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[209.132.180.67]) by mx.google.com with ESMTP id ec21si6420137ejb.26.2019.09.30.06.03.05; Mon, 30 Sep 2019 06:03:30 -0700 (PDT) Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67; Authentication-Results: mx.google.com; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731032AbfI3NCu (ORCPT + 99 others); Mon, 30 Sep 2019 09:02:50 -0400 Received: from foss.arm.com ([217.140.110.172]:53784 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730830AbfI3NCu (ORCPT ); Mon, 30 Sep 2019 09:02:50 -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 8E9751000; Mon, 30 Sep 2019 06:02:49 -0700 (PDT) Received: from arm.com (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 8C1FF3F706; Mon, 30 Sep 2019 06:02:48 -0700 (PDT) Date: Mon, 30 Sep 2019 14:02:46 +0100 From: Dave Martin To: Julien Grall Cc: Masayoshi Mizuma , Catalin Marinas , Will Deacon , linux-arm-kernel@lists.infradead.org, Masayoshi Mizuma , linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] arm64/sve: Fix wrong free for task->thread.sve_state Message-ID: <20190930130244.GT27757@arm.com> References: <20190927153949.29870-1-msys.mizuma@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Sep 30, 2019 at 01:23:18PM +0100, Julien Grall wrote: > 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 Good to have that confirmed -- thanks for verifying. [...] > >--- > > 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; Ack, some more explanation would be a good idea here. Maybe the following comments are sufficient? /* We rely on the above assingment to initialise dst's thread_flags: */ > >+ BUILD_BUG_ON(!IS_ENABLED(CONFIG_THREAD_INFO_IN_TASK)); > and /* * Detach src's sve_state (if any) from dst so that it does not * get erroneously used or freed prematurely. dst's sve_state * will be allocated on demand later on if dst uses SVE. * For consistency, also clear TIF_SVE here: this could be done * later in copy_process(), but to avoid tripping up future * maintainers it is best not to leave TIF_SVE and sve_state in * an inconsistent state, even temporarily. */ > >+ dst->thread.sve_state = NULL; > >+ clear_tsk_thread_flag(dst, TIF_SVE); (TIF_SVE should not usually be set in the first place of course, since we are in a fork() or clone() syscall in src. This may not be true if a task is created using kernel_thread() while running in the context of some user task that entered the kernel due to a trap or syscall -- though probably nobody should be doing that.) [...] Cheers ---DavE