From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Kirill A. Shutemov" Subject: Re: [PATCH] fs: fix data race on mnt.mnt_flags Date: Mon, 21 Sep 2015 17:06:15 +0300 Message-ID: <20150921140615.GA30755@node.dhcp.inet.fi> References: <1442837807-70839-1-git-send-email-dvyukov@google.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: viro@zeniv.linux.org.uk, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, akpm@linux-foundation.org, kirill.shutemov@linux.intel.com, riel@redhat.com, mhocko@suse.cz, oleg@redhat.com, sasha.levin@oracle.com, gang.chen.5i5j@gmail.com, pfeiner@google.com, aarcange@redhat.com, vishnu.ps@samsung.com, linux-mm@kvack.org, glider@google.com, kcc@google.com, andreyknvl@google.com, ktsan@googlegroups.com, paulmck@linux.vnet.ibm.com To: Dmitry Vyukov Return-path: Received: from mail-wi0-f178.google.com ([209.85.212.178]:37509 "EHLO mail-wi0-f178.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932583AbbIUOGT (ORCPT ); Mon, 21 Sep 2015 10:06:19 -0400 Received: by wicfx3 with SMTP id fx3so113082222wic.0 for ; Mon, 21 Sep 2015 07:06:18 -0700 (PDT) Content-Disposition: inline In-Reply-To: <1442837807-70839-1-git-send-email-dvyukov@google.com> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On Mon, Sep 21, 2015 at 02:16:47PM +0200, Dmitry Vyukov wrote: > do_remount() does: > > mnt_flags |= mnt->mnt.mnt_flags & ~MNT_USER_SETTABLE_MASK; > mnt->mnt.mnt_flags = mnt_flags; > > This can easily be compiled as: > > mnt->mnt.mnt_flags &= ~MNT_USER_SETTABLE_MASK; > mnt->mnt.mnt_flags |= mnt_flags; > > (also 2 memory accesses, less register pressure) > The flags are being concurrently read by e.g. do_mmap_pgoff() > which does: > > if (file->f_path.mnt->mnt_flags & MNT_NOEXEC) > > As the result we can allow to mmap a MNT_NOEXEC mount > as VM_EXEC. > > Use WRITE_ONCE() to set new flags. > > The data race was found with KernelThreadSanitizer (KTSAN). > > Signed-off-by: Dmitry Vyukov Acked-by: Kirill A. Shutemov -- Kirill A. Shutemov