Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758890AbcLUOnu (ORCPT ); Wed, 21 Dec 2016 09:43:50 -0500 Received: from mx2.suse.de ([195.135.220.15]:42399 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753384AbcLUOnr (ORCPT ); Wed, 21 Dec 2016 09:43:47 -0500 Date: Wed, 21 Dec 2016 15:43:43 +0100 From: Michal Hocko To: Dashi DS1 Cao Cc: "linux-mm@kvack.org" , "linux-kernel@vger.kernel.org" , Peter Zijlstra Subject: Re: A small window for a race condition in mm/rmap.c:page_lock_anon_vma_read Message-ID: <20161221144343.GD593@dhcp22.suse.cz> References: <23B7B563BA4E9446B962B142C86EF24ADBD62C@CNMAILEX03.lenovo.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <23B7B563BA4E9446B962B142C86EF24ADBD62C@CNMAILEX03.lenovo.com> User-Agent: Mutt/1.6.0 (2016-04-01) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3350 Lines: 76 anon_vma locking is clever^Wsubtle as hell. CC Peter... On Tue 20-12-16 09:32:27, Dashi DS1 Cao wrote: > I've collected four crash dumps with similar backtrace. > > PID: 247 TASK: ffff881fcfad8000 CPU: 14 COMMAND: "kswapd1" > #0 [ffff881fcfad7978] machine_kexec at ffffffff81051e9b > #1 [ffff881fcfad79d8] crash_kexec at ffffffff810f27e2 > #2 [ffff881fcfad7aa8] oops_end at ffffffff8163f448 > #3 [ffff881fcfad7ad0] die at ffffffff8101859b > #4 [ffff881fcfad7b00] do_general_protection at ffffffff8163ed3e > #5 [ffff881fcfad7b30] general_protection at ffffffff8163e5e8 > [exception RIP: down_read_trylock+9] > RIP: ffffffff810aa9f9 RSP: ffff881fcfad7be0 RFLAGS: 00010286 > RAX: 0000000000000000 RBX: ffff882b47ddadc0 RCX: 0000000000000000 > RDX: 0000000000000000 RSI: 0000000000000000 RDI: 91550b2b32f5a3e8 rdi is obviously a mess - smells like a string. So either sombody has overwritten root_anon_vma or this is really a use after free... > RBP: ffff881fcfad7be0 R8: ffffea00ecc28860 R9: ffff883fcffeae28 > R10: ffffffff81a691a0 R11: 0000000000000001 R12: ffff882b47ddadc1 > R13: ffffea00ecc28840 R14: 91550b2b32f5a3e8 R15: ffffea00ecc28840 > ORIG_RAX: ffffffffffffffff CS: 0010 SS: 0000 > #6 [ffff881fcfad7be8] page_lock_anon_vma_read at ffffffff811a3365 > #7 [ffff881fcfad7c18] page_referenced at ffffffff811a35e7 > #8 [ffff881fcfad7c90] shrink_active_list at ffffffff8117e8cc > #9 [ffff881fcfad7d48] balance_pgdat at ffffffff81180288 > #10 [ffff881fcfad7e20] kswapd at ffffffff81180813 > #11 [ffff881fcfad7ec8] kthread at ffffffff810a5b8f > #12 [ffff881fcfad7f50] ret_from_fork at ffffffff81646a98 > > I suspect my customer hits into a small window of a race condition in mm/rmap.c: page_lock_anon_vma_read. > struct anon_vma *page_lock_anon_vma_read(struct page *page) > { > struct anon_vma *anon_vma = NULL; > struct anon_vma *root_anon_vma; > unsigned long anon_mapping; > > rcu_read_lock(); > anon_mapping = (unsigned long)READ_ONCE(page->mapping); > if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON) > goto out; > if (!page_mapped(page)) > goto out; > > anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON); > root_anon_vma = READ_ONCE(anon_vma->root); Could you dump the anon_vma and struct page as well? > if (down_read_trylock(&root_anon_vma->rwsem)) { > /* > * If the page is still mapped, then this anon_vma is still > * its anon_vma, and holding the mutex ensures that it will > * not go away, see anon_vma_free(). > */ > if (!page_mapped(page)) { > up_read(&root_anon_vma->rwsem); > anon_vma = NULL; > } > goto out; > } > ... > } > > Between the time the two "page_mapped(page)" are checked, the address > (anon_mapping - PAGE_MAPPING_ANON) is unmapped! However it seems > that anon_vma->root could still be read in but the value is wild. So > the kernel crashes in down_read_trylock. But it's weird that all the > "struct page" has its member "_mapcount" still with value 0, not -1, > in the four crashes. -- Michal Hocko SUSE Labs