APPLIED: [SRU][J/N][PATCH 0/1] CVE-2025-38248

Thu Jan 8 14:14:50 UTC 2026

On 06/01/2026 23:44, Tim Whisonant wrote:
> SRU Justification:
> 
> [Impact]
> 
> 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_lvl+0x6f/0xa0
>   print_address_description.constprop.0+0x6f/0x350
>   print_report+0x108/0x205
>   kasan_report+0xdf/0x110
>   br_multicast_add_router.part.0+0x3f1/0x560
>   br_multicast_set_port_router+0x74e/0xac0
>   br_setport+0xa55/0x1870
>   br_port_slave_changelink+0x95/0x120
>   __rtnl_newlink+0x5e8/0xa40
>   rtnl_newlink+0x627/0xb00
>   rtnetlink_rcv_msg+0x6fb/0xb70
>   netlink_rcv_skb+0x11f/0x350
>   netlink_unicast+0x426/0x710
>   netlink_sendmsg+0x75a/0xc20
>   __sock_sendmsg+0xc1/0x150
>   ____sys_sendmsg+0x5aa/0x7b0
>   ___sys_sendmsg+0xfc/0x180
>   __sys_sendmsg+0x124/0x1c0
>   do_syscall_64+0xbb/0x360
>   entry_SYSCALL_64_after_hwframe+0x4b/0x53
> 
> [2]
> BUG: KASAN: slab-use-after-free in br_multicast_add_router.part.0+0x378/0x560
> Read of size 8 at addr ffff888009f00840 by task bridge/391
> [...]
> Call Trace:
>   <TASK>
>   dump_stack_lvl+0x6f/0xa0
>   print_address_description.constprop.0+0x6f/0x350
>   print_report+0x108/0x205
>   kasan_report+0xdf/0x110
>   br_multicast_add_router.part.0+0x378/0x560
>   br_multicast_set_port_router+0x6f9/0xac0
>   br_vlan_process_options+0x8b6/0x1430
>   br_vlan_rtm_process_one+0x605/0xa30
>   br_vlan_rtm_process+0x396/0x4c0
>   rtnetlink_rcv_msg+0x2f7/0xb70
>   netlink_rcv_skb+0x11f/0x350
>   netlink_unicast+0x426/0x710
>   netlink_sendmsg+0x75a/0xc20
>   __sock_sendmsg+0xc1/0x150
>   ____sys_sendmsg+0x5aa/0x7b0
>   ___sys_sendmsg+0xfc/0x180
>   __sys_sendmsg+0x124/0x1c0
>   do_syscall_64+0xbb/0x360
>   entry_SYSCALL_64_after_hwframe+0x4b/0x53
> 
> [Fix]
> 
> Questing: not affected
> Noble:    applied Jammy patch
> Jammy:    backported from upstream
> Focal:    not affected
> Bionic:   not affected
> Xenial:   not affected
> Trusty:   not affected
> 
> [Test Plan]
> 
> Compile and boot tested.
> 
> [Where problems could occur]
> 
> The changes resolve a use-after-free in the networking stack,
> specifically in the area of bridged ports and multicasting.
> An issue might appear as a premature freeing of this mcast
> state.
> 
> Ido Schimmel (1):
>    bridge: mcast: Fix use-after-free during router port configuration
> 
>   net/bridge/br_multicast.c | 9 +++++++++
>   1 file changed, 9 insertions(+)
> 

Applied to noble,jammy:linux/master-next. Thanks.

-Stefan

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