[PATCH] rtl8187: Fix potential buffer underflow in rtl8187_rx_cb()

[pip-izony <eeodqql09@gmail.com>]

From: Seungjin Bae <eeodqql09@gmail.com>

The rtl8187_rx_cb() calculates the rx descriptor header address
by subtracting its size from the skb tail pointer.
However, it does not validate if the received packet
(skb->len from urb->actual_length) is large enough to contain this
header.

If a truncated packet is received, this will lead to a buffer
underflow, reading memory before the start of the skb data area,
and causing a kernel panic.

This patch adds length checks for both rtl8187 and rtl8187b descriptor
headers before attempting to access them, dropping the packet cleanly
if the check fails.

Fixes: 6f7853f3cbe4 ("rtl8187: change rtl8187_dev.c to support RTL8187B (part 2)")
Signed-off-by: Seungjin Bae <eeodqql09@gmail.com>
---
 drivers/net/wireless/realtek/rtl818x/rtl8187/dev.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/net/wireless/realtek/rtl818x/rtl8187/dev.c b/drivers/net/wireless/realtek/rtl818x/rtl8187/dev.c
index 0c5c66401daa..eff42acc11a0 100644
--- a/drivers/net/wireless/realtek/rtl818x/rtl8187/dev.c
+++ b/drivers/net/wireless/realtek/rtl818x/rtl8187/dev.c
@@ -344,6 +344,10 @@ static void rtl8187_rx_cb(struct urb *urb)
 	}
 
 	if (!priv->is_rtl8187b) {
+		if (skb->len < sizeof(struct rtl8187_rx_hdr)) {
+			dev_kfree_skb_irq(skb);
+			return;
+		}
 		struct rtl8187_rx_hdr *hdr =
 			(typeof(hdr))(skb_tail_pointer(skb) - sizeof(*hdr));
 		flags = le32_to_cpu(hdr->flags);
@@ -355,6 +359,10 @@ static void rtl8187_rx_cb(struct urb *urb)
 		rx_status.antenna = (hdr->signal >> 7) & 1;
 		rx_status.mactime = le64_to_cpu(hdr->mac_time);
 	} else {
+		if (skb->len < sizeof(struct rtl8187b_rx_hdr)) {
+			dev_kfree_skb_irq(skb);
+			return;
+		}
 		struct rtl8187b_rx_hdr *hdr =
 			(typeof(hdr))(skb_tail_pointer(skb) - sizeof(*hdr));
 		/* The Realtek datasheet for the RTL8187B shows that the RX
-- 
2.43.0