Date: Tue, 16 Apr 2024 12:37:29 +0200
From: Roberto Sassu <roberto.sassu@...weicloud.com>
To: Jarkko Sakkinen <jarkko@...nel.org>, corbet@....net, paul@...l-moore.com,
 jmorris@...ei.org, serge@...lyn.com, akpm@...ux-foundation.org,
 shuah@...nel.org, mcoquelin.stm32@...il.com, alexandre.torgue@...s.st.com,
 mic@...ikod.net
Cc: linux-security-module@...r.kernel.org, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
 bpf@...r.kernel.org, zohar@...ux.ibm.com, dmitry.kasatkin@...il.com,
 linux-integrity@...r.kernel.org, wufan@...ux.microsoft.com,
 pbrobinson@...il.com, zbyszek@...waw.pl, hch@....de, mjg59@...f.ucam.org,
 pmatilai@...hat.com, jannh@...gle.com, dhowells@...hat.com,
 jikos@...nel.org, mkoutny@...e.com, ppavlu@...e.com, petr.vorel@...il.com,
 mzerqung@...inter.de, kgold@...ux.ibm.com,
 Roberto Sassu <roberto.sassu@...wei.com>
Subject: Re: [PATCH v4 11/14] digest_cache: Reset digest cache on
 file/directory change

On 4/15/2024 9:44 PM, Jarkko Sakkinen wrote:
> On Mon Apr 15, 2024 at 5:24 PM EEST, Roberto Sassu wrote:
>> From: Roberto Sassu <roberto.sassu@...wei.com>
>>
>> Register six new LSM hooks, path_truncate, file_release, inode_unlink,
>> inode_rename, inode_post_setxattr and inode_post_removexattr, to monitor
>> digest lists/directory modifications.
>>
>> If an action affects a digest list or the parent directory, the new LSM
>> hook implementations call digest_cache_reset_owner() to set the RESET bit
>> (if unset) on the digest cache pointed to by dig_owner in the inode
>> security blob. This will cause next calls to digest_cache_get() and
>> digest_cache_create() to respectively put and clear dig_user and dig_owner,
>> and request a new digest cache.
>>
>> If an action affects a file using a digest cache, the new LSM hook
>> implementations call digest_cache_reset_user() to set the RESET_USER bit
>> (if unset) on the digest cache pointed to by dig_user in the inode security
>> blob. This will cause next calls to digest_cache_get() to put and clear
>> dig_user, and retrieve the digest cache again.
>>
>> That does not affect other users of the old digest cache, since that one
>> remains valid as long as the reference count is greater than zero. However,
>> they will be notified in a subsequent patch about the reset, so that they
>> can eventually request a new digest cache.
>>
>> Recreating a file digest cache means reading the digest list again and
>> extracting the digests. Recreating a directory digest cache, instead, does
>> not mean recreating the digest cache for directory entries, since those
>> digest caches are likely already stored in the inode security blob. It
>> would happen however for new files.
>>
>> Dig_owner reset for file digest caches is done on path_truncate, when a
>> digest list is truncated (there is no inode_truncate, file_truncate does
>> not catch operations through the truncate() system call), file_release,
>> when a digest list opened for write is being closed, inode_unlink, when a
>> digest list is removed, and inode_rename when a digest list is renamed.
>>
>> Dig_owner reset for directory digest caches is done on file_release, when a
>> new digest list is written in the digest list directory, on inode_unlink,
>> when a digest list is deleted from that directory, and finally on
>> inode_rename, when a digest list is moved to/from that directory.
>>
>> Dig_user reset is always done on inode_post_setxattr and
>> inode_post_removexattr, when the security.digest_list xattr is respectively
>> set or removed from a file using a digest cache.
>>
>> With the exception of file_release, which will always be executed (cannot
>> be denied), and inode_post_setxattr and inode_post_removexattr, which are
>> executed after the actual operation, the other LSM hooks are not optimal,
>> since the digest_cache LSM does not know whether or not the operation will
>> be allowed also by other LSMs. If the operation is denied, the digest_cache
>> LSM would do an unnecessary reset.
>>
>> Signed-off-by: Roberto Sassu <roberto.sassu@...wei.com>
>> ---
>>   security/digest_cache/Kconfig    |   1 +
>>   security/digest_cache/Makefile   |   3 +-
>>   security/digest_cache/dir.c      |   6 +
>>   security/digest_cache/internal.h |  14 +++
>>   security/digest_cache/main.c     |  19 +++
>>   security/digest_cache/reset.c    | 197 +++++++++++++++++++++++++++++++
>>   6 files changed, 239 insertions(+), 1 deletion(-)
>>   create mode 100644 security/digest_cache/reset.c
>>
>> diff --git a/security/digest_cache/Kconfig b/security/digest_cache/Kconfig
>> index cb4fa44e8f2a..54ba3a585073 100644
>> --- a/security/digest_cache/Kconfig
>> +++ b/security/digest_cache/Kconfig
>> @@ -2,6 +2,7 @@
>>   config SECURITY_DIGEST_CACHE
>>   	bool "Digest_cache LSM"
>>   	select TLV_PARSER
>> +	select SECURITY_PATH
>>   	default n
>>   	help
>>   	  This option enables an LSM maintaining a cache of digests
>> diff --git a/security/digest_cache/Makefile b/security/digest_cache/Makefile
>> index e417da0383ab..3d5e600a2c45 100644
>> --- a/security/digest_cache/Makefile
>> +++ b/security/digest_cache/Makefile
>> @@ -4,7 +4,8 @@
>>   
>>   obj-$(CONFIG_SECURITY_DIGEST_CACHE) += digest_cache.o
>>   
>> -digest_cache-y := main.o secfs.o htable.o populate.o modsig.o verif.o dir.o
>> +digest_cache-y := main.o secfs.o htable.o populate.o modsig.o verif.o dir.o \
>> +		  reset.o
>>   
>>   digest_cache-y += parsers/tlv.o
>>   digest_cache-y += parsers/rpm.o
>> diff --git a/security/digest_cache/dir.c b/security/digest_cache/dir.c
>> index a7d203c15386..937177660242 100644
>> --- a/security/digest_cache/dir.c
>> +++ b/security/digest_cache/dir.c
>> @@ -148,6 +148,12 @@ digest_cache_dir_lookup_digest(struct dentry *dentry,
>>   
>>   	list_for_each_entry(dir_entry, &digest_cache->dir_entries, list) {
>>   		mutex_lock(&dir_entry->digest_cache_mutex);
>> +		if (dir_entry->digest_cache &&
>> +		    test_bit(RESET, &dir_entry->digest_cache->flags)) {
>> +			digest_cache_put(dir_entry->digest_cache);
>> +			dir_entry->digest_cache = NULL;
>> +		}
>> +
>>   		if (!dir_entry->digest_cache) {
>>   			cache = digest_cache_create(dentry, digest_list_path,
>>   						    digest_cache->path_str,
>> diff --git a/security/digest_cache/internal.h b/security/digest_cache/internal.h
>> index c13b35f6b2c0..c816929c4743 100644
>> --- a/security/digest_cache/internal.h
>> +++ b/security/digest_cache/internal.h
>> @@ -18,6 +18,8 @@
>>   #define INVALID			1	/* Digest cache marked as invalid. */
>>   #define IS_DIR			2	/* Digest cache created from dir. */
>>   #define DIR_PREFETCH		3	/* Prefetching requested for dir. */
>> +#define RESET			4	/* Digest cache to be recreated. */
>> +#define RESET_USER		5	/* Dig_user pointer to be released. */
>>   
>>   /**
>>    * struct readdir_callback - Structure to store information for dir iteration
>> @@ -247,4 +249,16 @@ digest_cache_dir_lookup_filename(struct dentry *dentry,
>>   				 char *filename);
>>   void digest_cache_dir_free(struct digest_cache *digest_cache);
>>   
>> +/* reset.c */
>> +int digest_cache_path_truncate(const struct path *path);
>> +void digest_cache_file_release(struct file *file);
>> +int digest_cache_inode_unlink(struct inode *dir, struct dentry *dentry);
>> +int digest_cache_inode_rename(struct inode *old_dir, struct dentry *old_dentry,
>> +			      struct inode *new_dir, struct dentry *new_dentry);
>> +void digest_cache_inode_post_setxattr(struct dentry *dentry, const char *name,
>> +				      const void *value, size_t size,
>> +				      int flags);
>> +void digest_cache_inode_post_removexattr(struct dentry *dentry,
>> +					 const char *name);
>> +
>>   #endif /* _DIGEST_CACHE_INTERNAL_H */
>> diff --git a/security/digest_cache/main.c b/security/digest_cache/main.c
>> index a5616fd07c1d..ce3518a33c80 100644
>> --- a/security/digest_cache/main.c
>> +++ b/security/digest_cache/main.c
>> @@ -169,6 +169,11 @@ struct digest_cache *digest_cache_create(struct dentry *dentry,
>>   
>>   	/* Serialize check and assignment of dig_owner. */
>>   	mutex_lock(&dig_sec->dig_owner_mutex);
>> +	if (dig_sec->dig_owner && test_bit(RESET, &dig_sec->dig_owner->flags)) {
>> +		digest_cache_put(dig_sec->dig_owner);
>> +		dig_sec->dig_owner = NULL;
>> +	}
>> +
>>   	if (dig_sec->dig_owner) {
>>   		/* Increment ref. count for reference returned to the caller. */
>>   		digest_cache = digest_cache_ref(dig_sec->dig_owner);
>> @@ -403,6 +408,13 @@ 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 &&
>> +	    (test_bit(RESET, &dig_sec->dig_user->flags) ||
>> +	     test_bit(RESET_USER, &dig_sec->dig_user->flags))) {
>> +		digest_cache_put(dig_sec->dig_user);
>> +		dig_sec->dig_user = NULL;
>> +	}
>> +
>>   	if (!dig_sec->dig_user) {
>>   		down_read(&default_path_sem);
>>   		/* Consume extra reference from digest_cache_create(). */
>> @@ -491,6 +503,13 @@ static void digest_cache_inode_free_security(struct inode *inode)
>>   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),
>> +	LSM_HOOK_INIT(path_truncate, digest_cache_path_truncate),
>> +	LSM_HOOK_INIT(file_release, digest_cache_file_release),
>> +	LSM_HOOK_INIT(inode_unlink, digest_cache_inode_unlink),
>> +	LSM_HOOK_INIT(inode_rename, digest_cache_inode_rename),
>> +	LSM_HOOK_INIT(inode_post_setxattr, digest_cache_inode_post_setxattr),
>> +	LSM_HOOK_INIT(inode_post_removexattr,
>> +		      digest_cache_inode_post_removexattr),
>>   };
>>   
>>   /**
>> diff --git a/security/digest_cache/reset.c b/security/digest_cache/reset.c
>> new file mode 100644
>> index 000000000000..e07222b0e0e8
>> --- /dev/null
>> +++ b/security/digest_cache/reset.c
>> @@ -0,0 +1,197 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +/*
>> + * Copyright (C) 2023-2024 Huawei Technologies Duesseldorf GmbH
>> + *
>> + * Author: Roberto Sassu <roberto.sassu@...wei.com>
>> + *
>> + * Reset digest cache on digest lists/directory modifications.
>> + */
>> +
>> +#define pr_fmt(fmt) "DIGEST CACHE: "fmt
>> +#include "internal.h"
>> +
>> +/**
>> + * digest_cache_reset_owner - Reset dig_owner
>> + * @inode: Inode of the digest list/directory containing the digest list
>> + * @reason: Reason for reset
>> + *
>> + * This function sets the RESET bit of the digest cache pointed to by dig_owner
>> + * (if unset), so that digest_cache_get() and digest_cache_create() respectively
>> + * release and clear dig_user and dig_owner in the inode security blob. This
>> + * causes new callers of digest_cache_get() to get a new digest cache.
>> + */
>> +static void digest_cache_reset_owner(struct inode *inode, const char *reason)
>> +{
>> +	struct digest_cache_security *dig_sec;
>> +
>> +	dig_sec = digest_cache_get_security(inode);
>> +	if (unlikely(!dig_sec))
>> +		return;
>> +
>> +	mutex_lock(&dig_sec->dig_owner_mutex);
>> +	if (dig_sec->dig_owner &&
>> +	    !test_and_set_bit(RESET, &dig_sec->dig_owner->flags))
>> +		pr_debug("Resetting %s (dig_owner), reason: %s\n",
>> +			 dig_sec->dig_owner->path_str, reason);
>> +	mutex_unlock(&dig_sec->dig_owner_mutex);
>> +}
>> +
>> +/**
>> + * digest_cache_reset_user - Reset dig_user
>> + * @inode: Inode of the file using the digest cache
>> + * @reason: Reason for reset
>> + *
>> + * This function sets the RESET_USER bit (if unset), so that digest_cache_get()
>> + * clears the dig_user pointer in the inode security blob and determines again
>> + * the digest list inode to get a digest cache from.
>> + */
>> +static void digest_cache_reset_user(struct inode *inode, const char *reason)
>> +{
>> +	struct digest_cache_security *dig_sec;
>> +
>> +	dig_sec = digest_cache_get_security(inode);
>> +	if (unlikely(!dig_sec))
>> +		return;
>> +
>> +	mutex_lock(&dig_sec->dig_user_mutex);
>> +	if (dig_sec->dig_user &&
>> +	    !test_and_set_bit(RESET_USER, &dig_sec->dig_user->flags))
>> +		pr_debug("Resetting %s (dig_user), reason: %s\n",
>> +			 dig_sec->dig_user->path_str, reason);
>> +	mutex_unlock(&dig_sec->dig_user_mutex);
>> +}
>> +
>> +/**
>> + * digest_cache_path_truncate - A file is being truncated
>> + * @path: File path
>> + *
>> + * This function is called when a file is being truncated. If the inode is a
>> + * digest list, it resets the inode dig_owner, to force rebuilding the digest
>> + * cache.
>> + *
>> + * Return: Zero.
>> + */
>> +int digest_cache_path_truncate(const struct path *path)
>> +{
>> +	struct inode *inode = d_backing_inode(path->dentry);
>> +
>> +	if (!S_ISREG(inode->i_mode))
>> +		return 0;
>> +
>> +	digest_cache_reset_owner(inode, "file_truncate");
>> +	return 0;
>> +}
>> +
>> +/**
>> + * digest_cache_file_release - Last reference of a file desc is being released
>> + * @file: File descriptor
>> + *
>> + * This function is called when the last reference of a file descriptor is
>> + * being released. If the inode is a regular file and was opened for write, or
>> + * the parent inode is the digest list directory and the file was created, it
>> + * resets the inode dig_owner, to force rebuilding the digest cache.
>> + */
>> +void digest_cache_file_release(struct file *file)
>> +{
>> +	struct inode *dir = d_backing_inode(file_dentry(file)->d_parent);
>> +
>> +	if (!S_ISREG(file_inode(file)->i_mode) || !(file->f_mode & FMODE_WRITE))
>> +		return;
>> +
>> +	digest_cache_reset_owner(file_inode(file), "file_file_release");
>> +	if (file->f_mode & FMODE_CREATED)
>> +		digest_cache_reset_owner(dir, "dir_file_release");
>> +}
>> +
>> +/**
>> + * digest_cache_inode_unlink - An inode is being removed
>> + * @dir: Inode of the affected directory
>> + * @dentry: Dentry of the inode being removed
>> + *
>> + * This function is called when an existing inode is being removed. If the
>> + * inode is a digest list, or the parent inode is the digest list directory and
>> + * the inode is a regular file, it resets the affected inode dig_owner, to force
>> + * rebuilding the digest cache.
>> + *
>> + * Return: Zero.
>> + */
>> +int digest_cache_inode_unlink(struct inode *dir, struct dentry *dentry)
>> +{
>> +	struct inode *inode = d_backing_inode(dentry);
>> +
>> +	if (!S_ISREG(inode->i_mode))
>> +		return 0;
>> +
>> +	digest_cache_reset_owner(inode, "file_unlink");
>> +	digest_cache_reset_owner(dir, "dir_unlink");
>> +	return 0;
>> +}
>> +
>> +/**
>> + * digest_cache_inode_rename - An inode is being renamed
>> + * @old_dir: Inode of the directory containing the inode being renamed
>> + * @old_dentry: Dentry of the inode being renamed
>> + * @new_dir: Directory where the inode will be placed into
>> + * @new_dentry: Dentry of the inode after being renamed
>> + *
>> + * This function is called when an existing inode is being moved from a
>> + * directory to another (rename). If the inode is a digest list, or that inode
>> + * is moved from/to the digest list directory, it resets the affected inode
>> + * dig_owner, to force rebuilding the digest cache.
>> + *
>> + * Return: Zero.
>> + */
>> +int digest_cache_inode_rename(struct inode *old_dir, struct dentry *old_dentry,
>> +			      struct inode *new_dir, struct dentry *new_dentry)
>> +{
>> +	struct inode *old_inode = d_backing_inode(old_dentry);
>> +
>> +	if (!S_ISREG(old_inode->i_mode))
>> +		return 0;
>> +
>> +	digest_cache_reset_owner(old_inode, "file_rename");
>> +	digest_cache_reset_owner(old_dir, "dir_rename_from");
>> +	digest_cache_reset_owner(new_dir, "dir_rename_to");
>> +	return 0;
>> +}
>> +
>> +/**
>> + * digest_cache_inode_post_setxattr() - An xattr was set
>> + * @dentry: file
>> + * @name: xattr name
>> + * @value: xattr value
>> + * @size: size of xattr value
>> + * @flags: flags
>> + *
>> + * This function is called after an xattr was set on an existing inode. If the
>> + * inode points to a digest cache and the xattr set is security.digest_list, it
>> + * resets dig_user in the inode security blob, to force retrieving a fresh
>> + * digest cache.
>> + */
>> +void digest_cache_inode_post_setxattr(struct dentry *dentry, const char *name,
>> +				      const void *value, size_t size, int flags)
>> +{
>> +	if (strcmp(name, XATTR_NAME_DIGEST_LIST))
>> +		return;
>> +
>> +	digest_cache_reset_user(d_backing_inode(dentry), "file_setxattr");
>> +}
>> +
>> +/**
>> + * digest_cache_inode_post_removexattr() - An xattr was removed
>> + * @dentry: file
>> + * @name: xattr name
>> + *
>> + * This function is called after an xattr was removed from an existing inode.
>> + * If the inode points to a digest cache and the xattr removed is
>> + * security.digest_list, it resets dig_user in the inode security blob, to
>> + * force retrieving a fresh digest cache.
>> + */
>> +void digest_cache_inode_post_removexattr(struct dentry *dentry,
>> +					 const char *name)
> 
> nit:
> 
> void digest_cache_inode_post_removexattr(struct dentry *dentry, const char *name)
> 
> would be fine too

I still prefer to stay within 80 characters.

Thanks

Roberto

>> +{
>> +	if (strcmp(name, XATTR_NAME_DIGEST_LIST))
>> +		return;
>> +
>> +	digest_cache_reset_user(d_backing_inode(dentry), "file_removexattr");
>> +}
> 
> 
> BR, Jarkko

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