CVE-2026-52925: vrf: Fix a potential NPD when removing a port from a VRF

In the Linux kernel, the following vulnerability has been resolved:

vrf: Fix a potential NPD when removing a port from a VRF

RCU readers that identified a net device as a VRF port using netifisl3slave() assume that a subsequent call to netdevmasterupperdevgetrcu() will return a VRF device. They then continue to dereference its l3mdev operations.

This assumption is not always correct and can result in a NPD [1]. There is no RCU synchronization when removing a port from a VRF, so it is possible for an RCU reader to see a new master device (e.g., a bridge) that does not have l3mdev operations.

Fix by adding RCU synchronization after clearing the IFFL3MDEVSLAVE flag. Skip this synchronization when a net device is removed from a VRF as part of its deletion and when the VRF device itself is deleted. In the latter case an RCU grace period will pass by the time RTNL is released.

[1] BUG: kernel NULL pointer dereference, address: 0000000000000000 [...] RIP: 0010:l3mdevfibtablercu (net/l3mdev/l3mdev.c:181) [...] Call Trace: <TASK> l3mdevfibtablebyindex (net/l3mdev/l3mdev.c:201 net/l3mdev/l3mdev.c:189) inetbind (net/ipv4/afinet.c:499 (discriminator 3)) inetbindsk (net/ipv4/afinet.c:469) sysbind (./include/linux/file.h:62 (discriminator 1) ./include/linux/file.h:83 (discriminator 1) net/socket.c:1951 (discriminator 1)) x64sysbind (net/socket.c:1969 (discriminator 1) net/socket.c:1967 (discriminator 1) net/socket.c:1967 (discriminator 1)) dosyscall64 (arch/x86/entry/syscall64.c:63 (discriminator 1) arch/x86/entry/syscall64.c:94 (discriminator 1)) entrySYSCALL64afterhwframe (arch/x86/entry/entry64.S:130)