This is a note to let you know that I've just added the patch titled veth: Fix race with AF_XDP exposing old or uninitialized descriptors to the 5.10-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: veth-fix-race-with-af_xdp-exposing-old-or-uninitiali.patch and it can be found in the queue-5.10 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. commit 19ad77894525f87a61981b22619cca345fdc9e79 Author: Shawn Bohrer <sbohrer@xxxxxxxxxxxxxx> Date: Tue Dec 20 12:59:03 2022 -0600 veth: Fix race with AF_XDP exposing old or uninitialized descriptors [ Upstream commit fa349e396e4886d742fd6501c599ec627ef1353b ] When AF_XDP is used on on a veth interface the RX ring is updated in two steps. veth_xdp_rcv() removes packet descriptors from the FILL ring fills them and places them in the RX ring updating the cached_prod pointer. Later xdp_do_flush() syncs the RX ring prod pointer with the cached_prod pointer allowing user-space to see the recently filled in descriptors. The rings are intended to be SPSC, however the existing order in veth_poll allows the xdp_do_flush() to run concurrently with another CPU creating a race condition that allows user-space to see old or uninitialized descriptors in the RX ring. This bug has been observed in production systems. To summarize, we are expecting this ordering: CPU 0 __xsk_rcv_zc() CPU 0 __xsk_map_flush() CPU 2 __xsk_rcv_zc() CPU 2 __xsk_map_flush() But we are seeing this order: CPU 0 __xsk_rcv_zc() CPU 2 __xsk_rcv_zc() CPU 0 __xsk_map_flush() CPU 2 __xsk_map_flush() This occurs because we rely on NAPI to ensure that only one napi_poll handler is running at a time for the given veth receive queue. napi_schedule_prep() will prevent multiple instances from getting scheduled. However calling napi_complete_done() signals that this napi_poll is complete and allows subsequent calls to napi_schedule_prep() and __napi_schedule() to succeed in scheduling a concurrent napi_poll before the xdp_do_flush() has been called. For the veth driver a concurrent call to napi_schedule_prep() and __napi_schedule() can occur on a different CPU because the veth xmit path can additionally schedule a napi_poll creating the race. The fix as suggested by Magnus Karlsson, is to simply move the xdp_do_flush() call before napi_complete_done(). This syncs the producer ring pointers before another instance of napi_poll can be scheduled on another CPU. It will also slightly improve performance by moving the flush closer to when the descriptors were placed in the RX ring. Fixes: d1396004dd86 ("veth: Add XDP TX and REDIRECT") Suggested-by: Magnus Karlsson <magnus.karlsson@xxxxxxxxx> Signed-off-by: Shawn Bohrer <sbohrer@xxxxxxxxxxxxxx> Link: https://lore.kernel.org/r/20221220185903.1105011-1-sbohrer@xxxxxxxxxxxxxx Signed-off-by: Paolo Abeni <pabeni@xxxxxxxxxx> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> diff --git a/drivers/net/veth.c b/drivers/net/veth.c index 5be8ed910553..5aa23a036ed3 100644 --- a/drivers/net/veth.c +++ b/drivers/net/veth.c @@ -849,6 +849,9 @@ static int veth_poll(struct napi_struct *napi, int budget) xdp_set_return_frame_no_direct(); done = veth_xdp_rcv(rq, budget, &bq, &stats); + if (stats.xdp_redirect > 0) + xdp_do_flush(); + if (done < budget && napi_complete_done(napi, done)) { /* Write rx_notify_masked before reading ptr_ring */ smp_store_mb(rq->rx_notify_masked, false); @@ -862,8 +865,6 @@ static int veth_poll(struct napi_struct *napi, int budget) if (stats.xdp_tx > 0) veth_xdp_flush(rq, &bq); - if (stats.xdp_redirect > 0) - xdp_do_flush(); xdp_clear_return_frame_no_direct(); return done;