stmmac_pci: Fix underflow size in stmmac_rx

This bug report came up when we were testing the device driver
by fuzzing. It shows that buf1_len can get underflowed and be
0xfffffffc (4294967292).

This bug is triggerable with a compromised/malfunctioning device.
We found the bug through QEMU emulation tested the patch with
emulation. We did NOT test it on real hardware.

Attached is the bug report by fuzzing.

BUG: KASAN: use-after-free in stmmac_napi_poll_rx+0x1c08/0x36e0 [stmmac]
Read of size 4294967292 at addr ffff888016358000 by task ksoftirqd/0/9

CPU: 0 PID: 9 Comm: ksoftirqd/0 Tainted: G        W         5.6.0 #1
Call Trace:
 dump_stack+0x76/0xa0
 print_address_description.constprop.0+0x16/0x200
 ? stmmac_napi_poll_rx+0x1c08/0x36e0 [stmmac]
 ? stmmac_napi_poll_rx+0x1c08/0x36e0 [stmmac]
 __kasan_report.cold+0x37/0x7c
 ? stmmac_napi_poll_rx+0x1c08/0x36e0 [stmmac]
 kasan_report+0xe/0x20
 check_memory_region+0x15a/0x1d0
 memcpy+0x20/0x50
 stmmac_napi_poll_rx+0x1c08/0x36e0 [stmmac]
 ? stmmac_suspend+0x850/0x850 [stmmac]
 ? __next_timer_interrupt+0xba/0xf0
 net_rx_action+0x363/0xbd0
 ? call_timer_fn+0x240/0x240
 ? __switch_to_asm+0x40/0x70
 ? napi_busy_loop+0x520/0x520
 ? __schedule+0x839/0x15a0
 __do_softirq+0x18c/0x634
 ? takeover_tasklets+0x5f0/0x5f0
 run_ksoftirqd+0x15/0x20
 smpboot_thread_fn+0x2f1/0x6b0
 ? smpboot_unregister_percpu_thread+0x160/0x160
 ? __kthread_parkme+0x80/0x100
 ? smpboot_unregister_percpu_thread+0x160/0x160
 kthread+0x2b5/0x3b0
 ? kthread_create_on_node+0xd0/0xd0
 ret_from_fork+0x22/0x40
Reported-by: Brendan Dolan-Gavitt <brendandg@nyu.edu>
Signed-off-by: Zekun Shen <bruceshenzk@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/stmicro/stmmac/stmmac_main.c

@@ -5161,12 +5161,13 @@ static int stmmac_rx(struct stmmac_priv *priv, int limit, u32 queue)
 		if (likely(!(status & rx_not_ls)) &&
 		    (likely(priv->synopsys_id >= DWMAC_CORE_4_00) ||
 		     unlikely(status != llc_snap))) {
-			if (buf2_len)
+			if (buf2_len) {
 				buf2_len -= ETH_FCS_LEN;
-			else
+				len -= ETH_FCS_LEN;
+			} else if (buf1_len) {
 				buf1_len -= ETH_FCS_LEN;
-
-			len -= ETH_FCS_LEN;
+				len -= ETH_FCS_LEN;
+			}
 		}
 
 		if (!skb) {