[PATCH] veth: Fix race with AF_XDP exposing old or uninitialized descriptors

Shawn Bohrer
2022-12-20T19:00:47Z

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>
---
drivers/net/veth.c | 5 +++--
1 file 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;
--
2.38.1