Re: [PATCH v2] kernel: bpf: syscall: fix a possible sleep-in-atomic bug in __bpf_prog_put()

[Yonghong Song <yhs@meta.com>]

On 5/30/23 10:30 PM, Teng Qi wrote:
>> I would really like you to create a test case
>> to demonstrate with a rcu or spin-lock warnings based on existing code
>> base. With a test case, it would hard to see whether we need this
>> patch or not.
> 
> Ok, I will try to construct a test case.
> 
>> Please put 'Fixes' right before 'Signed-off-by' in the above.
> 
> Ok.
> 
>> Could we have cases where in software context we have irqs_disabled()?
> 
> What do you mean about software context?

sorry. i mean softirq context.

> 
> On Wed, May 31, 2023 at 1:46 AM Yonghong Song <yhs@meta.com> wrote:
>>
>>
>>
>> On 5/30/23 12:06 AM, starmiku1207184332@gmail.com wrote:
>>> From: Teng Qi <starmiku1207184332@gmail.com>
>>>
>>> __bpf_prog_put() indirectly calls kvfree() through bpf_prog_put_deferred()
>>> which is unsafe under atomic context. The current
>>> condition ‘in_irq() || irqs_disabled()’ in __bpf_prog_put() to ensure safety
>>> does not cover cases involving the spin lock region and rcu read lock region.
>>> Since __bpf_prog_put() is called by various callers in kernel/, net/ and
>>> drivers/, and potentially more in future, it is necessary to handle those
>>> cases as well.
>>>
>>> Although we haven`t found a proper way to identify the rcu read lock region,
>>> we have noticed that vfree() calls vfree_atomic() with the
>>> condition 'in_interrupt()' to ensure safety.
>>
>> I would really like you to create a test case
>> to demonstrate with a rcu or spin-lock warnings based on existing code
>> base. With a test case, it would hard to see whether we need this
>> patch or not.
>>
>>>
>>> To make __bpf_prog_put() safe in practice, we propose calling
>>> bpf_prog_put_deferred() with the condition 'in_interrupt()' and
>>> using the work queue for any other context.
>>>
>>> We also added a comment to indicate that the safety of  __bpf_prog_put()
>>> relies implicitly on the implementation of vfree().
>>>
>>> Signed-off-by: Teng Qi <starmiku1207184332@gmail.com>
>>> ---
>>> v2:
>>> remove comments because of self explanatory of code.
>>>
>>> Fixes: d809e134be7a ("bpf: Prepare bpf_prog_put() to be called from irq context.")
>>
>> Please put 'Fixes' right before 'Signed-off-by' in the above.
>>
>>> ---
>>>    kernel/bpf/syscall.c | 2 +-
>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
>>> index 14f39c1e573e..96658e5874be 100644
>>> --- a/kernel/bpf/syscall.c
>>> +++ b/kernel/bpf/syscall.c
>>> @@ -2099,7 +2099,7 @@ static void __bpf_prog_put(struct bpf_prog *prog)
>>>        struct bpf_prog_aux *aux = prog->aux;
>>>
>>>        if (atomic64_dec_and_test(&aux->refcnt)) {
>>> -             if (in_irq() || irqs_disabled()) {
>>> +             if (!in_interrupt()) {
>>
>> Could we have cases where in software context we have irqs_disabled()?
>>
>>>                        INIT_WORK(&aux->work, bpf_prog_put_deferred);
>>>                        schedule_work(&aux->work);
>>>                } else {