to files is sort of old world.

Still like it if you don't mind...

What happens when you get the source code from non-git?

> > + *
> > + * Public API of the digest_cache LSM.
> > + */
> > +
> > +#ifndef _LINUX_DIGEST_CACHE_H
> > +#define _LINUX_DIGEST_CACHE_H
> > +
> > +#include <linux/fs.h>
> > +
> > +struct digest_cache;
> 
> Is this declaration necessary?

I thought so. Users of the digest_cache LSM do:

struct digest_cache *digest_cache;


digest_cache = digest_cache_get(dentry);


and so on. Those users don't know the internal layout of the
digest_cache structure, but still pass it to the various functions.

> I don't think you need forward declaration here as this does compile:
> 
> #include <stdio.h>
> 
> struct digest_cache *digest_cache_get(void)
> {
>         return NULL;
> }
> 
> int main(void)
> {
>         return (long)digest_cache_get();
> }
> 
> 
> > +
> > +#ifdef CONFIG_SECURITY_DIGEST_CACHE
> > +struct digest_cache *digest_cache_get(struct dentry *dentry);
> > +void digest_cache_put(struct digest_cache *digest_cache);
> > +
> > +#else
> > +static inline struct digest_cache *digest_cache_get(struct dentry *dentry)
> > +{
> > +	return NULL;
> > +}
> > +
> > +static inline void digest_cache_put(struct digest_cache *digest_cache)
> > +{
> > +}
> > +
> > +#endif /* CONFIG_SECURITY_DIGEST_CACHE */
> > +#endif /* _LINUX_DIGEST_CACHE_H */
> > diff --git a/include/uapi/linux/lsm.h b/include/uapi/linux/lsm.h
> > index 33d8c9f4aa6b..832b3aea5c26 100644
> > --- a/include/uapi/linux/lsm.h
> > +++ b/include/uapi/linux/lsm.h
> > @@ -64,6 +64,7 @@ struct lsm_ctx {
> >  #define LSM_ID_LANDLOCK		110
> >  #define LSM_ID_IMA		111
> >  #define LSM_ID_EVM		112
> > +#define LSM_ID_DIGEST_CACHE	113
> >  
> >  /*
> >   * LSM_ATTR_XXX definitions identify different LSM attributes
> > diff --git a/include/uapi/linux/xattr.h b/include/uapi/linux/xattr.h
> > index 9463db2dfa9d..8a58cf4bce65 100644
> > --- a/include/uapi/linux/xattr.h
> > +++ b/include/uapi/linux/xattr.h
> > @@ -54,6 +54,9 @@
> >  #define XATTR_IMA_SUFFIX "ima"
> >  #define XATTR_NAME_IMA XATTR_SECURITY_PREFIX XATTR_IMA_SUFFIX
> >  
> > +#define XATTR_DIGEST_LIST_SUFFIX "digest_list"
> > +#define XATTR_NAME_DIGEST_LIST XATTR_SECURITY_PREFIX XATTR_DIGEST_LIST_SUFFIX
> > +
> >  #define XATTR_SELINUX_SUFFIX "selinux"
> >  #define XATTR_NAME_SELINUX XATTR_SECURITY_PREFIX XATTR_SELINUX_SUFFIX
> >  
> > diff --git a/security/Kconfig b/security/Kconfig
> > index 52c9af08ad35..99f99cbd94cc 100644
> > --- a/security/Kconfig
> > +++ b/security/Kconfig
> > @@ -194,6 +194,7 @@ source "security/yama/Kconfig"
> >  source "security/safesetid/Kconfig"
> >  source "security/lockdown/Kconfig"
> >  source "security/landlock/Kconfig"
> > +source "security/digest_cache/Kconfig"
> >  
> >  source "security/integrity/Kconfig"
> >  
> > @@ -233,11 +234,11 @@ endchoice
> >  
> >  config LSM
> >  	string "Ordered list of enabled LSMs"
> > -	default "landlock,lockdown,yama,loadpin,safesetid,smack,selinux,tomoyo,apparmor,bpf" if DEFAULT_SECURITY_SMACK
> > -	default "landlock,lockdown,yama,loadpin,safesetid,apparmor,selinux,smack,tomoyo,bpf" if DEFAULT_SECURITY_APPARMOR
> > -	default "landlock,lockdown,yama,loadpin,safesetid,tomoyo,bpf" if DEFAULT_SECURITY_TOMOYO
> > -	default "landlock,lockdown,yama,loadpin,safesetid,bpf" if DEFAULT_SECURITY_DAC
> > -	default "landlock,lockdown,yama,loadpin,safesetid,selinux,smack,tomoyo,apparmor,bpf"
> > +	default "digest_cache,landlock,lockdown,yama,loadpin,safesetid,smack,selinux,tomoyo,apparmor,bpf" if DEFAULT_SECURITY_SMACK
> > +	default "digest_cache,landlock,lockdown,yama,loadpin,safesetid,apparmor,selinux,smack,tomoyo,bpf" if DEFAULT_SECURITY_APPARMOR
> > +	default "digest_cache,landlock,lockdown,yama,loadpin,safesetid,tomoyo,bpf" if DEFAULT_SECURITY_TOMOYO
> > +	default "digest_cache,landlock,lockdown,yama,loadpin,safesetid,bpf" if DEFAULT_SECURITY_DAC
> > +	default "digest_cache,landlock,lockdown,yama,loadpin,safesetid,selinux,smack,tomoyo,apparmor,bpf"
> >  	help
> >  	  A comma-separated list of LSMs, in initialization order.
> >  	  Any LSMs left off this list, except for those with order
> > diff --git a/security/Makefile b/security/Makefile
> > index 59f238490665..e9b43e7b715a 100644
> > --- a/security/Makefile
> > +++ b/security/Makefile
> > @@ -25,6 +25,7 @@ obj-$(CONFIG_SECURITY_LOCKDOWN_LSM)	+= lockdown/
> >  obj-$(CONFIG_CGROUPS)			+= device_cgroup.o
> >  obj-$(CONFIG_BPF_LSM)			+= bpf/
> >  obj-$(CONFIG_SECURITY_LANDLOCK)		+= landlock/
> > +obj-$(CONFIG_SECURITY_DIGEST_CACHE)	+= digest_cache/
> >  
> >  # Object integrity file lists
> >  obj-$(CONFIG_INTEGRITY)			+= integrity/
> > diff --git a/security/digest_cache/Kconfig b/security/digest_cache/Kconfig
> > new file mode 100644
> > index 000000000000..e53fbf0779d6
> > --- /dev/null
> > +++ b/security/digest_cache/Kconfig
> > @@ -0,0 +1,16 @@
> > +# SPDX-License-Identifier: GPL-2.0
> > +config SECURITY_DIGEST_CACHE
> > +	bool "Digest_cache LSM"
> > +	default n
> > +	help
> > +	  This option enables an LSM maintaining a cache of digests
> > +	  (e.g. of file data or metadata).
> > +
> > +	  This LSM can support other kernel components in making access
> > +	  control decisions.
> > +
> > +config DIGEST_LIST_DEFAULT_PATH
> > +	string
> > +	default "/etc/digest_lists"
> > +	help
> > +	  Default directory where digest_cache LSM expects to find digest lists.
> > diff --git a/security/digest_cache/Makefile b/security/digest_cache/Makefile
> > new file mode 100644
> > index 000000000000..48848c41253e
> > --- /dev/null
> > +++ b/security/digest_cache/Makefile
> > @@ -0,0 +1,7 @@
> > +# SPDX-License-Identifier: GPL-2.0
> > +#
> > +# Makefile for building the digest_cache LSM.
> > +
> > +obj-$(CONFIG_SECURITY_DIGEST_CACHE) += digest_cache.o
> > +
> > +digest_cache-y := main.o
> > diff --git a/security/digest_cache/internal.h b/security/digest_cache/internal.h
> > new file mode 100644
> > index 000000000000..5f04844af3a5
> > --- /dev/null
> > +++ b/security/digest_cache/internal.h
> > @@ -0,0 +1,86 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +/*
> > + * Copyright (C) 2023-2024 Huawei Technologies Duesseldorf GmbH
> > + *
> > + * Author: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> 
> ditto
> 
> > + *
> > + * Internal header of the digest_cache LSM.
> > + */
> > +
> > +#ifndef _DIGEST_CACHE_INTERNAL_H
> > +#define _DIGEST_CACHE_INTERNAL_H
> > +
> > +#include <linux/lsm_hooks.h>
> > +#include <linux/digest_cache.h>
> > +
> > +/* Digest cache bits in flags. */
> > +#define INIT_IN_PROGRESS	0	/* Digest cache being initialized. */
> > +
> > +/**
> > + * struct digest_cache - Digest cache
> > + * @ref_count: Number of references to the digest cache
> > + * @path_str: Path of the digest list the digest cache was created from
> > + * @flags: Control flags
> > + *
> > + * This structure represents a cache of digests extracted from a digest list.
> > + */
> > +struct digest_cache {
> > +	atomic_t ref_count;
> > +	char *path_str;
> > +	unsigned long flags;
> > +};
> > +
> > +/**
> > + * struct digest_cache_security - Digest cache pointers in inode security blob
> > + * @dig_owner: Digest cache created from this inode
> > + * @dig_owner_mutex: Protects @dig_owner
> > + * @dig_user: Digest cache requested for this inode
> > + * @dig_user_mutex: Protects @dig_user
> > + *
> > + * This structure contains references to digest caches, protected by their
> > + * respective mutex.
> > + */
> > +struct digest_cache_security {
> > +	struct digest_cache *dig_owner;
> > +	struct mutex dig_owner_mutex;
> > +	struct digest_cache *dig_user;
> > +	struct mutex dig_user_mutex;
> > +};
> > +
> > +extern struct lsm_blob_sizes digest_cache_blob_sizes;
> > +extern char *default_path_str;
> > +
> > +static inline struct digest_cache_security *
> > +digest_cache_get_security(const struct inode *inode)
> > +{
> > +	if (unlikely(!inode->i_security))
> > +		return NULL;
> > +
> > +	return inode->i_security + digest_cache_blob_sizes.lbs_inode;
> > +}
> > +
> > +static inline struct digest_cache *
> > +digest_cache_ref(struct digest_cache *digest_cache)
> > +{
> > +	atomic_inc(&digest_cache->ref_count);
> > +	pr_debug("Ref (+) digest cache %s (ref count: %d)\n",
> > +		 digest_cache->path_str, atomic_read(&digest_cache->ref_count));
> > +	return digest_cache;
> > +}
> > +
> > +static inline struct digest_cache *
> > +digest_cache_unref(struct digest_cache *digest_cache)
> > +{
> > +	bool ref_is_zero = atomic_dec_and_test(&digest_cache->ref_count);
> > +
> > +	pr_debug("Ref (-) digest cache %s (ref count: %d)\n",
> > +		 digest_cache->path_str, atomic_read(&digest_cache->ref_count));
> > +	return (ref_is_zero) ? digest_cache : NULL;
> > +}
> > +
> > +/* main.c */
> > +struct digest_cache *digest_cache_create(struct dentry *dentry,
> > +					 struct path *digest_list_path,
> > +					 char *path_str, char *filename);
> > +
> > +#endif /* _DIGEST_CACHE_INTERNAL_H */
> > diff --git a/security/digest_cache/main.c b/security/digest_cache/main.c
> > new file mode 100644
> > index 000000000000..14dba8915e99
> > --- /dev/null
> > +++ b/security/digest_cache/main.c
> > @@ -0,0 +1,404 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +/*
> > + * Copyright (C) 2023-2024 Huawei Technologies Duesseldorf GmbH
> > + *
> > + * Author: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> > + *
> > + * Implement the main code of the digest_cache LSM.
> > + */
> > +
> > +#define pr_fmt(fmt) "DIGEST CACHE: "fmt
> > +#include <linux/namei.h>
> > +#include <linux/xattr.h>
> > +
> > +#include "internal.h"
> > +
> > +static int digest_cache_enabled __ro_after_init = 1;
> > +static struct kmem_cache *digest_cache_cache __read_mostly;
> > +
> > +char *default_path_str = CONFIG_DIGEST_LIST_DEFAULT_PATH;
> > +
> > +/**
> > + * digest_cache_alloc_init - Allocate and initialize a new digest cache
> > + * @path_str: Path string of the digest list
> > + * @filename: Digest list file name (can be an empty string)
> > + *
> > + * This function allocates and initializes a new digest cache.
> > + *
> > + * Return: A digest_cache structure on success, NULL on error.
> > + */
> > +static struct digest_cache *digest_cache_alloc_init(char *path_str,
> > +						    char *filename)
> > +{
> > +	struct digest_cache *digest_cache;
> > +
> > +	digest_cache = kmem_cache_alloc(digest_cache_cache, GFP_KERNEL);
> > +	if (!digest_cache)
> > +		return digest_cache;
> > +
> > +	digest_cache->path_str = kasprintf(GFP_KERNEL, "%s%s%s", path_str,
> > +					   filename[0] ? "/" : "", filename);
> > +	if (!digest_cache->path_str) {
> > +		kmem_cache_free(digest_cache_cache, digest_cache);
> > +		return NULL;
> > +	}
> > +
> > +	atomic_set(&digest_cache->ref_count, 1);
> > +	digest_cache->flags = 0UL;
> > +
> > +	pr_debug("New digest cache %s (ref count: %d)\n",
> > +		 digest_cache->path_str, atomic_read(&digest_cache->ref_count));
> 
> Nit: kretprobe can be used to grab the same information easil and
> do e.g. statistics and stuff like that. Traces for return values
> are not very useful.

I like to do dynamic debug, and I can control what part I want to
debug. All these messages allowed me to debug efficiently when there
was any issue. Would be a pity to remove them.

Thanks

Roberto

> > +
> > +	return digest_cache;
> > +}
> > +
> > +/**
> > + * digest_cache_free - Free all memory occupied by the digest cache
> > + * @digest_cache: Digest cache
> > + *
> > + * This function frees the memory occupied by the digest cache.
> > + */
> > +static void digest_cache_free(struct digest_cache *digest_cache)
> > +{
> > +	pr_debug("Freed digest cache %s\n", digest_cache->path_str);
> 
> ditto for the above trace
> 
> > +	kfree(digest_cache->path_str);
> > +	kmem_cache_free(digest_cache_cache, digest_cache);
> > +}
> > +
> > +/**
> > + * digest_cache_create - Create a digest cache
> > + * @dentry: Dentry of the inode for which the digest cache will be used
> > + * @digest_list_path: Path structure of the digest list
> > + * @path_str: Path string of the digest list
> > + * @filename: Digest list file name (can be an empty string)
> > + *
> > + * This function first locates, from the passed path, the digest list inode
> > + * from which the digest cache will be created or retrieved (if it already
> > + * exists).
> > + *
> > + * If dig_owner is NULL in the inode security blob, this function creates a
> > + * new digest cache with reference count set to 1 (reference returned), sets
> > + * it to dig_owner and consequently increments again the digest cache reference
> > + * count.
> > + *
> > + * Otherwise, it simply increments the reference count of the existing
> > + * dig_owner, since that reference is returned to the caller.
> > + *
> > + * Incrementing the reference count twice before calling path_put() ensures
> > + * that the digest cache returned is valid even if the inode is evicted from
> > + * memory (which decreases the reference count).
> > + *
> > + * Releasing the dig_owner_mutex lock does not mean that the digest cache is
> > + * ready for use. digest_cache_create() callers that found a partially
> > + * instantiated digest cache have to wait until the INIT_IN_PROGRESS bit is
> > + * cleared by the caller that is actually creating that digest cache.
> > + *
> > + * Return: A new digest cache on success, NULL on error.
> > + */
> > +struct digest_cache *digest_cache_create(struct dentry *dentry,
> > +					 struct path *digest_list_path,
> > +					 char *path_str, char *filename)
> > +{
> > +	struct path file_path;
> > +	struct digest_cache *digest_cache = NULL;
> > +	struct digest_cache_security *dig_sec;
> > +	struct inode *inode = d_backing_inode(digest_list_path->dentry);
> > +	bool dig_owner_exists = false;
> > +	int ret;
> > +
> > +	if (S_ISDIR(d_backing_inode(digest_list_path->dentry)->i_mode) &&
> > +	    filename[0]) {
> > +		ret = vfs_path_lookup(digest_list_path->dentry,
> > +				      digest_list_path->mnt, filename, 0,
> > +				      &file_path);
> > +		if (ret < 0) {
> > +			pr_debug("Cannot find digest list %s/%s\n", path_str,
> > +				 filename);
> > +			return NULL;
> > +		}
> > +
> > +		digest_list_path = &file_path;
> > +		inode = d_backing_inode(file_path.dentry);
> > +
> > +		/*
> > +		 * Cannot request a digest cache for the same inode the
> > +		 * digest cache is populated from.
> > +		 */
> > +		if (d_backing_inode(dentry) == inode) {
> > +			pr_debug("Cannot request a digest cache for %s and use it as digest list\n",
> > +				 dentry->d_name.name);
> > +			goto out;
> > +		}
> > +
> > +		/* No support for nested directories. */
> > +		if (!S_ISREG(inode->i_mode)) {
> > +			pr_debug("%s is not a regular file (no support for nested directories)\n",
> > +				 dentry->d_name.name);
> > +			goto out;
> > +		}
> > +	}
> > +
> > +	dig_sec = digest_cache_get_security(inode);
> > +	if (unlikely(!dig_sec))
> > +		goto out;
> > +
> > +	/* Serialize check and assignment of dig_owner. */
> > +	mutex_lock(&dig_sec->dig_owner_mutex);
> > +	if (dig_sec->dig_owner) {
> > +		/* Increment ref. count for reference returned to the caller. */
> > +		digest_cache = digest_cache_ref(dig_sec->dig_owner);
> > +		dig_owner_exists = true;
> > +		mutex_unlock(&dig_sec->dig_owner_mutex);
> > +		goto exists;
> > +	}
> > +
> > +	/* Ref. count is already 1 for this reference. */
> > +	digest_cache = digest_cache_alloc_init(path_str, filename);
> > +	if (!digest_cache) {
> > +		mutex_unlock(&dig_sec->dig_owner_mutex);
> > +		goto out;
> > +	}
> > +
> > +	/* Increment ref. count for reference set to dig_owner. */
> > +	dig_sec->dig_owner = digest_cache_ref(digest_cache);
> > +
> > +	/* Make the other lock contenders wait until creation complete. */
> > +	set_bit(INIT_IN_PROGRESS, &dig_sec->dig_owner->flags);
> > +	mutex_unlock(&dig_sec->dig_owner_mutex);
> > +
> > +	/* Creation complete, notify the other lock contenders. */
> > +	clear_and_wake_up_bit(INIT_IN_PROGRESS, &dig_sec->dig_owner->flags);
> > +exists:
> > +	if (dig_owner_exists)
> > +		/* Wait until creation complete. */
> > +		wait_on_bit(&dig_sec->dig_owner->flags, INIT_IN_PROGRESS,
> > +			    TASK_UNINTERRUPTIBLE);
> > +out:
> > +	if (digest_list_path == &file_path)
> > +		path_put(&file_path);
> > +
> > +	return digest_cache;
> > +}
> > +
> > +/**
> > + * digest_cache_new - Retrieve digest list file name and request digest cache
> > + * @dentry: Dentry of the inode for which the digest cache will be used
> > + *
> > + * This function locates the default path. If it is a file, it directly creates
> > + * a digest cache from it. Otherwise, it reads the digest list file name from
> > + * the security.digest_list xattr and requests the creation of a digest cache
> > + * with that file name. If security.digest_list is not found, this function
> > + * requests the creation of a digest cache on the parent directory.
> > + *
> > + * Return: A new digest cache on success, NULL on error.
> > + */
> > +static struct digest_cache *digest_cache_new(struct dentry *dentry)
> > +{
> > +	char filename[NAME_MAX + 1] = { 0 };
> > +	struct digest_cache *digest_cache = NULL;
> > +	struct path default_path;
> > +	int ret;
> > +
> > +	ret = kern_path(default_path_str, 0, &default_path);
> > +	if (ret < 0) {
> > +		pr_debug("Cannot find path %s\n", default_path_str);
> > +		return NULL;
> > +	}
> > +
> > +	/* The default path is a file, no need to get xattr. */
> > +	if (S_ISREG(d_backing_inode(default_path.dentry)->i_mode)) {
> > +		pr_debug("Default path %s is a file, not reading %s xattr\n",
> > +			 default_path_str, XATTR_NAME_DIGEST_LIST);
> > +		goto create;
> > +	} else if (!S_ISDIR(d_backing_inode(default_path.dentry)->i_mode)) {
> > +		pr_debug("Default path %s must be either a file or a directory\n",
> > +			 default_path_str);
> > +		goto out;
> > +	}
> > +
> > +	ret = vfs_getxattr(&nop_mnt_idmap, dentry, XATTR_NAME_DIGEST_LIST,
> > +			   filename, sizeof(filename) - 1);
> > +	if (ret <= 0) {
> > +		pr_debug("Digest list path not found for file %s, using %s\n",
> > +			 dentry->d_name.name, default_path_str);
> > +		goto create;
> > +	}
> > +
> > +	if (strchr(filename, '/')) {
> > +		pr_debug("%s xattr should contain only a file name, got: %s\n",
> > +			 XATTR_NAME_DIGEST_LIST, filename);
> > +		goto out;
> > +	}
> > +
> > +	pr_debug("Found %s xattr in %s, default path: %s, digest list: %s\n",
> > +		 XATTR_NAME_DIGEST_LIST, dentry->d_name.name, default_path_str,
> > +		 filename);
> > +create:
> > +	digest_cache = digest_cache_create(dentry, &default_path,
> > +					   default_path_str, filename);
> > +out:
> > +	path_put(&default_path);
> > +	return digest_cache;
> > +}
> > +
> > +/**
> > + * digest_cache_get - Get a digest cache for a given inode
> > + * @dentry: Dentry of the inode for which the digest cache will be used
> > + *
> > + * This function tries to find a digest cache from the inode security blob of
> > + * the passed dentry (dig_user field). If a digest cache was not found, it calls
> > + * digest_cache_new() to create a new one. In both cases, it increments the
> > + * digest cache reference count before returning the reference to the caller.
> > + *
> > + * The caller is responsible to call digest_cache_put() to release the digest
> > + * cache reference returned.
> > + *
> > + * Lock dig_user_mutex to protect against concurrent requests to obtain a digest
> > + * cache for the same inode, and to make other contenders wait until the first
> > + * requester finishes the process.
> > + *
> > + * Return: A digest cache on success, NULL otherwise.
> > + */
> > +struct digest_cache *digest_cache_get(struct dentry *dentry)
> > +{
> > +	struct digest_cache_security *dig_sec;
> > +	struct digest_cache *digest_cache = NULL;
> > +	struct inode *inode = d_backing_inode(dentry);
> > +
> > +	if (!digest_cache_enabled)
> > +		return NULL;
> > +
> > +	dig_sec = digest_cache_get_security(inode);
> > +	if (unlikely(!dig_sec))
> > +		return NULL;
> > +
> > +	/* Serialize accesses to inode for which the digest cache is used. */
> > +	mutex_lock(&dig_sec->dig_user_mutex);
> > +	if (!dig_sec->dig_user)
> > +		/* Consume extra reference from digest_cache_create(). */
> > +		dig_sec->dig_user = digest_cache_new(dentry);
> > +
> > +	if (dig_sec->dig_user)
> > +		/* Increment ref. count for reference returned to the caller. */
> > +		digest_cache = digest_cache_ref(dig_sec->dig_user);
> > +
> > +	mutex_unlock(&dig_sec->dig_user_mutex);
> > +	return digest_cache;
> > +}
> > +EXPORT_SYMBOL_GPL(digest_cache_get);
> > +
> > +/**
> > + * digest_cache_put - Release a digest cache reference
> > + * @digest_cache: Digest cache
> > + *
> > + * This function decrements the reference count of the digest cache passed as
> > + * argument. If the reference count reaches zero, it calls digest_cache_free()
> > + * to free the digest cache.
> > + */
> > +void digest_cache_put(struct digest_cache *digest_cache)
> > +{
> > +	struct digest_cache *to_free;
> > +
> > +	to_free = digest_cache_unref(digest_cache);
> > +	if (!to_free)
> > +		return;
> > +
> > +	digest_cache_free(to_free);
> > +}
> > +EXPORT_SYMBOL_GPL(digest_cache_put);
> > +
> > +struct lsm_blob_sizes digest_cache_blob_sizes __ro_after_init = {
> > +	.lbs_inode = sizeof(struct digest_cache_security),
> > +};
> > +
> > +/**
> > + * digest_cache_inode_alloc_security - Initialize inode security blob
> > + * @inode: Inode for which the security blob is initialized
> > + *
> > + * This function initializes the digest_cache_security structure, directly
> > + * stored in the inode security blob.
> > + *
> > + * Return: Zero.
> > + */
> > +static int digest_cache_inode_alloc_security(struct inode *inode)
> > +{
> > +	struct digest_cache_security *dig_sec;
> > +
> > +	/* The inode security blob is always allocated here. */
> > +	dig_sec = digest_cache_get_security(inode);
> > +	mutex_init(&dig_sec->dig_owner_mutex);
> > +	mutex_init(&dig_sec->dig_user_mutex);
> > +	return 0;
> > +}
> > +
> > +/**
> > + * digest_cache_inode_free_security - Release the digest cache references
> > + * @inode: Inode for which the digest cache references are released
> > + *
> > + * Since the inode is being evicted, this function releases the non-needed
> > + * references to the digest caches stored in the digest_cache_security
> > + * structure.
> > + */
> > +static void digest_cache_inode_free_security(struct inode *inode)
> > +{
> > +	struct digest_cache_security *dig_sec;
> > +
> > +	dig_sec = digest_cache_get_security(inode);
> > +	if (!dig_sec)
> > +		return;
> > +
> > +	mutex_destroy(&dig_sec->dig_owner_mutex);
> > +	mutex_destroy(&dig_sec->dig_user_mutex);
> > +	if (dig_sec->dig_owner)
> > +		digest_cache_put(dig_sec->dig_owner);
> > +	if (dig_sec->dig_user)
> > +		digest_cache_put(dig_sec->dig_user);
> > +}
> > +
> > +static struct security_hook_list digest_cache_hooks[] __ro_after_init = {
> > +	LSM_HOOK_INIT(inode_alloc_security, digest_cache_inode_alloc_security),
> > +	LSM_HOOK_INIT(inode_free_security, digest_cache_inode_free_security),
> > +};
> > +
> > +/**
> > + * digest_cache_init_once - Initialize the digest cache structure
> > + * @foo: Digest cache structure to initialize
> > + *
> > + * This function fills the digest cache structure with zeros.
> > + */
> > +static void digest_cache_init_once(void *foo)
> > +{
> > +	struct digest_cache *digest_cache = (struct digest_cache *)foo;
> > +
> > +	memset(digest_cache, 0, sizeof(*digest_cache));
> > +}
> > +
> > +static const struct lsm_id digest_cache_lsmid = {
> > +	.name = "digest_cache",
> > +	.id = LSM_ID_DIGEST_CACHE,
> > +};
> > +
> > +/**
> > + * digest_cache_init - Initialize the digest_cache LSM
> > + *
> > + * Initialize the digest_cache LSM, by instantiating a cache for the
> > + * digest_cache structure and by registering the digest_cache LSM hooks.
> > + */
> > +static int __init digest_cache_init(void)
> > +{
> > +	digest_cache_cache = kmem_cache_create("digest_cache_cache",
> > +					       sizeof(struct digest_cache),
> > +					       0, SLAB_PANIC,
> > +					       digest_cache_init_once);
> > +
> > +	security_add_hooks(digest_cache_hooks, ARRAY_SIZE(digest_cache_hooks),
> > +			   &digest_cache_lsmid);
> > +	return 0;
> > +}
> > +
> > +DEFINE_LSM(digest_cache) = {
> > +	.name = "digest_cache",
> > +	.enabled = &digest_cache_enabled,
> > +	.init = digest_cache_init,
> > +	.blobs = &digest_cache_blob_sizes,
> > +};
> > diff --git a/security/security.c b/security/security.c
> > index cbdc9bebe802..cb084ed58617 100644
> > --- a/security/security.c
> > +++ b/security/security.c
> > @@ -50,7 +50,8 @@
> >  	(IS_ENABLED(CONFIG_BPF_LSM) ? 1 : 0) + \
> >  	(IS_ENABLED(CONFIG_SECURITY_LANDLOCK) ? 1 : 0) + \
> >  	(IS_ENABLED(CONFIG_IMA) ? 1 : 0) + \
> > -	(IS_ENABLED(CONFIG_EVM) ? 1 : 0))
> > +	(IS_ENABLED(CONFIG_EVM) ? 1 : 0) + \
> > +	(IS_ENABLED(CONFIG_SECURITY_DIGEST_CACHE) ? 1 : 0))
> >  
> >  /*
> >   * These are descriptions of the reasons that can be passed to the
> > diff --git a/tools/testing/selftests/lsm/lsm_list_modules_test.c b/tools/testing/selftests/lsm/lsm_list_modules_test.c
> > index 4d5d4cee2586..d00831edc582 100644
> > --- a/tools/testing/selftests/lsm/lsm_list_modules_test.c
> > +++ b/tools/testing/selftests/lsm/lsm_list_modules_test.c
> > @@ -128,6 +128,9 @@ TEST(correct_lsm_list_modules)
> >  		case LSM_ID_EVM:
> >  			name = "evm";
> >  			break;
> > +		case LSM_ID_DIGEST_CACHE:
> > +			name = "digest_cache";
> > +			break;
> >  		default:
> >  			name = "INVALID";
> >  			break;
> 
> BR, Jarkko