bpf: Fix race in devmap on PREEMPT_RT

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

bpf: Fix race in devmap on PREEMPT_RT

On PREEMPT_RT kernels, the per-CPU xdp_dev_bulk_queue (bq) can be

accessed concurrently by multiple preemptible tasks on the same CPU.

The original code assumes bq_enqueue() and __dev_flush() run atomically

with respect to each other on the same CPU, relying on

local_bh_disable() to prevent preemption. However, on PREEMPT_RT,

local_bh_disable() only calls migrate_disable() (when

PREEMPT_RT_NEEDS_BH_LOCK is not set) and does not disable

preemption, which allows CFS scheduling to preempt a task during

bq_xmit_all(), enabling another task on the same CPU to enter

bq_enqueue() and operate on the same per-CPU bq concurrently.

This leads to several races:

1. Double-free / use-after-free on bq->q[]: bq_xmit_all() snapshots

cnt = bq->count, then iterates bq->q[0..cnt-1] to transmit frames.

If preempted after the snapshot, a second task can call bq_enqueue()

-> bq_xmit_all() on the same bq, transmitting (and freeing) the

same frames. When the first task resumes, it operates on stale

pointers in bq->q[], causing use-after-free.

2. bq->count and bq->q[] corruption: concurrent bq_enqueue() modifying

bq->count and bq->q[] while bq_xmit_all() is reading them.

3. dev_rx/xdp_prog teardown race: __dev_flush() clears bq->dev_rx and

bq->xdp_prog after bq_xmit_all(). If preempted between

bq_xmit_all() return and bq->dev_rx = NULL, a preempting

bq_enqueue() sees dev_rx still set (non-NULL), skips adding bq to

the flush_list, and enqueues a frame. When __dev_flush() resumes,

it clears dev_rx and removes bq from the flush_list, orphaning the

newly enqueued frame.

4. __list_del_clearprev() on flush_node: similar to the cpumap race,

both tasks can call __list_del_clearprev() on the same flush_node,

the second dereferences the prev pointer already set to NULL.

The race between task A (__dev_flush -> bq_xmit_all) and task B

(bq_enqueue -> bq_xmit_all) on the same CPU:

Task A (xdp_do_flush) Task B (ndo_xdp_xmit redirect)

---------------------- --------------------------------

__dev_flush(flush_list)

bq_xmit_all(bq)

cnt = bq->count /* e.g. 16 */

/* start iterating bq->q[] */

<-- CFS preempts Task A -->

bq_enqueue(dev, xdpf)

bq->count == DEV_MAP_BULK_SIZE

bq_xmit_all(bq, 0)

cnt = bq->count /* same 16! */

ndo_xdp_xmit(bq->q[])

/* frames freed by driver */

bq->count = 0

<-- Task A resumes -->

ndo_xdp_xmit(bq->q[])

/* use-after-free: frames already freed! */

Fix this by adding a local_lock_t to xdp_dev_bulk_queue and acquiring

it in bq_enqueue() and __dev_flush(). These paths already run under

local_bh_disable(), so use local_lock_nested_bh() which on non-RT is

a pure annotation with no overhead, and on PREEMPT_RT provides a

per-CPU sleeping lock that serializes access to the bq.