On Mon, Oct 7, 2019 at 3:18 PM Andrea Parri <parri.and...@gmail.com> 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? > > > > > > 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+c5d03165a1bd1dead...@syzkaller.appspotmail.com > > Fixes: 34ec12349c8a ("taskstats: cleanup ->signal->stats allocation") > > Cc: sta...@vger.kernel.org > > Signed-off-by: Christian Brauner <christian.brau...@ubuntu.com> > > Reviewed-by: Dmitry Vyukov <dvyu...@google.com> > > Link: > > https://lore.kernel.org/r/20191006235216.7483-1-christian.brau...@ubuntu.com > > --- > > /* v1 */ > > Link: > > https://lore.kernel.org/r/20191005112806.13960-1-christian.brau...@ubuntu.com > > > > /* v2 */ > > - Dmitry Vyukov <dvyu...@google.com>, Marco Elver <el...@google.com>: > > - fix the original double-checked locking using memory barriers > > > > /* v3 */ > > - Andrea Parri <parri.and...@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?
I would assume callers of this function access *stats. So the dependency is between loading stats and accessing *stats. > > + 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? > > Thanks, > Andrea > > > > + stats_new = NULL; > > } > > spin_unlock_irq(&tsk->sighand->siglock); > > > > - if (stats) > > - kmem_cache_free(taskstats_cache, stats); > > -ret: > > + if (stats_new) > > + kmem_cache_free(taskstats_cache, stats_new); > > + > > return sig->stats; > > } > > > > -- > > 2.23.0 > >