Subject: Re: [net-next PATCH] page_pool: do not release pool until inflight == 0.

From: Jesper Dangaard Brouer <[email-masked]>
Date: Tue, 12 Nov 2019 17:48:22 +0100

On Tue, 12 Nov 2019 08:33:58 -0800
"Jonathan Lemon" <[email-masked]> wrote:

> On 12 Nov 2019, at 4:08, Jesper Dangaard Brouer wrote:
> 
> > On Mon, 11 Nov 2019 21:32:10 -0800
> > Jonathan Lemon <[email-masked]> wrote:
> >  
> >> The page pool keeps track of the number of pages in flight, and
> >> it isn't safe to remove the pool until all pages are returned.
> >>
> >> Disallow removing the pool until all pages are back, so the pool
> >> is always available for page producers.
> >>
> >> Make the page pool responsible for its own delayed destruction
> >> instead of relying on XDP, so the page pool can be used without
> >> xdp.  
> >
> > Can you please change this to:
> >  [... can be used without] xdp memory model.  
> 
> Okay.
> 
> 
> >> When all pages are returned, free the pool and notify xdp if the
> >> pool is registered with the xdp memory system.  Have the callback
> >> perform a table walk since some drivers (cpsw) may share the pool
> >> among multiple xdp_rxq_info.
> >>
> >> Fixes: d956a048cd3f ("xdp: force mem allocator removal and periodic 
> >> warning")
> >> Signed-off-by: Jonathan Lemon <[email-masked]>
> >> ---  
> > [...]  
> >> diff --git a/net/core/page_pool.c b/net/core/page_pool.c
> >> index 5bc65587f1c4..bfe96326335d 100644
> >> --- a/net/core/page_pool.c
> >> +++ b/net/core/page_pool.c  
> > [...]
> >
> > Found an issue, see below.
> >  
> >> @@ -338,31 +333,10 @@ static void __page_pool_empty_ring(struct 
> >> page_pool *pool)
> >>  	}
> >>  }
> >>
> >> -static void __warn_in_flight(struct page_pool *pool)
> >> +static void page_pool_free(struct page_pool *pool)
> >>  {
> >> -	u32 release_cnt = atomic_read(&pool->pages_state_release_cnt);
> >> -	u32 hold_cnt = READ_ONCE(pool->pages_state_hold_cnt);
> >> -	s32 distance;
> >> -
> >> -	distance = _distance(hold_cnt, release_cnt);
> >> -
> >> -	/* Drivers should fix this, but only problematic when DMA is used 
> >> */
> >> -	WARN(1, "Still in-flight pages:%d hold:%u released:%u",
> >> -	     distance, hold_cnt, release_cnt);
> >> -}
> >> -
> >> -void __page_pool_free(struct page_pool *pool)
> >> -{
> >> -	/* Only last user actually free/release resources */
> >> -	if (!page_pool_put(pool))
> >> -		return;
> >> -
> >> -	WARN(pool->alloc.count, "API usage violation");
> >> -	WARN(!ptr_ring_empty(&pool->ring), "ptr_ring is not empty");
> >> -
> >> -	/* Can happen due to forced shutdown */
> >> -	if (!__page_pool_safe_to_destroy(pool))
> >> -		__warn_in_flight(pool);
> >> +	if (pool->disconnect)
> >> +		pool->disconnect(pool);
> >>  	ptr_ring_cleanup(&pool->ring, NULL);
> >>
> >> @@ -371,12 +345,8 @@ void __page_pool_free(struct page_pool *pool)
> >>
> >>  	kfree(pool);
> >>  }
> >> -EXPORT_SYMBOL(__page_pool_free);  
> >
> > I don't think this is correct according to RCU.
> >
> > Let me reproduce the resulting version of page_pool_free():
> >
> >  static void page_pool_free(struct page_pool *pool)
> >  {
> > 	if (pool->disconnect)
> > 		pool->disconnect(pool);
> >
> > 	ptr_ring_cleanup(&pool->ring, NULL);
> >
> > 	if (pool->p.flags & PP_FLAG_DMA_MAP)
> > 		put_device(pool->p.dev);
> >
> > 	kfree(pool);
> >  }
> >
> > The issue is that pool->disconnect() will call into
> > mem_allocator_disconnect() -> mem_xa_remove(), and mem_xa_remove() 
> > does
> > a RCU delayed free.  And this function immediately does a kfree(pool).
> >
> > I do know that we can ONLY reach this page_pool_free() function, when
> > inflight == 0, but that can happen as soon as __page_pool_clean_page()
> > does the decrement, and after this trace_page_pool_state_release()
> > still have access the page_pool object (thus, hard to catch 
> > use-after-free).  
> 
> Is this an issue?  The RCU delayed free is for the xa object, it is held
> in an RCU-protected mem_id_ht, so it can't be freed until all the 
> readers
> are complete.
> 
> The change of &pool->pages_state_release_cnt can decrement the inflight
> pages to 0, and another thread could see inflight == 0 and immediately
> the remove the pool.  The atomic manipulation should be the last use of
> the pool - this should be documented, I'll add that as well:
> 
> skip_dma_unmap:
>          /* This may be the last page returned, releasing the pool, so
>           * it is not safe to reference pool afterwards.
>           */
>          count = atomic_inc_return(&pool->pages_state_release_cnt);
>          trace_page_pool_state_release(pool, page, count);
> 
> The trace_page_pool_state_release() does not dereference pool, it just
> reports the pointer value, so there shouldn't be any use-after-free.

In the tracepoint we can still dereference the pool object pointer.
This is made easier via using bpftrace for example see[1] (and with BTF
this will become more common to do so).

[1] https://github.com/xdp-project/xdp-project/blob/master/areas/mem/bpftrace/test_tp__xdp_mem_disconnect.bt

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer