[PATCH v9 11/25] security: Introduce inode_post_removexattr hook

[Roberto Sassu]

From: Roberto Sassu <roberto.sassu@xxxxxxxxxx>

In preparation for moving IMA and EVM to the LSM infrastructure, introduce
the inode_post_removexattr hook.

At inode_removexattr hook, EVM verifies the file's existing HMAC value. At
inode_post_removexattr, EVM re-calculates the file's HMAC with the passed
xattr removed and other file metadata.

Other LSMs could similarly take some action after successful xattr removal.

The new hook cannot return an error and cannot cause the operation to be
reverted.

Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
Reviewed-by: Stefan Berger <stefanb@xxxxxxxxxxxxx>
Reviewed-by: Mimi Zohar <zohar@xxxxxxxxxxxxx>
Reviewed-by: Casey Schaufler <casey@xxxxxxxxxxxxxxxx>
---
 fs/xattr.c                    |  9 +++++----
 include/linux/lsm_hook_defs.h |  2 ++
 include/linux/security.h      |  5 +++++
 security/security.c           | 14 ++++++++++++++
 4 files changed, 26 insertions(+), 4 deletions(-)

diff --git a/fs/xattr.c b/fs/xattr.c
index 09d927603433..f891c260a971 100644
--- a/fs/xattr.c
+++ b/fs/xattr.c
@@ -552,11 +552,12 @@ __vfs_removexattr_locked(struct mnt_idmap *idmap,
 		goto out;
 
 	error = __vfs_removexattr(idmap, dentry, name);
+	if (error)
+		return error;
 
-	if (!error) {
-		fsnotify_xattr(dentry);
-		evm_inode_post_removexattr(dentry, name);
-	}
+	fsnotify_xattr(dentry);
+	security_inode_post_removexattr(dentry, name);
+	evm_inode_post_removexattr(dentry, name);
 
 out:
 	return error;
diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
index e1731d0b63f8..fbca96523c18 100644
--- a/include/linux/lsm_hook_defs.h
+++ b/include/linux/lsm_hook_defs.h
@@ -149,6 +149,8 @@ LSM_HOOK(int, 0, inode_getxattr, struct dentry *dentry, const char *name)
 LSM_HOOK(int, 0, inode_listxattr, struct dentry *dentry)
 LSM_HOOK(int, 0, inode_removexattr, struct mnt_idmap *idmap,
 	 struct dentry *dentry, const char *name)
+LSM_HOOK(void, LSM_RET_VOID, inode_post_removexattr, struct dentry *dentry,
+	 const char *name)
 LSM_HOOK(int, 0, inode_set_acl, struct mnt_idmap *idmap,
 	 struct dentry *dentry, const char *acl_name, struct posix_acl *kacl)
 LSM_HOOK(int, 0, inode_get_acl, struct mnt_idmap *idmap,
diff --git a/include/linux/security.h b/include/linux/security.h
index 56c841aa3994..84ae03690340 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -380,6 +380,7 @@ int security_inode_getxattr(struct dentry *dentry, const char *name);
 int security_inode_listxattr(struct dentry *dentry);
 int security_inode_removexattr(struct mnt_idmap *idmap,
 			       struct dentry *dentry, const char *name);
+void security_inode_post_removexattr(struct dentry *dentry, const char *name);
 int security_inode_need_killpriv(struct dentry *dentry);
 int security_inode_killpriv(struct mnt_idmap *idmap, struct dentry *dentry);
 int security_inode_getsecurity(struct mnt_idmap *idmap,
@@ -942,6 +943,10 @@ static inline int security_inode_removexattr(struct mnt_idmap *idmap,
 	return cap_inode_removexattr(idmap, dentry, name);
 }
 
+static inline void security_inode_post_removexattr(struct dentry *dentry,
+						   const char *name)
+{ }
+
 static inline int security_inode_need_killpriv(struct dentry *dentry)
 {
 	return cap_inode_need_killpriv(dentry);
diff --git a/security/security.c b/security/security.c
index 06ffe0bcf711..d9d2636104db 100644
--- a/security/security.c
+++ b/security/security.c
@@ -2452,6 +2452,20 @@ int security_inode_removexattr(struct mnt_idmap *idmap,
 	return evm_inode_removexattr(idmap, dentry, name);
 }
 
+/**
+ * security_inode_post_removexattr() - Update the inode after a removexattr op
+ * @dentry: file
+ * @name: xattr name
+ *
+ * Update the inode after a successful removexattr operation.
+ */
+void security_inode_post_removexattr(struct dentry *dentry, const char *name)
+{
+	if (unlikely(IS_PRIVATE(d_backing_inode(dentry))))
+		return;
+	call_void_hook(inode_post_removexattr, dentry, name);
+}
+
 /**
  * security_inode_need_killpriv() - Check if security_inode_killpriv() required
  * @dentry: associated dentry
-- 
2.34.1