From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751943AbdGFR7k (ORCPT ); Thu, 6 Jul 2017 13:59:40 -0400 Received: from mail-pg0-f66.google.com ([74.125.83.66]:36822 "EHLO mail-pg0-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750778AbdGFR7h (ORCPT ); Thu, 6 Jul 2017 13:59:37 -0400 From: Eric Biggers To: linux-fscrypt@vger.kernel.org Cc: "Theodore Y . Ts'o" , Jaegeuk Kim , linux-kernel@vger.kernel.org, stable@vger.kernel.org, Eric Biggers Subject: [PATCH] fscrypt: lock mutex before checking for bounce page pool Date: Thu, 6 Jul 2017 10:57:48 -0700 Message-Id: <20170706175748.33093-1-ebiggers3@gmail.com> X-Mailer: git-send-email 2.13.2.725.g09c95d1e9-goog Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Eric Biggers fscrypt_initialize(), which allocates the global bounce page pool when an encrypted file is first accessed, uses "double-checked locking" to try to avoid locking fscrypt_init_mutex. However, it doesn't use any memory barriers, so it's theoretically possible for a thread to observe a bounce page pool which has not been fully initialized. This is a classic bug with "double-checked locking". While "only a theoretical issue" in the latest kernel, in pre-4.8 kernels the pointer that was checked was not even the last to be initialized, so it was easily possible for a crash (NULL pointer dereference) to happen. This was changed only incidentally by the large refactor to use fs/crypto/. Solve both problems in a trivial way that can easily be backported: just always take the mutex. It's theoretically less efficient, but it shouldn't be noticeable in practice as the mutex is only acquired very briefly once per encrypted file. Later I'd like to make this use a helper macro like DO_ONCE(). However, DO_ONCE() runs in atomic context, so we'd need to add a new macro that allows blocking. Cc: stable@vger.kernel.org # v4.1+ Signed-off-by: Eric Biggers --- fs/crypto/crypto.c | 7 ++----- 1 file changed, 2 insertions(+), 5 deletions(-) diff --git a/fs/crypto/crypto.c b/fs/crypto/crypto.c index c7835df7e7b8..d262a93d9b31 100644 --- a/fs/crypto/crypto.c +++ b/fs/crypto/crypto.c @@ -410,11 +410,8 @@ int fscrypt_initialize(unsigned int cop_flags) { int i, res = -ENOMEM; - /* - * No need to allocate a bounce page pool if there already is one or - * this FS won't use it. - */ - if (cop_flags & FS_CFLG_OWN_PAGES || fscrypt_bounce_page_pool) + /* No need to allocate a bounce page pool if this FS won't use it. */ + if (cop_flags & FS_CFLG_OWN_PAGES) return 0; mutex_lock(&fscrypt_init_mutex); -- 2.13.2.725.g09c95d1e9-goog