evm: don't copy up 'security.evm' xattr

The security.evm HMAC and the original file signatures contain
filesystem specific data.  As a result, the HMAC and signature
are not the same on the stacked and backing filesystems.

Don't copy up 'security.evm'.
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

include/linux/evm.h

@@ -31,6 +31,7 @@ extern void evm_inode_post_setxattr(struct dentry *dentry,
 				    const char *xattr_name,
 				    const void *xattr_value,
 				    size_t xattr_value_len);
+extern int evm_inode_copy_up_xattr(const char *name);
 extern int evm_inode_removexattr(struct mnt_idmap *idmap,
 				 struct dentry *dentry, const char *xattr_name);
 extern void evm_inode_post_removexattr(struct dentry *dentry,
@@ -117,6 +118,11 @@ static inline void evm_inode_post_setxattr(struct dentry *dentry,
 	return;
 }
 
+static inline int  evm_inode_copy_up_xattr(const char *name)
+{
+	return 0;
+}
+
 static inline int evm_inode_removexattr(struct mnt_idmap *idmap,
 					struct dentry *dentry,
 					const char *xattr_name)

security/integrity/evm/evm_main.c

@@ -863,6 +863,13 @@ void evm_inode_post_setattr(struct dentry *dentry, int ia_valid)
 		evm_update_evmxattr(dentry, NULL, NULL, 0);
 }
 
+int evm_inode_copy_up_xattr(const char *name)
+{
+	if (strcmp(name, XATTR_NAME_EVM) == 0)
+		return 1; /* Discard */
+	return -EOPNOTSUPP;
+}
+
 /*
  * evm_inode_init_security - initializes security.evm HMAC value
  */

security/security.c

@@ -2539,7 +2539,7 @@ int security_inode_copy_up_xattr(const char *name)
 			return rc;
 	}
 
-	return LSM_RET_DEFAULT(inode_copy_up_xattr);
+	return evm_inode_copy_up_xattr(name);
 }
 EXPORT_SYMBOL(security_inode_copy_up_xattr);