[PATCH] rsi: fix oob in rsi_prepare_skb

We found this bug while fuzzing the rsi_usb driver.
rsi_prepare_skb does not check for OOB memcpy. We
add the check in the caller to fix.

Although rsi_prepare_skb checks if length is larger
than (4 * RSI_RCV_BUFFER_LEN), it really can't because
length is 0xfff maximum. So the check in patch is sufficient.

This patch is created upon ath-next branch. It is
NOT tested with real device, but with QEMU emulator.

Following is the bug report

BUG: KASAN: use-after-free in rsi_read_pkt
(/linux/drivers/net/wireless/rsi/rsi_91x_main.c:206) rsi_91x
Read of size 3815 at addr ffff888031da736d by task RX-Thread/204

CPU: 0 PID: 204 Comm: RX-Thread Not tainted 5.6.0 #5
Call Trace:
dump_stack (/linux/lib/dump_stack.c:120)
 ? rsi_read_pkt (/linux/drivers/net/wireless/rsi/rsi_91x_main.c:206) rsi_91x
 print_address_description.constprop.6 (/linux/mm/kasan/report.c:377)
 ? rsi_read_pkt (/linux/drivers/net/wireless/rsi/rsi_91x_main.c:206) rsi_91x
 ? rsi_read_pkt (/linux/drivers/net/wireless/rsi/rsi_91x_main.c:206) rsi_91x
 __kasan_report.cold.9 (/linux/mm/kasan/report.c:510)
 ? syscall_return_via_sysret (/linux/arch/x86/entry/entry_64.S:253)
 ? rsi_read_pkt (/linux/drivers/net/wireless/rsi/rsi_91x_main.c:206) rsi_91x
 kasan_report (/linux/arch/x86/include/asm/smap.h:69 /linux/mm/kasan/common.c:644)
 check_memory_region (/linux/mm/kasan/generic.c:186 /linux/mm/kasan/generic.c:192)
 memcpy (/linux/mm/kasan/common.c:130)
 rsi_read_pkt (/linux/drivers/net/wireless/rsi/rsi_91x_main.c:206) rsi_91x
 ? skb_dequeue (/linux/net/core/skbuff.c:3042)
 rsi_usb_rx_thread (/linux/drivers/net/wireless/rsi/rsi_91x_usb_ops.c:47) rsi_usb

Reported-by: Brendan Dolan-Gavitt <redacted>
Signed-off-by: Zekun Shen <redacted>
---
 drivers/net/wireless/rsi/rsi_91x_main.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/net/wireless/rsi/rsi_91x_main.c b/drivers/net/wireless/rsi/rsi_91x_main.c
index 5d1490fc3..41d3c12e0 100644
--- a/drivers/net/wireless/rsi/rsi_91x_main.c
+++ b/drivers/net/wireless/rsi/rsi_91x_main.c

@@ -193,6 +193,10 @@ int rsi_read_pkt(struct rsi_common *common, u8 *rx_pkt, s32 rcv_pkt_len)
 			break;
 
 		case RSI_WIFI_DATA_Q:
+			if (!rcv_pkt_len && offset + length >
+				RSI_MAX_RX_USB_PKT_SIZE)
+				goto fail;
+
 			skb = rsi_prepare_skb(common,
 					      (frame_desc + offset),
 					      length,

-- 
2.25.1