bpf: Fix UAF due to concurrent consumption of ttrace lists in alloc_bulk

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

bpf: Fix UAF due to concurrent consumption of ttrace lists in alloc_bulk

Syzkaller repeatedly triggered UAF splats related to nodes in
waiting_for_gp_ttrace within the bpf memalloc:

BUG: KASAN: slab-use-after-free in llist_del_first+0x85/0x110 lib/llist.c:61
Read of size 8 at addr ffff8881572cd080 by task syz.4.470/5112
...
llist_del_first+0x85/0x110 lib/llist.c:61
alloc_bulk+0x193/0x460 kernel/bpf/memalloc.c:229
bpf_mem_refill+0x386/0x560 kernel/bpf/memalloc.c:436

Freed by task 14:
...
__free_rcu kernel/bpf/memalloc.c:281 [inline]
__free_rcu_tasks_trace+0x48/0xd0 kernel/bpf/memalloc.c:291
rcu_tasks_invoke_cbs+0x1ec/0x3e0 kernel/rcu/tasks.h:571
rcu_tasks_one_gp+0x13d/0x220 kernel/rcu/tasks.h:621
rcu_tasks_kthread+0xf3/0x120 kernel/rcu/tasks.h:651

The reason is that the UAF occurs after the RCU Tasks Trace GP expires:
when the __free_rcu() callback runs, there is no synchronization
protecting llist_del_all() against concurrent alloc_bulk() operating on
waiting_for_gp_ttrace, leading to the race condition below:

CPU0 CPU1
__free_rcu (RCU Tasks Trace callback)
alloc_bulk
llist_del_first(&c->waiting_for_gp_ttrace)
entry = smp_load_acquire(&head->first);
do {
if (entry == NULL)
return NULL;
free_all(llist_del_all(&c->waiting_for_gp_ttrace))
llist_for_each_safe(pos, t, llnode)
free_one(pos);
next = READ_ONCE(entry->next); <-- trigger UAF
} while (!try_cmpxchg(&head->first, &entry, next));

In addition, there is also a theoretical race condition on the
free_by_rcu_ttrace list. This race requires two preconditions: an
in-flight Tasks Trace GP keeping c->call_rcu_ttrace_in_progress == 1,
and concurrent cross-CPU frees repopulating c->free_by_rcu_ttrace with
new nodes. Under these conditions, the following scenario triggers UAF:

// CPU0
// irq work is still busy (on PREEMPT_RT)
alloc_bulk()
llist_del_first(&c->free_by_rcu_ttrace)
entry = smp_load_acquire(&head->first);
do {
if (entry == NULL)
return NULL;

// CPU1
bpf_mem_alloc_destroy()
WRITE_ONCE(c->draining, true)
// wait for CPU0
irq_work_sync()

// CPU2
do_call_rcu_ttrace(tgt(CPU0))
if (c->draining) {
llist_del_all(&c->free_by_rcu_ttrace)
free_all()
}

// CPU0 continue
next = READ_ONCE(entry->next); <-- trigger UAF
while (!try_cmpxchg(&head->first, &entry, next));

Fix this by introducing a raw spinlock to synchronize the concurrent
consumption on waiting_for_gp_ttrace and free_by_rcu_ttrace.