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    SubjectRe: KCSAN: data-race in taskstats_exit / taskstats_exit
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    On Wed, 2019-11-06 at 11:23 +0100, Marco Elver wrote:
    > On Wed, 6 Nov 2019 at 01:10, Balbir Singh <bsingharora@gmail.com> 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.

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