CVE-2026-63988

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

bridge: Fix sleep in atomic context in sysfs path

Since the start of the git history, brport_store() always acquired the

bridge lock. Back then this decision made sense: The bridge lock

protects the STP state of the bridge and its ports and at that time the

function was only used by two STP related attributes (cost and

priority).

Nowadays, brport_store() processes a lot more attributes and most of

them do not need the bridge lock:

* Bridge flags: Only require RTNL. Read locklessly by the data path.

Annotations can be added in net-next.

* FDB port flushing: Only requires the FDB lock.

* Multicast attributes: Only require the multicast lock.

* Group forward mask: Only requires RTNL. Read locklessly by the data

path. Annotations can be added in net-next.

* Backup port: Only requires RTNL. Read locklessly by the data path.

This is a problem as the bridge calls dev_set_promiscuity() when certain

bridge port flags change and this function can sleep since the commit

cited below, resulting in a splat such as [1].

Fix this by reducing the scope of the bridge lock and only take it when

processing the two STP related attributes that require it. Remove the

now stale comment from br_switchdev_set_port_flag(). The

SWITCHDEV_F_DEFER flag can be removed in net-next.

[1]

BUG: sleeping function called from invalid context at net/core/dev_addr_lists.c:1262

in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 372, name: bash

preempt_count: 201, expected: 0

RCU nest depth: 0, expected: 0

5 locks held by bash/372:

#0: ffff88810c51c3f0 (sb_writers#7){.+.+}-{0:0}, at: ksys_write (fs/read_write.c:740)

#1: ffff888115ce9480 (&of->mutex){+.+.}-{4:4}, at: kernfs_fop_write_iter (fs/kernfs/file.c:343)

#2: ffff88810b9fd330 (kn->active#37){.+.+}-{0:0}, at: kernfs_fop_write_iter (fs/kernfs/file.c:80 fs/kernfs/file.c:344)

#3: ffffffffa59473a0 (rtnl_mutex){+.+.}-{4:4}, at: brport_store (net/bridge/br_sysfs_if.c:326)

#4: ffff8881099d2d58 (&br->lock){+...}-{3:3}, at: brport_store (./include/linux/spinlock.h:348 net/bridge/br_sysfs_if.c:345)

Preemption disabled at:

0x0

Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011

Call Trace:

<TASK>

dump_stack_lvl (lib/dump_stack.c:94 lib/dump_stack.c:120)

__might_resched.cold (kernel/sched/core.c:9163)

netif_rx_mode_run (net/core/dev_addr_lists.c:1262)

netif_rx_mode_sync (net/core/dev_addr_lists.c:1428)

dev_set_promiscuity (net/core/dev_api.c:289)

br_manage_promisc (net/bridge/br_if.c:135 net/bridge/br_if.c:172)

br_port_flags_change (net/bridge/br_if.c:242 net/bridge/br_if.c:747)

store_learning (net/bridge/br_sysfs_if.c:79 net/bridge/br_sysfs_if.c:235)

brport_store (net/bridge/br_sysfs_if.c:346)

kernfs_fop_write_iter (fs/kernfs/file.c:352)

new_sync_write (fs/read_write.c:595)

vfs_write (fs/read_write.c:688)

ksys_write (fs/read_write.c:740)

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

entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)