[PATCH net] usbnet: gl620a: fix out-of-bounds read in genelink_rx_fixup()

[Xiang Mei]

genelink_rx_fixup() splits an aggregated RX frame into its individual
packets, using a per-packet length taken from device-supplied data. That
length is only bounded by GL_MAX_PACKET_LEN (1514); it is never compared
against how many bytes were actually received.

A malicious GeneLink (GL620A) device can therefore send a short URB whose
header claims packet_count > 1 and a first packet of up to 1514 bytes.

	skb_put_data(gl_skb, packet->packet_data, size);

then copies past the end of the receive buffer and hands the adjacent slab
contents up the network stack, an out-of-bounds read that leaks kernel heap.
No privilege is required: the path runs in the usbnet RX softirq as soon as
the interface is up.

  BUG: KASAN: slab-out-of-bounds in genelink_rx_fixup (drivers/net/usb/gl620a.c:112)
  Read of size 1514 at addr ffff888011309708 by task ksoftirqd/0/14
  Call Trace:
    ...
    __asan_memcpy (mm/kasan/shadow.c:105)
    genelink_rx_fixup (include/linux/skbuff.h:2814 drivers/net/usb/gl620a.c:112)
    usbnet_bh (drivers/net/usb/usbnet.c:572 drivers/net/usb/usbnet.c:1589)
    process_one_work (kernel/workqueue.c:3322)
    bh_worker (kernel/workqueue.c:3405)
    tasklet_action (kernel/softirq.c:965)
    handle_softirqs (kernel/softirq.c:622)
    run_ksoftirqd (kernel/softirq.c:1076)
    ...

skb_pull() already verifies that the requested length fits the buffer and
returns NULL otherwise. Move it ahead of the copy and check its result, so
a packet that overruns the received data is rejected before it is read.
Well-formed frames, whose packets are fully present, are unaffected.

Fixes: 47ee3051c856 ("[PATCH] USB: usbnet (5/9) module for genesys gl620a cables")
Reported-by: Weiming Shi <bestswngs@xxxxxxxxx>
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Xiang Mei <xmei5@xxxxxxx>
---
 drivers/net/usb/gl620a.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/net/usb/gl620a.c b/drivers/net/usb/gl620a.c
index 0bfa37c14059..09afd137b64e 100644
--- a/drivers/net/usb/gl620a.c
+++ b/drivers/net/usb/gl620a.c
@@ -104,6 +104,9 @@ static int genelink_rx_fixup(struct usbnet *dev, struct sk_buff *skb)
 			return 0;
 		}
 
+		if (!skb_pull(skb, size + 4))
+			return 0;
+
 		// allocate the skb for the individual packet
 		gl_skb = alloc_skb(size, GFP_ATOMIC);
 		if (gl_skb) {
@@ -116,9 +119,6 @@ static int genelink_rx_fixup(struct usbnet *dev, struct sk_buff *skb)
 		// advance to the next packet
 		packet = (struct gl_packet *)&packet->packet_data[size];
 		count--;
-
-		// shift the data pointer to the next gl_packet
-		skb_pull(skb, size + 4);
 	}
 
 	// skip the packet length field 4 bytes
-- 
2.43.0