Re: [PATCH net] page_pool: Fix use-after-free in page_pool_recycle_in_ring

[dongchenchen (A)]

> On Fri, May 23, 2025 at 1:31 AM Yunsheng Lin <linyunsheng@xxxxxxxxxx> wrote:
> > On 2025/5/23 14:45, Dong Chenchen wrote:
> >
> > >   static bool page_pool_recycle_in_ring(struct page_pool *pool, netmem_ref netmem)
> > >   {
> > > +     bool in_softirq;
> > >        int ret;
> > int -> bool?
> >
> > >        /* BH protection not needed if current is softirq */
> > > -     if (in_softirq())
> > > -             ret = ptr_ring_produce(&pool->ring, (__force void *)netmem);
> > > -     else
> > > -             ret = ptr_ring_produce_bh(&pool->ring, (__force void *)netmem);
> > > -
> > > -     if (!ret) {
> > > +     in_softirq = page_pool_producer_lock(pool);
> > > +     ret = !__ptr_ring_produce(&pool->ring, (__force void *)netmem);
> > > +     if (ret)
> > >                recycle_stat_inc(pool, ring);
> > > -             return true;
> > > -     }
> > > +     page_pool_producer_unlock(pool, in_softirq);
> > >
> > > -     return false;
> > > +     return ret;
> > >   }
> > >
> > >   /* Only allow direct recycling in special circumstances, into the
> > > @@ -1091,10 +1088,14 @@ static void page_pool_scrub(struct page_pool *pool)
> > >
> > >   static int page_pool_release(struct page_pool *pool)
> > >   {
> > > +     bool in_softirq;
> > >        int inflight;
> > >
> > >        page_pool_scrub(pool);
> > >        inflight = page_pool_inflight(pool, true);
> > > +     /* Acquire producer lock to make sure producers have exited. */
> > > +     in_softirq = page_pool_producer_lock(pool);
> > > +     page_pool_producer_unlock(pool, in_softirq);
> > Is a compiler barrier needed to ensure compiler doesn't optimize away
> > the above code?
> I don't want to derail this conversation too much, and I suggested a
> similar fix to this initially, but now I'm not sure I understand why
> it works.
>
> Why is the existing barrier not working and acquiring/releasing the
> producer lock fixes this issue instead? The existing barrier is the
> producer thread incrementing pool->pages_state_release_cnt, and
> page_pool_release() is supposed to block the freeing of the page_pool
> until it sees the
> `atomic_inc_return_relaxed(&pool->pages_state_release_cnt);` from the
> producer thread. Any idea why this barrier is not working? AFAIU it
> should do the exact same thing as acquiring/dropping the producer
> lock.

Hi, Mina
As previously mentioned:
page_pool_recycle_in_ring
  ptr_ring_produce
    spin_lock(&r->producer_lock);
    WRITE_ONCE(r->queue[r->producer++], ptr)
      //recycle last page to pool, producer + release_cnt = hold_cnt
				page_pool_release
				  page_pool_scrub
				    page_pool_empty_ring
				      ptr_ring_consume
				        page_pool_return_page
				       //release_cnt=hold_cnt
				  __page_pool_destroy //inflight=0
				     free_percpu(pool->recycle_stats);
				     free(pool) //free
     spin_unlock(&r->producer_lock); //pool->ring uaf read
  recycle_stat_inc(pool, ring);

release_cnt can block the freeing of the page_pool until it sees the
(release_cnt = hold_cnt) from the producer thread.
However, page_pool_release() can be executed simultaneously when a page
is recycle (e.g. kfree_skb). page_pool release_cnt will increase after
the producer is written, then pool can be free and pool read in producer
will trigger UAF.
So adding a producer lock barrier to wait for recycle process to
complete can fix it.

Best Regards,
Dong Chenchen

>