From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751170AbbIGLkx (ORCPT ); Mon, 7 Sep 2015 07:40:53 -0400 Received: from mail-wi0-f174.google.com ([209.85.212.174]:38622 "EHLO mail-wi0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750997AbbIGLkv (ORCPT ); Mon, 7 Sep 2015 07:40:51 -0400 Date: Mon, 7 Sep 2015 14:40:48 +0300 From: "Kirill A. Shutemov" To: Sasha Levin , Rik van Riel , Andrew Morton Cc: Andrey Konovalov , Dmitry Vyukov , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Multiple potential races on vma->vm_flags Message-ID: <20150907114048.GA5016@node.dhcp.inet.fi> References: <55EC9221.4040603@oracle.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <55EC9221.4040603@oracle.com> User-Agent: Mutt/1.5.23.1 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Sep 06, 2015 at 03:21:05PM -0400, Sasha Levin wrote: > ================================================================== > ThreadSanitizer: data-race in munlock_vma_pages_range > > Write of size 8 by thread T378 (K2633, CPU3): > [] munlock_vma_pages_range+0x59/0x3e0 mm/mlock.c:425 > [] mlock_fixup+0x1c9/0x280 mm/mlock.c:549 > [] do_mlock+0x14c/0x180 mm/mlock.c:589 > [< inlined >] SyS_munlock+0x74/0xb0 SYSC_munlock mm/mlock.c:651 > [] SyS_munlock+0x74/0xb0 mm/mlock.c:643 > [] entry_SYSCALL_64_fastpath+0x12/0x71 > arch/x86/entry/entry_64.S:186 ... > Previous read of size 8 by thread T398 (K2623, CPU2): > [] try_to_unmap_one+0x78/0x4f0 mm/rmap.c:1208 > [< inlined >] rmap_walk+0x147/0x450 rmap_walk_file mm/rmap.c:1540 > [] rmap_walk+0x147/0x450 mm/rmap.c:1559 > [] try_to_munlock+0xa2/0xc0 mm/rmap.c:1423 > [] __munlock_isolated_page+0x30/0x60 mm/mlock.c:129 > [] __munlock_pagevec+0x236/0x3f0 mm/mlock.c:331 > [] munlock_vma_pages_range+0x380/0x3e0 mm/mlock.c:476 > [] mlock_fixup+0x1c9/0x280 mm/mlock.c:549 > [] do_mlock+0x14c/0x180 mm/mlock.c:589 > [< inlined >] SyS_munlock+0x74/0xb0 SYSC_munlock mm/mlock.c:651 > [] SyS_munlock+0x74/0xb0 mm/mlock.c:643 > [] entry_SYSCALL_64_fastpath+0x12/0x71 > arch/x86/entry/entry_64.S:186 Okay, the detected race is mlock/munlock vs. rmap. On rmap side we check vma->vm_flags in few places without taking vma->vm_mm->mmap_sem. The vma cannot be freed since we hold i_mmap_rwsem or anon_vma_lock, but nothing prevent vma->vm_flags from changing under us. In this particular case, speculative check in beginning of try_to_unmap_one() is fine, since we re-check it under mmap_sem later in the function. False-negative is fine too here, since we will mlock the page in __mm_populate() on mlock side after mlock_fixup(). BUT. We *must* have all speculative vm_flags accesses wrapped READ_ONCE() to avoid all compiler trickery, like duplication vm_flags access with inconsistent results. I looked only on VM_LOCKED checks, but there are few other flags checked in rmap. All of them must be handled carefully. At least READ_ONCE() is required. Other solution would be to introduce per-vma spinlock to protect vma->vm_flags and probably other vma fields and offload this duty from mmap_sem. But that's much bigger project. -- Kirill A. Shutemov