evm: fix potential race when removing xattrs

EVM needs to be atomically updated when removing xattrs.
Otherwise concurrent EVM verification may fail in between.
This patch fixes by moving i_mutex unlocking after calling
EVM hook. fsnotify_xattr() is also now called while locked
the same way as it is done in __vfs_setxattr_noperm.

Changelog:
- remove unused 'inode' variable.
Signed-off-by: Dmitry Kasatkin <d.kasatkin@samsung.com>
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>

fs/xattr.c

@@ -298,18 +298,18 @@ vfs_removexattr(struct dentry *dentry, const char *name)
 
 	mutex_lock(&inode->i_mutex);
 	error = security_inode_removexattr(dentry, name);
-	if (error) {
-		mutex_unlock(&inode->i_mutex);
-		return error;
-	}
+	if (error)
+		goto out;
 
 	error = inode->i_op->removexattr(dentry, name);
-	mutex_unlock(&inode->i_mutex);
 
 	if (!error) {
 		fsnotify_xattr(dentry);
 		evm_inode_post_removexattr(dentry, name);
 	}
+
+out:
+	mutex_unlock(&inode->i_mutex);
 	return error;
 }
 EXPORT_SYMBOL_GPL(vfs_removexattr);

security/integrity/evm/evm_main.c

@@ -387,17 +387,16 @@ void evm_inode_post_setxattr(struct dentry *dentry, const char *xattr_name,
  * @xattr_name: pointer to the affected extended attribute name
  *
  * Update the HMAC stored in 'security.evm' to reflect removal of the xattr.
+ *
+ * No need to take the i_mutex lock here, as this function is called from
+ * vfs_removexattr() which takes the i_mutex.
  */
 void evm_inode_post_removexattr(struct dentry *dentry, const char *xattr_name)
 {
-	struct inode *inode = d_backing_inode(dentry);
-
 	if (!evm_initialized || !evm_protected_xattr(xattr_name))
 		return;
 
-	mutex_lock(&inode->i_mutex);
 	evm_update_evmxattr(dentry, xattr_name, NULL, 0);
-	mutex_unlock(&inode->i_mutex);
 }
 
 /**