diff options
author | Shawn Bohrer <sbohrer@cloudflare.com> | 2022-12-20 19:59:03 +0100 |
---|---|---|
committer | Paolo Abeni <pabeni@redhat.com> | 2022-12-22 15:06:10 +0100 |
commit | fa349e396e4886d742fd6501c599ec627ef1353b (patch) | |
tree | 5f85c1b9cd47a8ce23c3007a6c2cb7b9c79ccd6b /drivers | |
parent | net: lan966x: Fix configuration of the PCS (diff) | |
download | linux-fa349e396e4886d742fd6501c599ec627ef1353b.tar.xz linux-fa349e396e4886d742fd6501c599ec627ef1353b.zip |
veth: Fix race with AF_XDP exposing old or uninitialized descriptors
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@gmail.com>
Signed-off-by: Shawn Bohrer <sbohrer@cloudflare.com>
Link: https://lore.kernel.org/r/20221220185903.1105011-1-sbohrer@cloudflare.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Diffstat (limited to 'drivers')
-rw-r--r-- | drivers/net/veth.c | 5 |
1 files changed, 3 insertions, 2 deletions
diff --git a/drivers/net/veth.c b/drivers/net/veth.c index ac7c0653695f..dfc7d87fad59 100644 --- a/drivers/net/veth.c +++ b/drivers/net/veth.c @@ -974,6 +974,9 @@ 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); @@ -987,8 +990,6 @@ 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; |