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

netif_is_l3_slave() assume that a subsequent call to

netdev_master_upper_dev_get_rcu() 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 IFF_L3MDEV_SLAVE

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:l3mdev_fib_table_rcu (net/l3mdev/l3mdev.c:181)

[...]

Call Trace:

<TASK>

l3mdev_fib_table_by_index (net/l3mdev/l3mdev.c:201 net/l3mdev/l3mdev.c:189)

__inet_bind (net/ipv4/af_inet.c:499 (discriminator 3))

inet_bind_sk (net/ipv4/af_inet.c:469)

__sys_bind (./include/linux/file.h:62 (discriminator 1) ./include/linux/file.h:83 (discriminator 1) net/socket.c:1951 (discriminator 1))

__x64_sys_bind (net/socket.c:1969 (discriminator 1) net/socket.c:1967 (discriminator 1) net/socket.c:1967 (discriminator 1))

do_syscall_64 (arch/x86/entry/syscall_64.c:63 (discriminator 1) arch/x86/entry/syscall_64.c:94 (discriminator 1))

entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)