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Date:   Tue, 24 Mar 2020 17:59:38 +0100
From:   Greg KH <greg@kroah.com>
To:     Jann Horn <jannh@google.com>
Cc:     Will Deacon <will@kernel.org>,
        kernel list <linux-kernel@vger.kernel.org>,
        Eric Dumazet <edumazet@google.com>,
        Kees Cook <keescook@chromium.org>,
        Maddie Stone <maddiestone@google.com>,
        Marco Elver <elver@google.com>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        kernel-team <kernel-team@android.com>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>
Subject: Re: [RFC PATCH 03/21] list: Annotate lockless list primitives with
 data_race()
Message-ID: <20200324165938.GA2521386@kroah.com>
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On Tue, Mar 24, 2020 at 05:38:30PM +0100, Jann Horn wrote:
> On Tue, Mar 24, 2020 at 5:26 PM Greg KH <greg@kroah.com> wrote:
> > On Tue, Mar 24, 2020 at 05:20:45PM +0100, Jann Horn wrote:
> > > On Tue, Mar 24, 2020 at 4:37 PM Will Deacon <will@kernel.org> wrote:
> > > > Some list predicates can be used locklessly even with the non-RCU list
> > > > implementations, since they effectively boil down to a test against
> > > > NULL. For example, checking whether or not a list is empty is safe even
> > > > in the presence of a concurrent, tearing write to the list head pointer.
> > > > Similarly, checking whether or not an hlist node has been hashed is safe
> > > > as well.
> > > >
> > > > Annotate these lockless list predicates with data_race() and READ_ONCE()
> > > > so that KCSAN and the compiler are aware of what's going on. The writer
> > > > side can then avoid having to use WRITE_ONCE() in the non-RCU
> > > > implementation.
> > > [...]
> > > >  static inline int list_empty(const struct list_head *head)
> > > >  {
> > > > -       return READ_ONCE(head->next) == head;
> > > > +       return data_race(READ_ONCE(head->next) == head);
> > > >  }
> > > [...]
> > > >  static inline int hlist_unhashed(const struct hlist_node *h)
> > > >  {
> > > > -       return !READ_ONCE(h->pprev);
> > > > +       return data_race(!READ_ONCE(h->pprev));
> > > >  }
> > >
> > > This is probably valid in practice for hlist_unhashed(), which
> > > compares with NULL, as long as the most significant byte of all kernel
> > > pointers is non-zero; but I think list_empty() could realistically
> > > return false positives in the presence of a concurrent tearing store?
> > > This could break the following code pattern:
> > >
> > > /* optimistic lockless check */
> > > if (!list_empty(&some_list)) {
> > >   /* slowpath */
> > >   mutex_lock(&some_mutex);
> > >   list_for_each(tmp, &some_list) {
> > >     ...
> > >   }
> > >   mutex_unlock(&some_mutex);
> > > }
> > >
> > > (I'm not sure whether patterns like this appear commonly though.)
> >
> >
> > I would hope not as the list could go "empty" before the lock is
> > grabbed.  That pattern would be wrong.
> 
> If the list becomes empty in between, the loop just iterates over
> nothing, and the effect is no different from what you'd get if you had
> bailed out before. But sure, you have to be aware that that can
> happen.

Doh, yeah, so it is safe, crazy, but safe :)