netdev - Re: [PATCH bpf-next] bpf/test_run: fix unkillable BPF_PROG_TEST

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Date:   Mon, 25 Feb 2019 22:36:05 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Stanislav Fomichev <sdf@...gle.com>, netdev@...r.kernel.org
Cc:     davem@...emloft.net, ast@...nel.org,
        syzbot <syzkaller@...glegroups.com>
Subject: Re: [PATCH bpf-next] bpf/test_run: fix unkillable BPF_PROG_TEST_RUN
 for flow dissector

On 02/19/2019 07:54 PM, Stanislav Fomichev wrote:
> Syzbot found out that running BPF_PROG_TEST_RUN with repeat=0xffffffff
> makes process unkillable. The problem is that when CONFIG_PREEMPT is
> enabled, we never see need_resched() return true. This is due to the
> fact that preempt_enable() (which we do in bpf_test_run_one on each
> iteration) now handles resched if it's needed.
> 
> Let's disable preemption for the whole run, not per test. In this case
> we can properly see whether resched is needed.
> Let's also properly return -EINTR to the userspace in case of a signal
> interrupt.
> 
> This is a follow up for a recently fixed issue in bpf_test_run, see
> commit df1a2cb7c74b ("bpf/test_run: fix unkillable
> BPF_PROG_TEST_RUN").
> 
> Reported-by: syzbot <syzkaller@...glegroups.com>
> Signed-off-by: Stanislav Fomichev <sdf@...gle.com>

Applied, thanks! Would be nice to consolidate at least some of these bits
so it's not duplicated, perhaps __always_inline might help for function args
in avoiding indirect calls (iirc, perf's rb handling uses this heavily).

> ---
>  net/bpf/test_run.c | 26 ++++++++++++++++++++------
>  1 file changed, 20 insertions(+), 6 deletions(-)
> 
> diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
> index 2c5172b33209..619655db8d9e 100644
> --- a/net/bpf/test_run.c
> +++ b/net/bpf/test_run.c
> @@ -293,31 +293,45 @@ int bpf_prog_test_run_flow_dissector(struct bpf_prog *prog,
>  	if (!repeat)
>  		repeat = 1;
>  
> +	rcu_read_lock();
> +	preempt_disable();
>  	time_start = ktime_get_ns();
>  	for (i = 0; i < repeat; i++) {
> -		preempt_disable();
> -		rcu_read_lock();
>  		retval = __skb_flow_bpf_dissect(prog, skb,
>  						&flow_keys_dissector,
>  						&flow_keys);
> -		rcu_read_unlock();
> -		preempt_enable();
> +
> +		if (signal_pending(current)) {
> +			preempt_enable();
> +			rcu_read_unlock();
> +
> +			ret = -EINTR;
> +			goto out;
> +		}
>  
>  		if (need_resched()) {
> -			if (signal_pending(current))
> -				break;
>  			time_spent += ktime_get_ns() - time_start;
> +			preempt_enable();
> +			rcu_read_unlock();
> +
>  			cond_resched();
> +
> +			rcu_read_lock();
> +			preempt_disable();
>  			time_start = ktime_get_ns();
>  		}
>  	}
>  	time_spent += ktime_get_ns() - time_start;
> +	preempt_enable();
> +	rcu_read_unlock();
> +
>  	do_div(time_spent, repeat);
>  	duration = time_spent > U32_MAX ? U32_MAX : (u32)time_spent;
>  
>  	ret = bpf_test_finish(kattr, uattr, &flow_keys, sizeof(flow_keys),
>  			      retval, duration);
>  
> +out:
>  	kfree_skb(skb);
>  	kfree(sk);
>  	return ret;
>