Re: [PATCH net] net: dev: Detect dev_hold() after netdev_wait_allrefs()

[Eric Dumazet]

On Thu, Jan 27, 2022 at 6:14 PM Jann Horn <jannh@xxxxxxxxxx> wrote:
>
> On Fri, Jan 28, 2022 at 3:09 AM Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
> > On Thu, Jan 27, 2022 at 5:43 PM Jann Horn <jannh@xxxxxxxxxx> wrote:
> > >
> > > I've run into a bug where dev_hold() was being called after
> > > netdev_wait_allrefs(). But at that point, the device is already going
> > > away, and dev_hold() can't stop that anymore.
> > >
> > > To make such problems easier to diagnose in the future:
> > >
> > >  - For CONFIG_PCPU_DEV_REFCNT builds: Recheck in free_netdev() whether
> > >    the net refcount has been elevated. If this is detected, WARN() and
> > >    leak the object (to prevent worse consequences from a
> > >    use-after-free).
> > >  - For builds without CONFIG_PCPU_DEV_REFCNT: Set the refcount to zero.
> > >    This signals to the generic refcount infrastructure that any attempt
> > >    to increment the refcount later is a bug.
> > >
> > > Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
> > > ---
> > >  net/core/dev.c | 18 +++++++++++++++++-
> > >  1 file changed, 17 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/net/core/dev.c b/net/core/dev.c
> > > index 1baab07820f6..f7916c0d226d 100644
> > > --- a/net/core/dev.c
> > > +++ b/net/core/dev.c
> > > @@ -9949,8 +9949,18 @@ void netdev_run_todo(void)
> > >
> > >                 netdev_wait_allrefs(dev);
> > >
> > > +               /* Drop the netdev refcount (which should be 1 at this point)
> > > +                * to zero. If we're using the generic refcount code, this will
> > > +                * tell it that any dev_hold() after this point is a bug.
> > > +                */
> > > +#ifdef CONFIG_PCPU_DEV_REFCNT
> > > +               this_cpu_dec(*dev->pcpu_refcnt);
> > > +               BUG_ON(netdev_refcnt_read(dev) != 0);
> > > +#else
> > > +               BUG_ON(!refcount_dec_and_test(&dev->dev_refcnt));
> > > +#endif
> > > +
> > >                 /* paranoia */
> > > -               BUG_ON(netdev_refcnt_read(dev) != 1);
> > >                 BUG_ON(!list_empty(&dev->ptype_all));
> > >                 BUG_ON(!list_empty(&dev->ptype_specific));
> > >                 WARN_ON(rcu_access_pointer(dev->ip_ptr));
> > > @@ -10293,6 +10303,12 @@ void free_netdev(struct net_device *dev)
> > >         free_percpu(dev->xdp_bulkq);
> > >         dev->xdp_bulkq = NULL;
> > >
> > > +       /* Recheck in case someone called dev_hold() between
> > > +        * netdev_wait_allrefs() and here.
> > > +        */
> >
> > At this point, dev->pcpu_refcnt per-cpu data has been freed already
> > (CONFIG_PCPU_DEV_REFCNT=y)
> >
> > So this should probably crash, or at least UAF ?
>
> Oh. Whoops. That's what I get for only testing without CONFIG_PCPU_DEV_REFCNT...
>
> I guess a better place to put the new check would be directly after
> checking for "dev->reg_state == NETREG_UNREGISTERING"? Like this?
>
>         if (dev->reg_state == NETREG_UNREGISTERING) {
>                 ASSERT_RTNL();
>                 dev->needs_free_netdev = true;
>                 return;
>         }
>
>         /* Recheck in case someone called dev_hold() between
>          * netdev_wait_allrefs() and here.
>          */
>         if (WARN_ON(netdev_refcnt_read(dev) != 0))
>                 return; /* leak memory, otherwise we might get UAF */
>
>         netif_free_tx_queues(dev);
>         netif_free_rx_queues(dev);

Maybe another solution would be to leverage the recent dev_hold_track().

We could add a  dead boolean to 'struct  ref_tracker_dir ' (dev->refcnt_tracker)

diff --git a/include/linux/ref_tracker.h b/include/linux/ref_tracker.h
index c11c9db5825cf933acf529c83db441a818135f29..d907759b2fa1dd6b2ef22f883d55963c410dc71b
100644
--- a/include/linux/ref_tracker.h
+++ b/include/linux/ref_tracker.h
@@ -12,6 +12,7 @@ struct ref_tracker_dir {
        spinlock_t              lock;
        unsigned int            quarantine_avail;
        refcount_t              untracked;
+       bool                    dead;
        struct list_head        list; /* List of active trackers */
        struct list_head        quarantine; /* List of dead trackers */
 #endif
@@ -24,6 +25,7 @@ static inline void ref_tracker_dir_init(struct
ref_tracker_dir *dir,
        INIT_LIST_HEAD(&dir->list);
        INIT_LIST_HEAD(&dir->quarantine);
        spin_lock_init(&dir->lock);
+       dir->dead = false;
        dir->quarantine_avail = quarantine_count;
        refcount_set(&dir->untracked, 1);
 }
diff --git a/lib/ref_tracker.c b/lib/ref_tracker.c
index a6789c0c626b0f68ad67c264cd19177a63fb82d2..cbc798e5b97674a389ea3bbf17d9bb39fbf63328
100644
--- a/lib/ref_tracker.c
+++ b/lib/ref_tracker.c
@@ -20,6 +20,7 @@ void ref_tracker_dir_exit(struct ref_tracker_dir *dir)
        unsigned long flags;
        bool leak = false;

+       dir->dead = true;
        spin_lock_irqsave(&dir->lock, flags);
        list_for_each_entry_safe(tracker, n, &dir->quarantine, head) {
                list_del(&tracker->head);
@@ -72,6 +73,8 @@ int ref_tracker_alloc(struct ref_tracker_dir *dir,
        gfp_t gfp_mask = gfp;
        unsigned long flags;

+       WARN_ON_ONCE(dir->dead);
+
        if (gfp & __GFP_DIRECT_RECLAIM)
                gfp_mask |= __GFP_NOFAIL;
        *trackerp = tracker = kzalloc(sizeof(*tracker), gfp_mask);