[PATCH v4 11/14] digest_cache: Reset digest cache on file/directory change

[Roberto Sassu]

From: Roberto Sassu <roberto.sassu@xxxxxxxxxx>

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@xxxxxxxxxx>
---
 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@xxxxxxxxxx>
+ *
+ * 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)
+{
+	if (strcmp(name, XATTR_NAME_DIGEST_LIST))
+		return;
+
+	digest_cache_reset_user(d_backing_inode(dentry), "file_removexattr");
+}
-- 
2.34.1