linux-kernel - Re: [PATCH v2] kernel: bpf: syscall: fix a possible sleep-in-atomic bug in __bpf_prog

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Message-ID: <4f37f760-048b-9d54-14ae-d1f979898625@meta.com>
Date:   Tue, 30 May 2023 10:45:53 -0700
From:   Yonghong Song <yhs@...a.com>
To:     starmiku1207184332@...il.com, ast@...nel.org, daniel@...earbox.net,
        john.fastabend@...il.com, andrii@...nel.org, martin.lau@...ux.dev,
        song@...nel.org, yhs@...com, kpsingh@...nel.org, sdf@...gle.com,
        haoluo@...gle.com, jolsa@...nel.org, davem@...emloft.net,
        kuba@...nel.org, hawk@...nel.org
Cc:     bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org
Subject: Re: [PATCH v2] kernel: bpf: syscall: fix a possible sleep-in-atomic
 bug in __bpf_prog_put()



On 5/30/23 12:06 AM, starmiku1207184332@...il.com wrote:
> From: Teng Qi <starmiku1207184332@...il.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@...il.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 {