Re: [PATCH net-next 1/5] hv_netvsc: fix the program refcount when the VF refuses XDP

ndo_bpf() takes over the reference it is passed only on success, the
caller puts it back itself on failure. netvsc_xdp_set() takes that one
plus num_chn - 1 more for its channels, and the rollback after the VF
refuses the program clears the channels again, putting all num_chn of
them. dev_xdp_install() then puts the one it passed in a second time,
and the program can be freed while the fd which loaded it still points
at it.

The VF refuses a program netvsc has already committed to when the
program is single-buffer and the VF has header-data split enabled, when
the VF has a memory provider bound, or when the VF's driver has
conditions of its own.

Reported by Sashiko during core rework. Unverified and untested.

Cc: stable+noautosel@kernel.org # untested fix to unlikely driver error path
Fixes: 184367dce4f7 ("hv_netvsc: Fix XDP refcnt for synthetic and VF NICs")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
---
 drivers/net/hyperv/netvsc_bpf.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/net/hyperv/netvsc_bpf.c b/drivers/net/hyperv/netvsc_bpf.c
index 1dd3755d9e6d..731bb7721fe2 100644
--- a/drivers/net/hyperv/netvsc_bpf.c
+++ b/drivers/net/hyperv/netvsc_bpf.c

@@ -216,6 +216,13 @@ int netvsc_bpf(struct net_device *dev, struct netdev_bpf *bpf)
 			netdev_err(dev, "vf_setxdp failed:%d\n", ret);
 			NL_SET_ERR_MSG_MOD(extack, "vf_setxdp failed");
 
+			/* Since we haven't completed the installation
+			 * of bpf->prog the reference core implicitly
+			 * transfers to us on success isn't ours.
+			 * Take a reference to balance the accounting.
+			 */
+			if (bpf->prog)
+				bpf_prog_inc(bpf->prog);
 			netvsc_xdp_set(dev, NULL, extack, nvdev);
 		}