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=-8.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 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 1EA9BC32792 for ; Mon, 30 Sep 2019 12:23:23 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id F088120815 for ; Mon, 30 Sep 2019 12:23:22 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730397AbfI3MXW (ORCPT ); Mon, 30 Sep 2019 08:23:22 -0400 Received: from foss.arm.com ([217.140.110.172]:53116 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726072AbfI3MXV (ORCPT ); Mon, 30 Sep 2019 08:23:21 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 056921000; Mon, 30 Sep 2019 05:23:21 -0700 (PDT) Received: from [10.1.196.50] (e108454-lin.cambridge.arm.com [10.1.196.50]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 1819B3F706; Mon, 30 Sep 2019 05:23:19 -0700 (PDT) Subject: Re: [PATCH v2] arm64/sve: Fix wrong free for task->thread.sve_state To: Masayoshi Mizuma , Dave Martin , Catalin Marinas , Will Deacon , linux-arm-kernel@lists.infradead.org Cc: Masayoshi Mizuma , linux-kernel@vger.kernel.org References: <20190927153949.29870-1-msys.mizuma@gmail.com> From: Julien Grall Message-ID: Date: Mon, 30 Sep 2019 13:23:18 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.9.0 MIME-Version: 1.0 In-Reply-To: <20190927153949.29870-1-msys.mizuma@gmail.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, On 27/09/2019 16:39, Masayoshi Mizuma wrote: > From: Masayoshi Mizuma > > The system which has SVE feature crashed because of > the memory pointed by task->thread.sve_state was destroyed > by someone. > > That is because sve_state is freed while the forking the > child process. The child process has the pointer of sve_state > which is same as the parent's because the child's task_struct > is copied from the parent's one. If the copy_process() > fails as an error on somewhere, for example, copy_creds(), > then the sve_state is freed even if the parent is alive. > The flow is as follows. > > copy_process > p = dup_task_struct > => arch_dup_task_struct > *dst = *src; // copy the entire region. > : > retval = copy_creds > if (retval < 0) > goto bad_fork_free; > : > bad_fork_free: > ... > delayed_free_task(p); > => free_task > => arch_release_task_struct > => fpsimd_release_task > => __sve_free > => kfree(task->thread.sve_state); > // free the parent's sve_state > > Move child's sve_state = NULL and clearing TIF_SVE flag > to arch_dup_task_struct() so that the child doesn't free the > parent's one. > > Cc: stable@vger.kernel.org > Fixes: bc0ee4760364 ("arm64/sve: Core task context handling") Looking at the log, it looks like THREAD_INFO_IN_TASK was selected before the bc0ee4760364. So it should be fine to backport for all the Linux tree contain this commit. > Signed-off-by: Masayoshi Mizuma > Reported-by: Hidetoshi Seto > Suggested-by: Dave Martin I have tested the patch and can confirm that double-free disappeared after the patch is applied: Tested-by: Julien Grall See below for a few comments. > --- > arch/arm64/kernel/process.c | 21 ++++----------------- > 1 file changed, 4 insertions(+), 17 deletions(-) > > diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c > index f674f28df..6937f5935 100644 > --- a/arch/arm64/kernel/process.c > +++ b/arch/arm64/kernel/process.c > @@ -323,22 +323,16 @@ void arch_release_task_struct(struct task_struct *tsk) > fpsimd_release_task(tsk); > } > > -/* > - * src and dst may temporarily have aliased sve_state after task_struct > - * is copied. We cannot fix this properly here, because src may have > - * live SVE state and dst's thread_info may not exist yet, so tweaking > - * either src's or dst's TIF_SVE is not safe. > - * > - * The unaliasing is done in copy_thread() instead. This works because > - * dst is not schedulable or traceable until both of these functions > - * have been called. > - */ It would be good to explain in the commit message why tweaking "dst" in arch_dup_task_struct() is fine. From my understanding, Arm64 used to have thread_info on the stack. So it would not be possible to clear TIF_SVE until the stack is initialized. Now that the thread_info is part of the task, it should be valid to modify the flag from arch_dup_task_struct(). Note that technically, TIF_SVE does not need to be cleared from arch_dup_task_struct(). It could also be done from copy_thread(). But it is easier to keep the both changes together. > int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src) > { > if (current->mm) > fpsimd_preserve_current_state(); > *dst = *src; > > + BUILD_BUG_ON(!IS_ENABLED(CONFIG_THREAD_INFO_IN_TASK)); You may want to add a comment on top explaining why TIF_SVE is cleared here. > + dst->thread.sve_state = NULL; > + clear_tsk_thread_flag(dst, TIF_SVE); > + > return 0; > } > > @@ -351,13 +345,6 @@ int copy_thread(unsigned long clone_flags, unsigned long stack_start, > > memset(&p->thread.cpu_context, 0, sizeof(struct cpu_context)); > > - /* > - * Unalias p->thread.sve_state (if any) from the parent task > - * and disable discard SVE state for p: > - */ > - clear_tsk_thread_flag(p, TIF_SVE); > - p->thread.sve_state = NULL; > - > /* > * In case p was allocated the same task_struct pointer as some > * other recently-exited task, make sure p is disassociated from > Cheers, -- Julien Grall