kernel: net: phy: realtek: fix NULL pointer dereference

The previous attempt to replace an open coded paged read in the RealTek
Ethernet PHY driver was too naive and resulted in breaking the r8169
PCIe Ethernet driver which also makes use of the RealTek Ethernet PHY
driver.
Fix this by instead of using the (not yet populated) paged operations
rather use rtl821x_write_page and protect the whole paged read operation
using the MDIO bus mutex.

Fixes: 998b973157 ("kernel: net: phy: realtek: improve RealTek 2.5G PHY driver")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>

1 files changed, 35 insertions, 0 deletions

diff --git a/target/linux/generic/pending-5.15/726-net-phy-realtek-make-sure-paged-read-is-protected-by.patch b/target/linux/generic/pending-5.15/726-net-phy-realtek-make-sure-paged-read-is-protected-by.patch
new file mode 100644
index 0000000000..31f0622327
--- /dev/null
+++ b/target/linux/generic/pending-5.15/726-net-phy-realtek-make-sure-paged-read-is-protected-by.patch
@@ -0,0 +1,35 @@
+From 4dd2cc9b91ecb25f278a2c55e07e6455e9000e6b Mon Sep 17 00:00:00 2001
+From: Daniel Golle <daniel@makrotopia.org>
+Date: Sat, 22 Apr 2023 01:21:14 +0100
+Subject: [PATCH] net: phy: realtek: make sure paged read is protected by mutex
+
+As we cannot rely on phy_read_paged function before the PHY is
+identified, the paged read in rtlgen_supports_2_5gbps needs to be open
+coded as it is being called by the match_phy_device function, ie. before
+.read_page and .write_page have been populated.
+
+Make sure it is also protected by the MDIO bus mutex and use
+rtl821x_write_page instead of 3 individually locked MDIO bus operations.
+
+Signed-off-by: Daniel Golle <daniel@makrotopia.org>
+---
+ drivers/net/phy/realtek.c | 8 +++++---
+ 1 file changed, 5 insertions(+), 3 deletions(-)
+
+--- a/drivers/net/phy/realtek.c
++++ b/drivers/net/phy/realtek.c
+@@ -727,9 +727,11 @@ static bool rtlgen_supports_2_5gbps(stru
+ {
+ 	int val;
+ 
+-	phy_write(phydev, RTL821x_PAGE_SELECT, 0xa61);
+-	val = phy_read(phydev, 0x13);
+-	phy_write(phydev, RTL821x_PAGE_SELECT, 0);
++	mutex_lock(&phydev->mdio.bus->mdio_lock);
++	rtl821x_write_page(phydev, 0xa61);
++	val = __phy_read(phydev, 0x13);
++	rtl821x_write_page(phydev, 0);
++	mutex_unlock(&phydev->mdio.bus->mdio_lock);
+ 
+ 	return val >= 0 && val & RTL_SUPPORTS_2500FULL;
+ }