hw/net: Fix NULL dereference with software RSS

When an eBPF program cannot be attached, virtio_net_load_ebpf() returns
false, and virtio_net_device_realize() enters the code path to handle
errors because of this, but it causes NULL dereference because no error
is generated.

Change virtio_net_load_ebpf() to return false only when a fatal error
occurred.

Fixes: b5900dff14e5 ("hw/net: report errors from failing to use eBPF RSS FDs")
Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Message-Id: <20250116-software-v1-1-9e5161b534d8@daynix.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

1 files changed, 17 insertions, 26 deletions

diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index 85e14b7..d64941b 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -1352,18 +1352,25 @@ exit:
 
 static bool virtio_net_load_ebpf(VirtIONet *n, Error **errp)
 {
-    bool ret = false;
+    if (!virtio_net_attach_ebpf_to_backend(n->nic, -1)) {
+        return true;
+    }
 
-    if (virtio_net_attach_ebpf_to_backend(n->nic, -1)) {
-        trace_virtio_net_rss_load(n, n->nr_ebpf_rss_fds, n->ebpf_rss_fds);
-        if (n->ebpf_rss_fds) {
-            ret = virtio_net_load_ebpf_fds(n, errp);
-        } else {
-            ret = ebpf_rss_load(&n->ebpf_rss, errp);
-        }
+    trace_virtio_net_rss_load(n, n->nr_ebpf_rss_fds, n->ebpf_rss_fds);
+
+    /*
+     * If user explicitly gave QEMU RSS FDs to use, then
+     * failing to use them must be considered a fatal
+     * error. If no RSS FDs were provided, QEMU is trying
+     * eBPF on a "best effort" basis only, so report a
+     * warning and allow fallback to software RSS.
+     */
+    if (n->ebpf_rss_fds) {
+        return virtio_net_load_ebpf_fds(n, errp);
     }
 
-    return ret;
+    ebpf_rss_load(&n->ebpf_rss, &error_warn);
+    return true;
 }
 
 static void virtio_net_unload_ebpf(VirtIONet *n)
@@ -3913,23 +3920,7 @@ static void virtio_net_device_realize(DeviceState *dev, Error **errp)
     net_rx_pkt_init(&n->rx_pkt);
 
     if (virtio_has_feature(n->host_features, VIRTIO_NET_F_RSS)) {
-        Error *err = NULL;
-        if (!virtio_net_load_ebpf(n, &err)) {
-            /*
-             * If user explicitly gave QEMU RSS FDs to use, then
-             * failing to use them must be considered a fatal
-             * error. If no RSS FDs were provided, QEMU is trying
-             * eBPF on a "best effort" basis only, so report a
-             * warning and allow fallback to software RSS.
-             */
-            if (n->ebpf_rss_fds) {
-                error_propagate(errp, err);
-            } else {
-                warn_report("unable to load eBPF RSS: %s",
-                            error_get_pretty(err));
-                error_free(err);
-            }
-        }
+        virtio_net_load_ebpf(n, errp);
     }
 }