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Commit ad0ab027 authored by Jesper Dangaard Brouer's avatar Jesper Dangaard Brouer Committed by Daniel Borkmann
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xdp: fix bug in cpumap teardown code path 



When removing a cpumap entry, a number of syncronization steps happen.
Eventually the teardown code __cpu_map_entry_free is invoked from/via
call_rcu.

The teardown code __cpu_map_entry_free() flushes remaining xdp_frames,
by invoking bq_flush_to_queue, which calls xdp_return_frame_rx_napi().
The issues is that the teardown code is not running in the RX NAPI
code path.  Thus, it is not allowed to invoke the NAPI variant of
xdp_return_frame.

This bug was found and triggered by using the --stress-mode option to
the samples/bpf program xdp_redirect_cpu.  It is hard to trigger,
because the ptr_ring have to be full and cpumap bulk queue max
contains 8 packets, and a remote CPU is racing to empty the ptr_ring
queue.

Fixes: 389ab7f0 ("xdp: introduce xdp_return_frame_rx_napi")
Tested-by: default avatarJean-Tsung Hsiao <jhsiao@redhat.com>
Signed-off-by: default avatarJesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
parent bf9bae0e

kernel/bpf/cpumap.c

+9 −6
Original line number Diff line number Diff line
@@ -69,7 +69,7 @@ struct bpf_cpu_map { 
};



static int bq_flush_to_queue(struct bpf_cpu_map_entry *rcpu,

			     struct xdp_bulk_queue *bq);

			     struct xdp_bulk_queue *bq, bool in_napi_ctx);



static u64 cpu_map_bitmap_size(const union bpf_attr *attr)

{
@@ -375,7 +375,7 @@ static void __cpu_map_entry_free(struct rcu_head *rcu) 
		struct xdp_bulk_queue *bq = per_cpu_ptr(rcpu->bulkq, cpu);



		/* No concurrent bq_enqueue can run at this point */

		bq_flush_to_queue(rcpu, bq);

		bq_flush_to_queue(rcpu, bq, false);

	}

	free_percpu(rcpu->bulkq);

	/* Cannot kthread_stop() here, last put free rcpu resources */
@@ -558,7 +558,7 @@ const struct bpf_map_ops cpu_map_ops = { 
};



static int bq_flush_to_queue(struct bpf_cpu_map_entry *rcpu,

			     struct xdp_bulk_queue *bq)

			     struct xdp_bulk_queue *bq, bool in_napi_ctx)

{

	unsigned int processed = 0, drops = 0;

	const int to_cpu = rcpu->cpu;
@@ -578,7 +578,10 @@ static int bq_flush_to_queue(struct bpf_cpu_map_entry *rcpu, 
		err = __ptr_ring_produce(q, xdpf);

		if (err) {

			drops++;

			if (likely(in_napi_ctx))

				xdp_return_frame_rx_napi(xdpf);

			else

				xdp_return_frame(xdpf);

		}

		processed++;

	}
@@ -598,7 +601,7 @@ static int bq_enqueue(struct bpf_cpu_map_entry *rcpu, struct xdp_frame *xdpf) 
	struct xdp_bulk_queue *bq = this_cpu_ptr(rcpu->bulkq);



	if (unlikely(bq->count == CPU_MAP_BULK_SIZE))

		bq_flush_to_queue(rcpu, bq);

		bq_flush_to_queue(rcpu, bq, true);



	/* Notice, xdp_buff/page MUST be queued here, long enough for

	 * driver to code invoking us to finished, due to driver
@@ -661,7 +664,7 @@ void __cpu_map_flush(struct bpf_map *map) 


		/* Flush all frames in bulkq to real queue */

		bq = this_cpu_ptr(rcpu->bulkq);

		bq_flush_to_queue(rcpu, bq);

		bq_flush_to_queue(rcpu, bq, true);



		/* If already running, costs spin_lock_irqsave + smb_mb */

		wake_up_process(rcpu->kthread);