[PATCH v2] xen-netfront: handle NULL returned by xdp_convert_buff_to_frame()

[Alexey Nepomnyashih <sdl@nppct.ru>]

The function xdp_convert_buff_to_frame() may return NULL if it fails
to correctly convert the XDP buffer into an XDP frame due to memory
constraints, internal errors, or invalid data. Failing to check for NULL
may lead to a NULL pointer dereference if the result is used later in
processing, potentially causing crashes, data corruption, or undefined
behavior.

On XDP redirect failure, the associated page must be released explicitly
if it was previously retained via get_page(). Failing to do so may result
in a memory leak, as the pages reference count is not decremented.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Cc: stable@vger.kernel.org # v5.9+
Fixes: 6c5aa6fc4def ("xen networking: add basic XDP support for xen-netfront")
Signed-off-by: Alexey Nepomnyashih <sdl@nppct.ru>
---
 drivers/net/xen-netfront.c | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
index 63fe51d0e64d..1d3ff57a6125 100644
--- a/drivers/net/xen-netfront.c
+++ b/drivers/net/xen-netfront.c
@@ -985,20 +985,27 @@ static u32 xennet_run_xdp(struct netfront_queue *queue, struct page *pdata,
 	act = bpf_prog_run_xdp(prog, xdp);
 	switch (act) {
 	case XDP_TX:
-		get_page(pdata);
 		xdpf = xdp_convert_buff_to_frame(xdp);
-		err = xennet_xdp_xmit(queue->info->netdev, 1, &xdpf, 0);
-		if (unlikely(!err))
-			xdp_return_frame_rx_napi(xdpf);
-		else if (unlikely(err < 0))
+		if (unlikely(!xdpf)) {
 			trace_xdp_exception(queue->info->netdev, prog, act);
+			break;
+		}
+		get_page(pdata);
+		err = xennet_xdp_xmit(queue->info->netdev, 1, &xdpf, 0);
+		if (unlikely(err <= 0)) {
+			if (err < 0)
+				trace_xdp_exception(queue->info->netdev, prog, act);
+			xdp_return_frame_rx_napi(xdpf);
+		}
 		break;
 	case XDP_REDIRECT:
 		get_page(pdata);
 		err = xdp_do_redirect(queue->info->netdev, xdp, prog);
 		*need_xdp_flush = true;
-		if (unlikely(err))
+		if (unlikely(err)) {
 			trace_xdp_exception(queue->info->netdev, prog, act);
+			xdp_return_buff(xdp);
+		}
 		break;
 	case XDP_PASS:
 	case XDP_DROP:
-- 
2.43.0