[PATCH] [stable, netdev 4.4+] lan78xx: make sure RX_ADDRL & RX_ADDRH regs are always up to date

[Paolo Pisati <p.pisati@gmail.com>]

[partial backport upstream 760db29bdc97b73ff60b091315ad787b1deb5cf5]

Upon invocation, lan78xx_init_mac_address() checks that the mac address present
in the RX_ADDRL & RX_ADDRH registers is a valid address, if not, it first tries
to read a new address from an external eeprom or the otp area, and in case both
read fail (or the address read back is invalid), it randomly generates a new
one.

Unfortunately, due to the way the above logic is laid out,
if both read_eeprom() and read_otp() fail, a new mac address is correctly
generated but is never written back to RX_ADDRL & RX_ADDRH, leaving the chip in an
incosistent state and with an invalid mac address (e.g. the nic appears to be
completely dead, and doesn't receive any packet, etc):

lan78xx_init_mac_address()
...
if (lan78xx_read_eeprom(addr ...) || lan78xx_read_otp(addr ...)) {
	if (is_valid_ether_addr(addr) {
		// nop...
	} else {
		random_ether_addr(addr);
	}

	// correctly writes back the new address
	lan78xx_write_reg(RX_ADDRL, addr ...);
	lan78xx_write_reg(RX_ADDRH, addr ...);
} else {
	// XXX if both eeprom and otp read fail, we land here and skip
	// XXX the RX_ADDRL & RX_ADDRH update completely
	random_ether_addr(addr);
}

This bug went unnoticed because lan78xx_read_otp() was buggy itself and would
never fail, up until 4bfc338 "lan78xx: Correctly indicate invalid OTP"
fixed it and as a side effect uncovered this bug.

4.18+ is fine, since the bug was implicitly fixed in 760db29 "lan78xx: Read MAC
address from DT if present" when the address change logic was reorganized, but
it's still present in all stable trees below that: linux-4.4.y, linux-4.9.y,
linux-4.14.y, etc up to linux-4.18.y (not included).

Signed-off-by: Paolo Pisati <p.pisati@gmail.com>
---
 drivers/net/usb/lan78xx.c | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/drivers/net/usb/lan78xx.c b/drivers/net/usb/lan78xx.c
index 50e2e10a..114dc55 100644
--- a/drivers/net/usb/lan78xx.c
+++ b/drivers/net/usb/lan78xx.c
@@ -1660,13 +1660,6 @@ static void lan78xx_init_mac_address(struct lan78xx_net *dev)
 				netif_dbg(dev, ifup, dev->net,
 					  "MAC address set to random addr");
 			}
-
-			addr_lo = addr[0] | (addr[1] << 8) |
-				  (addr[2] << 16) | (addr[3] << 24);
-			addr_hi = addr[4] | (addr[5] << 8);
-
-			ret = lan78xx_write_reg(dev, RX_ADDRL, addr_lo);
-			ret = lan78xx_write_reg(dev, RX_ADDRH, addr_hi);
 		} else {
 			/* generate random MAC */
 			random_ether_addr(addr);
@@ -1674,6 +1667,11 @@ static void lan78xx_init_mac_address(struct lan78xx_net *dev)
 				  "MAC address set to random addr");
 		}
 	}
+	addr_lo = addr[0] | (addr[1] << 8) | (addr[2] << 16) | (addr[3] << 24);
+	addr_hi = addr[4] | (addr[5] << 8);
+
+	ret = lan78xx_write_reg(dev, RX_ADDRL, addr_lo);
+	ret = lan78xx_write_reg(dev, RX_ADDRH, addr_hi);
 
 	ret = lan78xx_write_reg(dev, MAF_LO(0), addr_lo);
 	ret = lan78xx_write_reg(dev, MAF_HI(0), addr_hi | MAF_HI_VALID_);
-- 
2.7.4