[PATCH v2] kernel: bpf: stackmap: fix a possible sleep-in-atomic bug in bpf_mmap_unlock_get_irq_work()

[starmiku1207184332@gmail.com]

From: Teng Qi <starmiku1207184332@gmail.com>

bpf_mmap_unlock_get_irq_work() and bpf_mmap_unlock_mm() cooperate to safely
acquire mm->mmap_lock safely. The code in bpf_mmap_unlock_get_irq_work():
 struct mmap_unlock_irq_work *work = NULL;
 bool irq_work_busy = false;
 if (irqs_disabled()) {
 	if (!IS_ENABLED(CONFIG_PREEMPT_RT)) {
 		work = this_cpu_ptr(&mmap_unlock_work);
 		if (irq_work_is_busy(&work->irq_work)) {
 			irq_work_busy = true;
 		}
 	} else {
 		irq_work_busy = true;
 	}
 }
 *work_ptr = work;

shows that the pointer of struct mmap_unlock_irq_work "work" is not NULL if
irqs_disabled() == true and IS_ENABLED(CONFIG_PREEMPT_RT) == false or NULL in
other cases. The "work" will be passed to bpf_mmap_unlock_mm() as the argument.
The code in bpf_mmap_unlock_mm():
 if (!work) {
 	mmap_read_unlock(mm);
 } else {
 	work->mm = mm;
 	rwsem_release(&mm->mmap_lock.dep_map, _RET_IP_);
 	irq_work_queue(&work->irq_work);
 }

shows that mm->mmap_lock is released directly if "work" is NULL. Otherwise,
irq_work_queue is called to avoid calling mmap_read_unlock() in an irq disabled
context because of its possible sleep operation. However, mmap_read_unlock()
is unsafely called in a preempt disabled context when spin_lock() or
rcu_read_lock() has been called.

We found that some ebpf helpers that call these two functions may be invoked in
preempt disabled contexts through various hooks. We can give an example:
 SEC("kprobe/kmem_cache_free")
 int bpf_prog1(struct pt_regs *ctx)
 {
 	char buff[50];
 	bpf_get_stack(ctx, buff, sizeof(struct bpf_stack_build_id),
	              BPF_F_USER_BUILD_ID | BPF_F_USER_STACK);
 	return 0;
 }

The hook "kprobe/kmem_cache_free" is often called in preempt disabled contexts
by many modules. To fix this possible bug, we add in_atomic() in
bpf_mmap_unlock_get_irq_work().


Signed-off-by: Teng Qi <starmiku1207184332@gmail.com>
---
v2:
Thank for John Fastabend`s friendly response.

We are currently developing a static analysis tool to detect eBPF bugs in the
kernel. During our work, we discovered several possible bugs, including this
one. Unfortunately, we do not have enough information to provide a runnable
case (e.g. selftest case) that would trigger this bug, nor do we have a stack 
trace to offer. Going forward, we plan to improve our tool to provide necessary
information to construct a runnable case thst could reproduce this bug.

Fixes: 7c7e3d31e785 ("bpf: Introduce helper bpf_find_vma")
---
 kernel/bpf/mmap_unlock_work.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/bpf/mmap_unlock_work.h b/kernel/bpf/mmap_unlock_work.h
index 5d18d7d85bef..3d472d24d88f 100644
--- a/kernel/bpf/mmap_unlock_work.h
+++ b/kernel/bpf/mmap_unlock_work.h
@@ -26,7 +26,7 @@ static inline bool bpf_mmap_unlock_get_irq_work(struct mmap_unlock_irq_work **wo
 	struct mmap_unlock_irq_work *work = NULL;
 	bool irq_work_busy = false;
 
-	if (irqs_disabled()) {
+	if (in_atomic() || irqs_disabled()) {
 		if (!IS_ENABLED(CONFIG_PREEMPT_RT)) {
 			work = this_cpu_ptr(&mmap_unlock_work);
 			if (irq_work_is_busy(&work->irq_work)) {
-- 
2.25.1