[PATCH net v1] IB/core: Fix use-after-free of ipvlan phy_dev in ib_get_eth_speed

[Jiayuan Chen <jiayuan.chen@linux.dev>]

From: Jiayuan Chen <jiayuan.chen@shopee.com>

Jianzhou Zhao reported a NULL pointer dereference in
__ethtool_get_link_ksettings [1]. The root cause is a use-after-free
of ipvlan->phy_dev.

In ib_get_eth_speed(), ib_device_get_netdev() obtains a reference to the
ipvlan device outside of rtnl_lock(). This creates a race window: between
ib_device_get_netdev() and rtnl_lock(), the underlying phy_dev (e.g. a
dummy device) can be unregistered and freed by another thread. When the
ethtool call later recurses through ipvlan_ethtool_get_link_ksettings()
into the freed phy_dev, it dereferences freed memory whose ethtool_ops
reads as NULL, causing the crash at offset 0x1f8.

Fix this by moving ib_device_get_netdev() inside the rtnl_lock() section
so that netdev lookup and the ethtool call are atomic with respect to
device unregistration. Under RTNL, if the phy_dev has been deleted, the
ipvlan device is also unregistered and ib_device_get_netdev() returns NULL
safely.

None of the existing callers of ib_get_eth_speed() hold rtnl_lock, so this
change does not introduce any deadlock.

[1] https://lore.kernel.org/netdev/94089b74-def5-4dd0-9143-1cfbc722fe73@linux.dev/T/#t

Fixes: d41861942fc5 ("IB/core: Add generic function to extract IB speed from netdev")
Reported-by: Jianzhou Zhao <luckd0g@163.com>
Closes: https://lore.kernel.org/netdev/94089b74-def5-4dd0-9143-1cfbc722fe73@linux.dev/T/#t
Cc: Jiayuan Chen <jiayuan.chen@linux.dev>
Signed-off-by: Jiayuan Chen <jiayuan.chen@shopee.com>
---
 drivers/infiniband/core/verbs.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/infiniband/core/verbs.c b/drivers/infiniband/core/verbs.c
index 575b4a4b200b..f16d11e7c2e3 100644
--- a/drivers/infiniband/core/verbs.c
+++ b/drivers/infiniband/core/verbs.c
@@ -2046,11 +2046,13 @@ int ib_get_eth_speed(struct ib_device *dev, u32 port_num, u16 *speed, u8 *width)
 	if (rdma_port_get_link_layer(dev, port_num) != IB_LINK_LAYER_ETHERNET)
 		return -EINVAL;
 
+	rtnl_lock();
 	netdev = ib_device_get_netdev(dev, port_num);
-	if (!netdev)
+	if (!netdev) {
+		rtnl_unlock();
 		return -ENODEV;
+	}
 
-	rtnl_lock();
 	rc = __ethtool_get_link_ksettings(netdev, &lksettings);
 	rtnl_unlock();
 
-- 
2.43.0