[PATCH net-next v2 2/5] hv_netvsc: hold the VF's instance lock when installing XDP on it

[Jakub Kicinski <kuba@kernel.org>]

netvsc_vf_setxdp() has two kinds of callers. netvsc_register_vf() runs
with the VF's instance lock already held, either by the NETDEV_REGISTER
notifier or by netvsc_probe() taking it, which is why the propagation
moved to the caller-locked netif_xdp_propagate(). netvsc_bpf() runs with
just RTNL, so the VF's ndo_bpf() gets called unlocked. For a VF with an
ops lock that trips the lockdep assertion in
dev_get_min_mp_channel_count(), which netif_xdp_propagate() calls, and
races with binding a memory provider, which takes only the instance
lock.

Take the VF's lock in netvsc_bpf().

Reported by Sashiko during core rework, the race was not reproduced.

Cc: stable+noautosel@kernel.org # LLM report + LLM fix, untested
Fixes: 3ec523304976 ("hv_netvsc: fix potential deadlock in netvsc_vf_setxdp()")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Tested-by: Kameron Carr <kameroncarr@linux.microsoft.com>
Tested-by: Erni Sri Satya Vennela <ernis@linux.microsoft.com>
---
v2:
 - commit msg: say that the race was not reproduced, rather than that
   the fix is untested, as functional testing covered it
v1: https://lore.kernel.org/20261001014131.310771-3-kuba@kernel.org
---
 drivers/net/hyperv/netvsc_bpf.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/drivers/net/hyperv/netvsc_bpf.c b/drivers/net/hyperv/netvsc_bpf.c
index fcc60729384e..ec8edfaebf0b 100644
--- a/drivers/net/hyperv/netvsc_bpf.c
+++ b/drivers/net/hyperv/netvsc_bpf.c
@@ -14,6 +14,7 @@
 #include <linux/bpf.h>
 #include <linux/bpf_trace.h>
 #include <linux/kernel.h>
+#include <net/netdev_lock.h>
 #include <net/xdp.h>
 
 #include <linux/mutex.h>
@@ -162,6 +163,7 @@ int netvsc_xdp_set(struct net_device *dev, struct bpf_prog *prog,
 	return 0;
 }
 
+/* Caller holds the VF's lock, see netdev_lock_ops() */
 int netvsc_vf_setxdp(struct net_device *vf_netdev, struct bpf_prog *prog)
 {
 	struct netdev_bpf xdp;
@@ -172,6 +174,8 @@ int netvsc_vf_setxdp(struct net_device *vf_netdev, struct bpf_prog *prog)
 	if (!vf_netdev)
 		return 0;
 
+	netdev_assert_locked_ops_compat(vf_netdev);
+
 	if (!vf_netdev->netdev_ops->ndo_bpf)
 		return 0;
 
@@ -218,7 +222,15 @@ int netvsc_bpf(struct net_device *dev, struct netdev_bpf *bpf)
 		if (ret)
 			goto put_old_prog;
 
-		ret = netvsc_vf_setxdp(vf_netdev, bpf->prog);
+		/* Unlike netvsc_register_vf() we don't get the VF's lock
+		 * handed to us here.
+		 */
+		ret = 0;
+		if (vf_netdev) {
+			netdev_lock_ops(vf_netdev);
+			ret = netvsc_vf_setxdp(vf_netdev, bpf->prog);
+			netdev_unlock_ops(vf_netdev);
+		}
 
 		if (ret) {
 			netdev_err(dev, "vf_setxdp failed:%d\n", ret);
-- 
2.56.0