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Thu, 07 Nov 2019 02:39:37 -0800 (PST) Date: Thu, 7 Nov 2019 21:39:34 +1100 From: Balbir Singh To: Marco Elver Cc: syzbot , LKML , syzkaller-bugs@googlegroups.com Subject: Re: KCSAN: data-race in taskstats_exit / taskstats_exit Message-ID: <20191107103934.GA9122@balbir-desktop> References: <0000000000009b403005942237bf@google.com> <27b57b11aadf1bd41ad8326101713ca0be7b8edf.camel@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Nov 06, 2019 at 11:23:52AM +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/ > Thanks for the explanation I presume or presumed that accessing a pointer is atomic, in this case sig->stats, I'll double check my assumption Balbir