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([2a09:bac5:d459:16d2::246:73]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-3ae8441a6f0sm16691a91.1.2026.10.29.07.24.26 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 29 Oct 2026 07:24:29 -0700 (PDT) From: Nguyen Ngoc Thang To: Andrew Lunn , Heiner Kallweit , Russell King Cc: "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Kory Maincent , netdev@vger.kernel.org, linux-kernel@vger.kernel.org, syzbot+583f42a41879bb60de32@syzkaller.appspotmail.com Subject: [PATCH net] net: phy: don't require RTNL to detach a PHY from an unregistered netdev Date: Thu, 29 Oct 2026 21:24:23 +0700 Message-ID: <20261029142423.93573-1-ngocthang2710.1999@gmail.com> X-Mailer: git-send-email 2.43.0 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit phy_detach_internal() reads dev->hwprov with rtnl_dereference(), but MAC drivers commonly attach and detach PHYs while their net_device is not registered, without holding RTNL. Before register_netdev(), a failed phylink_connect_phy() or phy_attach_direct() unwinds through phy_detach_internal(). After unregister_netdev(), a driver's remove path may call phy_disconnect(). Both trip lockdep: WARNING: suspicious RCU usage drivers/net/phy/phy_device.c:1831 suspicious rcu_dereference_protected() usage! phy_detach_internal+0x6a5/0x830 drivers/net/phy/phy_device.c:1831 phylink_connect_phy+0x22e/0x2b0 drivers/net/phy/phylink.c:2452 ax88772_bind+0xbb1/0x1520 drivers/net/usb/asix_devices.c:925 usbnet_probe+0xbb3/0x27e0 drivers/net/usb/usbnet.c:1812 syzbot hits this with an emulated asix device. phy_eee_rx_clock_stop() fails on a USB MDIO error, so phylink_bringup_phy() fails and the PHY is detached from inside usbnet_probe(), before the netdev is registered. hwprov is only set through ethtool, under RTNL, on a registered device. reg_state also changes only under RTNL. So if the device is not registered, no one else can reach hwprov, and RTNL is not needed. Tell lockdep that. A detach from a registered device without RTNL still warns. Fixes: 35f7cad1743e ("net: Add the possibility to support a selected hwtstamp in netdevice") Reported-by: syzbot+583f42a41879bb60de32@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=583f42a41879bb60de32 Signed-off-by: Nguyen Ngoc Thang --- Tested in QEMU (x86_64, syzbot config, KASAN + PROVE_RCU) with the syzbot C reproducer on 6b4b543fa187: - before: "WARNING: suspicious RCU usage" at phy_device.c:1831 via phylink_connect_phy() -> phy_detach_internal() from ax88772_bind() - after: 11/11 runs hit the same "Could not connect PHY" failure path with no lockdep/KASAN report drivers/net/phy/phy_device.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-) diff --git a/drivers/net/phy/phy_device.c b/drivers/net/phy/phy_device.c index 5d4508bcfbe9..06c5a8c8cc2a 100644 --- a/drivers/net/phy/phy_device.c +++ b/drivers/net/phy/phy_device.c @@ -1826,9 +1826,13 @@ static void phy_detach_internal(struct phy_device *phydev, bool notify_bus) struct hwtstamp_provider *hwprov; /* hwprov may technically be protected by ops lock but - * not for devices with a phydev, see phy_link_topo_add_phy() + * not for devices with a phydev, see phy_link_topo_add_phy(). + * Drivers may detach without RTNL while dev is unregistered, + * but then nothing else can reach hwprov. */ - hwprov = rtnl_dereference(dev->hwprov); + hwprov = rcu_dereference_protected(dev->hwprov, + lockdep_rtnl_is_held() || + dev->reg_state != NETREG_REGISTERED); /* Disable timestamp if it is the one selected */ if (hwprov && hwprov->phydev == phydev) { rcu_assign_pointer(dev->hwprov, NULL); -- 2.43.0