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=-7.2 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_2 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 4C6A2C43331 for ; Fri, 8 Nov 2019 00:54:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 0B4392084D for ; Fri, 8 Nov 2019 00:54:47 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="Bnyc/x8R" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727543AbfKHAyp (ORCPT ); Thu, 7 Nov 2019 19:54:45 -0500 Received: from mail-pf1-f194.google.com ([209.85.210.194]:45538 "EHLO mail-pf1-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725930AbfKHAyp (ORCPT ); Thu, 7 Nov 2019 19:54:45 -0500 Received: by mail-pf1-f194.google.com with SMTP id z4so3546415pfn.12 for ; Thu, 07 Nov 2019 16:54:43 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=message-id:subject:from:to:cc:date:in-reply-to:references :mime-version:content-transfer-encoding; bh=RwwCfPj9kkXT/tgPNBKXIQ7hGpqfevKBcJxOL853a+o=; b=Bnyc/x8RgDN22tHTNccOzxXcbeqyCFSZEudqb044t0pCgqzb6m4VsO8rEqeq0Ilipa xi62vOifeZH0Jais2ka51HXOWAR3B+NKDPcQ47d23MZ4pEtm0VOWh4yVIC5TWg9NTOZB LhWk26/+e3lLVNI39BL0DX9a+bBm+MDdyPHSpUczQTtgVpwg2T0HukTWuozC06qMt+Ap YU+d67FubOqnkC3T4aT9cvik5rLCPIibktuI04o9tiHkUYe3IEOBy8DIPhYqonslsoLc 9vJLmD/1lighvX/5L2bpt/O9e/M9tcTY2EKUWUu2MP8sKJjJiFI+Cn/+72RkjOAduHho SMfw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:message-id:subject:from:to:cc:date:in-reply-to :references:mime-version:content-transfer-encoding; bh=RwwCfPj9kkXT/tgPNBKXIQ7hGpqfevKBcJxOL853a+o=; b=qHCFziVVhOK3h63mwnMBhSEk7sGx5yrvswCaaQLT7LU2sYXB7ZY22LdBMShhBq+fsF lix1GpJBevRCBrWUNyrRZ7prQ75i5q3YGFJCsnLKrghB0LKvlSDuRghhoO+gNooSCBRc D1bp6viNnTwRYH89MIjzOWNHAzZdHJ8CYk5qTpzdhGAJcY3lgOn3B862Q0gc/oZc0GQ0 fOyk9h+3+48enpVCCgvX3o4iGFm1TtLn/8DB5Hw8dIymJbnQ7p1O8UibTs+nR9ite9lr Jh7GnzQKoKhFwGXYpZ5AAu1dVXosdgSyF4mBhfa5gDRMCwBb8qN83HWZhSf4lIuUsYUG BwsA== X-Gm-Message-State: APjAAAV10JSJ/R4LBz94ERcxGjLrTOxIT9E+X8UEI74xQqDAXFt5sIAe zEweCNzg9zDVACy6FFecoZQ= X-Google-Smtp-Source: APXvYqwbwdUvkn0aiqWIkFptXc7o8wUplF9P7eaHrcBHFY6F3XtojjOGk6duBwebyy7kMRcqO7B1rA== X-Received: by 2002:a62:2a14:: with SMTP id q20mr8118766pfq.148.1573174482785; Thu, 07 Nov 2019 16:54:42 -0800 (PST) Received: from u2f459ca2e0dd5b.ant.amazon.com ([61.69.148.113]) by smtp.googlemail.com with ESMTPSA id z18sm3894528pgv.90.2019.11.07.16.54.39 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Thu, 07 Nov 2019 16:54:41 -0800 (PST) Message-ID: Subject: Re: KCSAN: data-race in taskstats_exit / taskstats_exit From: Balbir Singh To: Marco Elver Cc: syzbot , LKML , syzkaller-bugs@googlegroups.com Date: Fri, 08 Nov 2019 11:54:36 +1100 In-Reply-To: References: <0000000000009b403005942237bf@google.com> <27b57b11aadf1bd41ad8326101713ca0be7b8edf.camel@gmail.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.28.5-0ubuntu0.18.04.1 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 2019-11-06 at 11:23 +0100, Marco Elver wrote: > On Wed, 6 Nov 2019 at 01:10, Balbir Singh wrote: > > > > On Fri, 2019-10-04 at 21:26 -0700, syzbot wrote: > > > Hello, > > > > > > syzbot found the following crash on: > > > > > > HEAD commit: b4bd9343 x86, kcsan: Enable KCSAN for x86 > > > git tree: https://github.com/google/ktsan.git kcsan > > > console output: https://syzkaller.appspot.com/x/log.txt?x=125329db600000 > > > kernel config: > > > https://syzkaller.appspot.com/x/.config?x=c0906aa620713d80 > > > dashboard link: > > > https://syzkaller.appspot.com/bug?extid=c5d03165a1bd1dead0c1 > > > compiler: gcc (GCC) 9.0.0 20181231 (experimental) > > > > > > Unfortunately, I don't have any reproducer for this crash yet. > > > > > > IMPORTANT: if you fix the bug, please add the following tag to the > > > commit: > > > Reported-by: syzbot+c5d03165a1bd1dead0c1@syzkaller.appspotmail.com > > > > > > ================================================================== > > > BUG: KCSAN: data-race in taskstats_exit / taskstats_exit > > > > > > write to 0xffff8881157bbe10 of 8 bytes by task 7951 on cpu 0: > > > taskstats_tgid_alloc kernel/taskstats.c:567 [inline] > > > taskstats_exit+0x6b7/0x717 kernel/taskstats.c:596 > > > do_exit+0x2c2/0x18e0 kernel/exit.c:864 > > > do_group_exit+0xb4/0x1c0 kernel/exit.c:983 > > > get_signal+0x2a2/0x1320 kernel/signal.c:2734 > > > do_signal+0x3b/0xc00 arch/x86/kernel/signal.c:815 > > > exit_to_usermode_loop+0x250/0x2c0 arch/x86/entry/common.c:159 > > > prepare_exit_to_usermode arch/x86/entry/common.c:194 [inline] > > > syscall_return_slowpath arch/x86/entry/common.c:274 [inline] > > > do_syscall_64+0x2d7/0x2f0 arch/x86/entry/common.c:299 > > > entry_SYSCALL_64_after_hwframe+0x44/0xa9 > > > > > > read to 0xffff8881157bbe10 of 8 bytes by task 7949 on cpu 1: > > > taskstats_tgid_alloc kernel/taskstats.c:559 [inline] > > > taskstats_exit+0xb2/0x717 kernel/taskstats.c:596 > > > do_exit+0x2c2/0x18e0 kernel/exit.c:864 > > > do_group_exit+0xb4/0x1c0 kernel/exit.c:983 > > > __do_sys_exit_group kernel/exit.c:994 [inline] > > > __se_sys_exit_group kernel/exit.c:992 [inline] > > > __x64_sys_exit_group+0x2e/0x30 kernel/exit.c:992 > > > do_syscall_64+0xcf/0x2f0 arch/x86/entry/common.c:296 > > > entry_SYSCALL_64_after_hwframe+0x44/0xa9 > > > > > > > Sorry I've been away and just catching up with email > > > > I don't think this is a bug, if I interpret the report correctly it shows > > a > > race > > > > static struct taskstats *taskstats_tgid_alloc(struct task_struct *tsk) > > { > > struct signal_struct *sig = tsk->signal; > > struct taskstats *stats; > > > > #1 if (sig->stats || thread_group_empty(tsk)) <- the check of sig- > > >stats > > goto ret; > > > > /* No problem if kmem_cache_zalloc() fails */ > > stats = kmem_cache_zalloc(taskstats_cache, GFP_KERNEL); > > > > spin_lock_irq(&tsk->sighand->siglock); > > if (!sig->stats) { > > #2 sig->stats = stats; <- here in setting sig->stats > > stats = NULL; > > } > > spin_unlock_irq(&tsk->sighand->siglock); > > > > if (stats) > > kmem_cache_free(taskstats_cache, stats); > > ret: > > return sig->stats; > > } > > > > The worst case scenario is that we might see sig->stats as being NULL when > > two > > threads belonging to the same tgid. We do free up stats if we got that > > wrong > > > > Am I misinterpreting the report? > > > > Balbir Singh. > > The plain concurrent reads/writes are a data race, which may manifest > in various undefined behaviour due to compiler optimizations [1, 2]. > Note that, "data race" does not necessarily imply "race condition"; > some data races are race conditions (usually the more interesting > bugs) -- but not *all* data races are race conditions (sometimes > referred to as "benign races"). KCSAN reports data races according to > the LKMM. > [1] https://lwn.net/Articles/793253/ > [2] https://lwn.net/Articles/799218/ > > If there is no race condition here that warrants heavier > synchronization (locking etc.), we still have the data race which > needs fixing by using marked atomic operations (READ_ONCE, WRITE_ONCE, > atomic_t, etc.). We also need to consider memory ordering requirements > (do we need smp_*mb(), smp_load_acquire/smp_store_release, ..)? > > In the case here, the pattern is double-checked locking, which is > incorrect without atomic operations and the correct memory ordering. > There is a lengthy discussion here: > https://lore.kernel.org/lkml/20191021113327.22365-1-christian.brauner@ubuntu.com/ > > I am still not convinced unless someone can prove that unsigned long reads are non-atomic, acquire/release and barriers semantics don't matter because the code deals with the race inside of a lock if the read was spurious, The assumption is based on the face that sig->stats can be NULL or the kzalloc'ed value in all cases. Balbir Singh.