Hi Christian, On Mon, Oct 07, 2019 at 01:52:16AM +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. > > 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 > Cc: Dmitry Vyukov <dvyu...@google.com> > Signed-off-by: Christian Brauner <christian.brau...@ubuntu.com>
FYI, checkpatch.pl says: WARNING: memory barrier without comment #62: FILE: kernel/taskstats.c:568: + smp_store_release(&sig->stats, stats_new); Maybe you can make checkpatch.pl happy ;-) and add a comment to stress the 'pairing' between this barrier and the added READ_ONCE() (as Dmitry was alluding to in a previous post)? Thanks, Andrea > --- > /* 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 > --- > kernel/taskstats.c | 19 ++++++++++--------- > 1 file changed, 10 insertions(+), 9 deletions(-) > > diff --git a/kernel/taskstats.c b/kernel/taskstats.c > index 13a0f2e6ebc2..8ee046e8a792 100644 > --- a/kernel/taskstats.c > +++ b/kernel/taskstats.c > @@ -554,24 +554,25 @@ 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; > + stats = READ_ONCE(sig->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; > + smp_store_release(&sig->stats, stats_new); > + 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 >
