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    SubjectRe: [PATCH v2] taskstats: fix data-race
    On Mon, Oct 07, 2019 at 03:18:04PM +0200, Andrea Parri wrote:
    > On Mon, Oct 07, 2019 at 01:01:17PM +0200, Christian Brauner wrote:
    > > When assiging and testing taskstats in taskstats_exit() there's a race
    > > when writing and reading sig->stats when a thread-group with more than
    > > one thread exits:
    > >
    > > cpu0:
    > > thread catches fatal signal and whole thread-group gets taken down
    > > do_exit()
    > > do_group_exit()
    > > taskstats_exit()
    > > taskstats_tgid_alloc()
    > > The tasks reads sig->stats holding sighand lock seeing garbage.
    >
    > You meant "without holding sighand lock" here, right?

    Correct, thanks for noticing!

    >
    >
    > >
    > > cpu1:
    > > task calls exit_group()
    > > do_exit()
    > > do_group_exit()
    > > taskstats_exit()
    > > taskstats_tgid_alloc()
    > > The task takes sighand lock and assigns new stats to sig->stats.
    > >
    > > Fix this by using READ_ONCE() and smp_store_release().
    > >
    > > Reported-by: syzbot+c5d03165a1bd1dead0c1@syzkaller.appspotmail.com
    > > Fixes: 34ec12349c8a ("taskstats: cleanup ->signal->stats allocation")
    > > Cc: stable@vger.kernel.org
    > > Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
    > > Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
    > > Link: https://lore.kernel.org/r/20191006235216.7483-1-christian.brauner@ubuntu.com
    > > ---
    > > /* v1 */
    > > Link: https://lore.kernel.org/r/20191005112806.13960-1-christian.brauner@ubuntu.com
    > >
    > > /* v2 */
    > > - Dmitry Vyukov <dvyukov@google.com>, Marco Elver <elver@google.com>:
    > > - fix the original double-checked locking using memory barriers
    > >
    > > /* v3 */
    > > - Andrea Parri <parri.andrea@gmail.com>:
    > > - document memory barriers to make checkpatch happy
    > > ---
    > > kernel/taskstats.c | 21 ++++++++++++---------
    > > 1 file changed, 12 insertions(+), 9 deletions(-)
    > >
    > > diff --git a/kernel/taskstats.c b/kernel/taskstats.c
    > > index 13a0f2e6ebc2..978d7931fb65 100644
    > > --- a/kernel/taskstats.c
    > > +++ b/kernel/taskstats.c
    > > @@ -554,24 +554,27 @@ static int taskstats_user_cmd(struct sk_buff *skb, struct genl_info *info)
    > > static struct taskstats *taskstats_tgid_alloc(struct task_struct *tsk)
    > > {
    > > struct signal_struct *sig = tsk->signal;
    > > - struct taskstats *stats;
    > > + struct taskstats *stats_new, *stats;
    > >
    > > - if (sig->stats || thread_group_empty(tsk))
    > > - goto ret;
    > > + /* Pairs with smp_store_release() below. */
    > > + stats = READ_ONCE(sig->stats);
    >
    > This pairing suggests that the READ_ONCE() is heading an address
    > dependency, but I fail to identify it: what is the target memory
    > access of such a (putative) dependency?
    >
    >
    > > + if (stats || thread_group_empty(tsk))
    > > + return stats;
    > >
    > > /* No problem if kmem_cache_zalloc() fails */
    > > - stats = kmem_cache_zalloc(taskstats_cache, GFP_KERNEL);
    > > + stats_new = kmem_cache_zalloc(taskstats_cache, GFP_KERNEL);
    > >
    > > spin_lock_irq(&tsk->sighand->siglock);
    > > if (!sig->stats) {
    > > - sig->stats = stats;
    > > - stats = NULL;
    > > + /* Pairs with READ_ONCE() above. */
    > > + smp_store_release(&sig->stats, stats_new);
    >
    > This is intended to 'order' the _zalloc() (zero initializazion)
    > before the update of sig->stats, right? what else am I missing?

    Right, I should've mentioned that. I'll change the comment.
    But I thought this also paired with smp_read_barrier_depends() that's
    placed alongside READ_ONCE()?

    Christian

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