Re: [PATCH V3] netrom: Prevent race conditions between neighbor operations

Eric Dumazet
2025-10-23T11:54:24Z

On Thu, Oct 23, 2025 at 4:44 AM Paolo Abeni <pabeni@redhat.com> wrote:
>
> On 10/21/25 10:35 AM, Lizhi Xu wrote:
> > The root cause of the problem is that multiple different tasks initiate
> > SIOCADDRT & NETROM_NODE commands to add new routes, there is no lock
> > between them to protect the same nr_neigh.
> >
> > Task0 can add the nr_neigh.refcount value of 1 on Task1 to routes[2].
> > When Task2 executes nr_neigh_put(nr_node->routes[2].neighbour), it will
> > release the neighbour because its refcount value is 1.
> >
> > In this case, the following situation causes a UAF on Task2:
> >
> > Task0 Task1 Task2
> > ===== ===== =====
> > nr_add_node()
> > nr_neigh_get_dev() nr_add_node()
> > nr_node_lock()
> > nr_node->routes[2].neighbour->count--
> > nr_neigh_put(nr_node->routes[2].neighbour);
> > nr_remove_neigh(nr_node->routes[2].neighbour)
> > nr_node_unlock()
> > nr_node_lock()
> > nr_node->routes[2].neighbour = nr_neigh
> > nr_neigh_hold(nr_neigh); nr_add_node()
> > nr_neigh_put()
> > if (nr_node->routes[2].neighbour->count
> > Description of the UAF triggering process:
> > First, Task 0 executes nr_neigh_get_dev() to set neighbor refcount to 3.
> > Then, Task 1 puts the same neighbor from its routes[2] and executes
> > nr_remove_neigh() because the count is 0. After these two operations,
> > the neighbor's refcount becomes 1. Then, Task 0 acquires the nr node
> > lock and writes it to its routes[2].neighbour.
> > Finally, Task 2 executes nr_neigh_put(nr_node->routes[2].neighbour) to
> > release the neighbor. The subsequent execution of the neighbor->count
> > check triggers a UAF.
> >
> > The solution to the problem is to use a lock to synchronize each add a
> > route to node, but for rigor, I'll add locks to related ioctl and route
> > frame operations to maintain synchronization.
>
> I think that adding another locking mechanism on top of an already
> complex and not well understood locking and reference infra is not the
> right direction.
>
> Why reordering the statements as:
>
> if (nr_node->routes[2].neighbour->count == 0 &&
> !nr_node->routes[2].neighbour->locked)
> nr_remove_neigh(nr_node->routes[2].neighbour);
> nr_neigh_put(nr_node->routes[2].neighbour);
>
> is not enough?
>
> > syzbot reported:
> > BUG: KASAN: slab-use-after-free in nr_add_node+0x25db/0x2c00 net/netrom/nr_route.c:248
> > Read of size 4 at addr ffff888051e6e9b0 by task syz.1.2539/8741
> >
> > Call Trace:
> > <TASK>
> > nr_add_node+0x25db/0x2c00 net/netrom/nr_route.c:248
> >
> > Reported-by: syzbot+2860e75836a08b172755@syzkaller.appspotmail.com
> > Closes: https://syzkaller.appspot.com/bug?extid=2860e75836a08b172755
> > Signed-off-by: Lizhi Xu <lizhi.xu@windriver.com>
>
>
>
> > ---
> > V1 -> V2: update comments for cause uaf
> > V2 -> V3: sync neighbor operations in ioctl and route frame, update comments
> >
> > net/netrom/nr_route.c | 4 ++++
> > 1 file changed, 4 insertions(+)
> >
> > diff --git a/net/netrom/nr_route.c b/net/netrom/nr_route.c
> > index b94cb2ffbaf8..debe3e925338 100644
> > --- a/net/netrom/nr_route.c
> > +++ b/net/netrom/nr_route.c
> > @@ -40,6 +40,7 @@ static HLIST_HEAD(nr_node_list);
> > static DEFINE_SPINLOCK(nr_node_list_lock);
> > static HLIST_HEAD(nr_neigh_list);
> > static DEFINE_SPINLOCK(nr_neigh_list_lock);
> > +static DEFINE_MUTEX(neighbor_lock);
> >
> > static struct nr_node *nr_node_get(ax25_address *callsign)
> > {
> > @@ -633,6 +634,8 @@ int nr_rt_ioctl(unsigned int cmd, void __user *arg)
> > ax25_digi digi;
> > int ret;
> >
> > + guard(mutex)(&neighbor_lock);
>
> See:
>
> https://elixir.bootlin.com/linux/v6.18-rc1/source/Documentation/process/maintainer-netdev.rst#L395
>

I would also try to use a single spinlock : ie fuse together
nr_node_list_lock and nr_neigh_list_lock

Having two locks for something that is primarily used by fuzzers
nowadays is wasting our time.