From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753638AbdLUSmL (ORCPT ); Thu, 21 Dec 2017 13:42:11 -0500 Received: from mail-io0-f196.google.com ([209.85.223.196]:34315 "EHLO mail-io0-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752629AbdLUSmF (ORCPT ); Thu, 21 Dec 2017 13:42:05 -0500 X-Google-Smtp-Source: ACJfBosJ4BbaCGzAv2b90k+iVztqgtO7bhnyCa+rtDhpmUButS0hY7+kSbdf9OYHM2QShqYseSIwEENEXZANhpEb/ug= MIME-Version: 1.0 In-Reply-To: <001a113ef748cc1ee50560c7b718@google.com> References: <001a113ef748cc1ee50560c7b718@google.com> From: Linus Torvalds Date: Thu, 21 Dec 2017 10:42:04 -0800 X-Google-Sender-Auth: dzr2H-S9pYcAttTZEyD-uj4ZxhY Message-ID: Subject: Re: general protection fault in finish_task_switch To: syzbot , Ingo Molnar , Peter Zijlstra Cc: Andrew Morton , Dave Jiang , Hugh Dickins , Jan Kara , Jerome Glisse , "Kirill A. Shutemov" , Linux Kernel Mailing List , linux-mm , tcharding , Michal Hocko , Minchan Kim , Ross Zwisler , syzkaller-bugs@googlegroups.com, Matthew Wilcox Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Dec 20, 2017 at 8:03 AM, syzbot wrote: > Hello, > > syzkaller hit the following crash on > 7dc9f647127d6955ffacaf51cb6a627b31dceec2 > git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/master > > kasan: CONFIG_KASAN_INLINE enabled > kasan: GPF could be caused by NULL-ptr deref or user memory access > general protection fault: 0000 [#1] SMP KASAN > Dumping ftrace buffer: > (ftrace buffer empty) > Modules linked in: > CPU: 0 PID: 4227 Comm: syzkaller244813 Not tainted 4.15.0-rc4-next-20171220+ > #77 > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS > Google 01/01/2011 > RIP: __fire_sched_in_preempt_notifiers kernel/sched/core.c:2534 [inline] That line 2534 is the call inside the hlist_for_each_entry() loop: hlist_for_each_entry(notifier, &curr->preempt_notifiers, link) notifier->ops->sched_in(notifier, raw_smp_processor_id()); and the Code: line disassembly is 0: ff 11 callq *(%rcx) 2: 4c 89 f9 mov %r15,%rcx 5: 48 c1 e9 03 shr $0x3,%rcx 9: 42 80 3c 31 00 cmpb $0x0,(%rcx,%r14,1) e: 0f 85 1b 02 00 00 jne 0x22f 14: 4d 8b 3f mov (%r15),%r15 17: 4d 85 ff test %r15,%r15 1a: 0f 84 c0 fd ff ff je 0xfffffffffffffde0 20: 49 8d 7f 10 lea 0x10(%r15),%rdi 24: 48 89 f9 mov %rdi,%rcx 27: 48 c1 e9 03 shr $0x3,%rcx 2b:* 42 80 3c 31 00 cmpb $0x0,(%rcx,%r14,1) <-- trapping instruction 30: 74 ae je 0xffffffffffffffe0 32: e8 a7 cc 5b 00 callq 0x5bccde 37: eb a7 jmp 0xffffffffffffffe0 39: 4c 89 fe mov %r15,%rsi 3c: 4c 89 e7 mov %r12,%rdi and while the "callq *(%rcx)" might be just the end part of some previous instruction, I think it may be right (there is indeed an indirect call in that function - that very "->sched_in()" call). So I think the oops happens after the indirect call returns. I think the second "callq" is call __asan_report_load8_noabort and the actual trapping instruction is loading the KASAN byte state. As far as I can tell, the kasan check is trying to check this part of hlist_for_each_entry(): movq (%r15), %r15 # notifier_110->link.next, and %r15 is dead000000000100, which is LIST_POISON1. End result: KASAN actually makes these things harder to debug, because it's trying to "validate" the list poison values before they are used, and takes a much more complex and indirect fault in the process, instead of just getting a page-fault on the LIST_POISON1 that would have made it more obvious. Oh well. There is nothing in this that indicates that it's actually related to KASAN, and it _should_ oops even without KASAN enabled. But the reproducer does nothing for me. Of course, I didn't actually run it on linux-next at all, so it is quite possibly related to scheduler work (or the TLB/pagetable work) that just hasn't hit mainstream yet. None of the scheduler people seem to have been on the report, though. Adding some in. Linus