CVE-2025-21986

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

net: switchdev: Convert blocking notification chain to a raw one

A blocking notification chain uses a read-write semaphore to protect the integrity of the chain. The semaphore is acquired for writing when adding / removing notifiers to / from the chain and acquired for reading when traversing the chain and informing notifiers about an event.

In case of the blocking switchdev notification chain, recursive notifications are possible which leads to the semaphore being acquired twice for reading and to lockdep warnings being generated [1].

Specifically, this can happen when the bridge driver processes a SWITCHDEV_BRPORT_UNOFFLOADED event which causes it to emit notifications about deferred events when calling switchdev_deferred_process().

Fix this by converting the notification chain to a raw notification chain in a similar fashion to the netdev notification chain. Protect the chain using the RTNL mutex by acquiring it when modifying the chain. Events are always informed under the RTNL mutex, but add an assertion in call_switchdev_blocking_notifiers() to make sure this is not violated in the future.

Maintain the blocking prefix as events are always emitted from process context and listeners are allowed to block.

[1]: WARNING: possible recursive locking detected 6.14.0-rc4-custom-g079270089484 #1 Not tainted

ip/52731 is trying to acquire lock: ffffffff850918d8 ((switchdev_blocking_notif_chain).rwsem){++++}-{4:4}, at: blocking_notifier_call_chain+0x58/0xa0

but task is already holding lock: ffffffff850918d8 ((switchdev_blocking_notif_chain).rwsem){++++}-{4:4}, at: blocking_notifier_call_chain+0x58/0xa0

other info that might help us debug this: Possible unsafe locking scenario: CPU0

lock((switchdev_blocking_notif_chain).rwsem); lock((switchdev_blocking_notif_chain).rwsem);

*** DEADLOCK *** May be due to missing lock nesting notation 3 locks held by ip/52731: #0: ffffffff84f795b0 (rtnl_mutex){+.+.}-{4:4}, at: rtnl_newlink+0x727/0x1dc0 #1: ffffffff8731f628 (&net->rtnl_mutex){+.+.}-{4:4}, at: rtnl_newlink+0x790/0x1dc0 #2: ffffffff850918d8 ((switchdev_blocking_notif_chain).rwsem){++++}-{4:4}, at: blocking_notifier_call_chain+0x58/0xa0

stack backtrace: … ? __pfx_down_read+0x10/0x10 ? __pfx_mark_lock+0x10/0x10 ? __pfx_switchdev_port_attr_set_deferred+0x10/0x10 blocking_notifier_call_chain+0x58/0xa0 switchdev_port_attr_notify.constprop.0+0xb3/0x1b0 ? __pfx_switchdev_port_attr_notify.constprop.0+0x10/0x10 ? mark_held_locks+0x94/0xe0 ? switchdev_deferred_process+0x11a/0x340 switchdev_port_attr_set_deferred+0x27/0xd0 switchdev_deferred_process+0x164/0x340 br_switchdev_port_unoffload+0xc8/0x100 [bridge] br_switchdev_blocking_event+0x29f/0x580 [bridge] notifier_call_chain+0xa2/0x440 blocking_notifier_call_chain+0x6e/0xa0 switchdev_bridge_port_unoffload+0xde/0x1a0 …

Weakness

The product does not properly acquire or release a lock on a resource, leading to unexpected resource state changes and behaviors.

Affected Software

Extended Description

Locking is a type of synchronization behavior that ensures that multiple independently-operating processes or threads do not interfere with each other when accessing the same resource. All processes/threads are expected to follow the same steps for locking. If these steps are not followed precisely - or if no locking is done at all - then another process/thread could modify the shared resource in a way that is not visible or predictable to the original process. This can lead to data or memory corruption, denial of service, etc.

Potential Mitigations

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/25.html
• https://cwe.mitre.org/data/definitions/26.html
• https://cwe.mitre.org/data/definitions/27.html

References

• https://git.kernel.org/stable/c/1f7d051814e7a0cb1f0717ed5527c1059992129d
• https://git.kernel.org/stable/c/62531a1effa87bdab12d5104015af72e60d926ff
• https://git.kernel.org/stable/c/a597d4b75669ec82c72cbee9fe75a15d04b35b2b
• https://git.kernel.org/stable/c/af757f5ee3f754c5dceefb05c12ff37cb46fc682
• https://git.kernel.org/stable/c/f9ed3fb50b872bd78bcb01f25087f9e4e25085d8
• https://lists.debian.org/debian-lts-announce/2025/05/msg00045.html