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.

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.")
---
 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()) {
 			INIT_WORK(&aux->work, bpf_prog_put_deferred);
 			schedule_work(&aux->work);
 		} else {
-- 
2.25.1

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 {

> 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?

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 {

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 {