From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 6E697C28CC0 for ; Wed, 29 May 2019 04:22:07 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 283CB2075C for ; Wed, 29 May 2019 04:22:07 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726008AbfE2EWG (ORCPT ); Wed, 29 May 2019 00:22:06 -0400 Received: from ozlabs.org ([203.11.71.1]:36779 "EHLO ozlabs.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725856AbfE2EWF (ORCPT ); Wed, 29 May 2019 00:22:05 -0400 Received: from authenticated.ozlabs.org (localhost [127.0.0.1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange ECDHE (P-256) server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by mail.ozlabs.org (Postfix) with ESMTPSA id 45DHbm6RTRz9s4V; Wed, 29 May 2019 14:21:55 +1000 (AEST) From: Michael Ellerman To: Will Deacon , Peter Zijlstra Cc: Young Xiao <92siuyang@gmail.com>, linux@armlinux.org.uk, mark.rutland@arm.com, mingo@redhat.com, bp@alien8.de, hpa@zytor.com, x86@kernel.org, kan.liang@linux.intel.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, ravi.bangoria@linux.vnet.ibm.com Subject: Re: [PATCH] perf: Fix oops when kthread execs user process In-Reply-To: <20190528153224.GE20758@fuggles.cambridge.arm.com> References: <1559046689-24091-1-git-send-email-92siuyang@gmail.com> <20190528140103.GT2623@hirez.programming.kicks-ass.net> <20190528153224.GE20758@fuggles.cambridge.arm.com> Date: Wed, 29 May 2019 14:21:54 +1000 Message-ID: <877ea9q49p.fsf@concordia.ellerman.id.au> MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Will Deacon writes: > On Tue, May 28, 2019 at 04:01:03PM +0200, Peter Zijlstra wrote: >> On Tue, May 28, 2019 at 08:31:29PM +0800, Young Xiao wrote: >> > When a kthread calls call_usermodehelper() the steps are: >> > 1. allocate current->mm >> > 2. load_elf_binary() >> > 3. populate current->thread.regs >> > >> > While doing this, interrupts are not disabled. If there is a perf >> > interrupt in the middle of this process (i.e. step 1 has completed >> > but not yet reached to step 3) and if perf tries to read userspace >> > regs, kernel oops. > > This seems to be because pt_regs(current) gives NULL for kthreads on Power. Right, we've done that since roughly forever in copy_thread(): int copy_thread(unsigned long clone_flags, unsigned long usp, unsigned long kthread_arg, struct task_struct *p) { ... /* Copy registers */ sp -= sizeof(struct pt_regs); childregs = (struct pt_regs *) sp; if (unlikely(p->flags & PF_KTHREAD)) { /* kernel thread */ memset(childregs, 0, sizeof(struct pt_regs)); childregs->gpr[1] = sp + sizeof(struct pt_regs); ... p->thread.regs = NULL; /* no user register state */ See commit from 2002: https://github.com/mpe/linux-fullhistory/commit/c0a96c0918d21d8a99270e94d9c4a4a322d04581#diff-edb76bfcc84905163f34d24d2aad3f3aR187 > From the initial report [1], it doesn't look like the mm isn't initialised, > but rather than we're dereferencing a NULL pt_regs pointer somehow for the > current task (see previous comment). I don't see how that can happen on > arm64, given that we put the pt_regs on the kernel stack which is allocated > during fork. We have the regs on the stack too (see above), but we're explicitly NULL'ing the link from task->thread. Looks like on arm64 and x86 there is no link from task->thread, instead you get from task to pt_regs via task_stack_page(). That actually seems potentially fishy given the comment on task_stack_page() about the stack going away for exiting tasks. We should probably be NULL'ing the regs pointer in free_thread_stack() or similar. Though that race mustn't be happening because other arches would see it. Or are we just wrong and kthreads should have non-NULL regs? I can't find another arch that does the same as us. cheers