bridge: mcast: Fix use-after-free during router port configuration

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

bridge: mcast: Fix use-after-free during router port configuration

The bridge maintains a global list of ports behind which a multicast

router resides. The list is consulted during forwarding to ensure

multicast packets are forwarded to these ports even if the ports are not

member in the matching MDB entry.

When per-VLAN multicast snooping is enabled, the per-port multicast

context is disabled on each port and the port is removed from the global

router port list:

# ip link add name br1 up type bridge vlan_filtering 1 mcast_snooping 1

# ip link add name dummy1 up master br1 type dummy

# ip link set dev dummy1 type bridge_slave mcast_router 2

$ bridge -d mdb show | grep router

router ports on br1: dummy1

# ip link set dev br1 type bridge mcast_vlan_snooping 1

$ bridge -d mdb show | grep router

However, the port can be re-added to the global list even when per-VLAN

multicast snooping is enabled:

# ip link set dev dummy1 type bridge_slave mcast_router 0

# ip link set dev dummy1 type bridge_slave mcast_router 2

$ bridge -d mdb show | grep router

router ports on br1: dummy1

Since commit 4b30ae9adb04 ("net: bridge: mcast: re-implement

br_multicast_{enable, disable}_port functions"), when per-VLAN multicast

snooping is enabled, multicast disablement on a port will disable the

per-{port, VLAN} multicast contexts and not the per-port one. As a

result, a port will remain in the global router port list even after it

is deleted. This will lead to a use-after-free [1] when the list is

traversed (when adding a new port to the list, for example):

# ip link del dev dummy1

# ip link add name dummy2 up master br1 type dummy

# ip link set dev dummy2 type bridge_slave mcast_router 2

Similarly, stale entries can also be found in the per-VLAN router port

list. When per-VLAN multicast snooping is disabled, the per-{port, VLAN}

contexts are disabled on each port and the port is removed from the

per-VLAN router port list:

# ip link add name br1 up type bridge vlan_filtering 1 mcast_snooping 1 mcast_vlan_snooping 1

# ip link add name dummy1 up master br1 type dummy

# bridge vlan add vid 2 dev dummy1

# bridge vlan global set vid 2 dev br1 mcast_snooping 1

# bridge vlan set vid 2 dev dummy1 mcast_router 2

$ bridge vlan global show dev br1 vid 2 | grep router

router ports: dummy1

# ip link set dev br1 type bridge mcast_vlan_snooping 0

$ bridge vlan global show dev br1 vid 2 | grep router

However, the port can be re-added to the per-VLAN list even when

per-VLAN multicast snooping is disabled:

# bridge vlan set vid 2 dev dummy1 mcast_router 0

# bridge vlan set vid 2 dev dummy1 mcast_router 2

$ bridge vlan global show dev br1 vid 2 | grep router

router ports: dummy1

When the VLAN is deleted from the port, the per-{port, VLAN} multicast

context will not be disabled since multicast snooping is not enabled

on the VLAN. As a result, the port will remain in the per-VLAN router

port list even after it is no longer member in the VLAN. This will lead

to a use-after-free [2] when the list is traversed (when adding a new

port to the list, for example):

# ip link add name dummy2 up master br1 type dummy

# bridge vlan add vid 2 dev dummy2

# bridge vlan del vid 2 dev dummy1

# bridge vlan set vid 2 dev dummy2 mcast_router 2

Fix these issues by removing the port from the relevant (global or

per-VLAN) router port list in br_multicast_port_ctx_deinit(). The

function is invoked during port deletion with the per-port multicast

context and during VLAN deletion with the per-{port, VLAN} multicast

context.

Note that deleting the multicast router timer is not enough as it only

takes care of the temporary multicast router states (1 or 3) and not the

permanent one (2).

[1]

BUG: KASAN: slab-out-of-bounds in br_multicast_add_router.part.0+0x3f1/0x560

Write of size 8 at addr ffff888004a67328 by task ip/384

[...]

Call Trace:

<TASK>

dump_stack

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