[PATCH v31 06/12] fs,security: Add sb_delete hook

[Mickaël Salaün]

From: Mickaël Salaün <mic at linux.microsoft.com>

The sb_delete security hook is called when shutting down a superblock,
which may be useful to release kernel objects tied to the superblock's
lifetime (e.g. inodes).

This new hook is needed by Landlock to release (ephemerally) tagged
struct inodes.  This comes from the unprivileged nature of Landlock
described in the next commit.

Cc: Al Viro <viro at zeniv.linux.org.uk>
Cc: James Morris <jmorris at namei.org>
Signed-off-by: Mickaël Salaün <mic at linux.microsoft.com>
Reviewed-by: Jann Horn <jannh at google.com>
Acked-by: Serge Hallyn <serge at hallyn.com>
Reviewed-by: Kees Cook <keescook at chromium.org>
Link: https://lore.kernel.org/r/20210324191520.125779-7-mic@digikod.net
---

Changes since v30:
* Add Reviewed-by Kees Cook.

Changes since v28:
* Extend hook description (suggested by Serge Hallyn).
* Add Acked-by Serge Hallyn.

Changes since v22:
* Add Reviewed-by Jann Horn.

Changes since v17:
* Initial patch to replace the direct call to landlock_release_inodes()
  (requested by James Morris).
  https://lore.kernel.org/lkml/alpine.LRH.2.21.2005150536440.7929@namei.org/
---
 fs/super.c                    | 1 +
 include/linux/lsm_hook_defs.h | 1 +
 include/linux/lsm_hooks.h     | 3 +++
 include/linux/security.h      | 4 ++++
 security/security.c           | 5 +++++
 5 files changed, 14 insertions(+)

diff --git a/fs/super.c b/fs/super.c
index 8c1baca35c16..11b7e7213fd1 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -454,6 +454,7 @@ void generic_shutdown_super(struct super_block *sb)
 		evict_inodes(sb);
 		/* only nonzero refcount inodes can have marks */
 		fsnotify_sb_delete(sb);
+		security_sb_delete(sb);
 
 		if (sb->s_dio_done_wq) {
 			destroy_workqueue(sb->s_dio_done_wq);
diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
index 477a597db013..e8adadbf9581 100644
--- a/include/linux/lsm_hook_defs.h
+++ b/include/linux/lsm_hook_defs.h
@@ -59,6 +59,7 @@ LSM_HOOK(int, 0, fs_context_dup, struct fs_context *fc,
 LSM_HOOK(int, -ENOPARAM, fs_context_parse_param, struct fs_context *fc,
 	 struct fs_parameter *param)
 LSM_HOOK(int, 0, sb_alloc_security, struct super_block *sb)
+LSM_HOOK(void, LSM_RET_VOID, sb_delete, struct super_block *sb)
 LSM_HOOK(void, LSM_RET_VOID, sb_free_security, struct super_block *sb)
 LSM_HOOK(void, LSM_RET_VOID, sb_free_mnt_opts, void *mnt_opts)
 LSM_HOOK(int, 0, sb_eat_lsm_opts, char *orig, void **mnt_opts)
diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
index 75715998a95f..cc2eaaaec0e4 100644
--- a/include/linux/lsm_hooks.h
+++ b/include/linux/lsm_hooks.h
@@ -108,6 +108,9 @@
  *	allocated.
  *	@sb contains the super_block structure to be modified.
  *	Return 0 if operation was successful.
+ * @sb_delete:
+ *	Release objects tied to a superblock (e.g. inodes).
+ *	@sb contains the super_block structure being released.
  * @sb_free_security:
  *	Deallocate and clear the sb->s_security field.
  *	@sb contains the super_block structure to be modified.
diff --git a/include/linux/security.h b/include/linux/security.h
index 8aeebd6646dc..90298baa4551 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -291,6 +291,7 @@ void security_bprm_committed_creds(struct linux_binprm *bprm);
 int security_fs_context_dup(struct fs_context *fc, struct fs_context *src_fc);
 int security_fs_context_parse_param(struct fs_context *fc, struct fs_parameter *param);
 int security_sb_alloc(struct super_block *sb);
+void security_sb_delete(struct super_block *sb);
 void security_sb_free(struct super_block *sb);
 void security_free_mnt_opts(void **mnt_opts);
 int security_sb_eat_lsm_opts(char *options, void **mnt_opts);
@@ -631,6 +632,9 @@ static inline int security_sb_alloc(struct super_block *sb)
 	return 0;
 }
 
+static inline void security_sb_delete(struct super_block *sb)
+{ }
+
 static inline void security_sb_free(struct super_block *sb)
 { }
 
diff --git a/security/security.c b/security/security.c
index e9c29480eb18..bb666f992497 100644
--- a/security/security.c
+++ b/security/security.c
@@ -900,6 +900,11 @@ int security_sb_alloc(struct super_block *sb)
 	return rc;
 }
 
+void security_sb_delete(struct super_block *sb)
+{
+	call_void_hook(sb_delete, sb);
+}
+
 void security_sb_free(struct super_block *sb)
 {
 	call_void_hook(sb_free_security, sb);
-- 
2.30.2