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Tue, 05 Nov 2019 16:09:58 -0800 (PST) Message-ID: <27b57b11aadf1bd41ad8326101713ca0be7b8edf.camel@gmail.com> Subject: Re: KCSAN: data-race in taskstats_exit / taskstats_exit From: Balbir Singh To: syzbot , elver@google.com, linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com Date: Wed, 06 Nov 2019 11:09:54 +1100 In-Reply-To: <0000000000009b403005942237bf@google.com> References: <0000000000009b403005942237bf@google.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 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.