netdev - Re: [PATCH bpf] bpf: run bpf programs with preemption disabled

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Date:   Tue, 29 Jan 2019 07:01:49 +0000
From:   Song Liu <songliubraving@...com>
To:     Alexei Starovoitov <ast@...nel.org>
CC:     David Miller <davem@...emloft.net>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "jannh@...gle.com" <jannh@...gle.com>,
        "paulmck@...ux.ibm.com" <paulmck@...ux.ibm.com>,
        "will.deacon@....com" <will.deacon@....com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH bpf] bpf: run bpf programs with preemption disabled



> On Jan 28, 2019, at 6:43 PM, Alexei Starovoitov <ast@...nel.org> wrote:
> 
> Disabled preemption is necessary for proper access to per-cpu maps
> from BPF programs.
> 
> But the sender side of socket filters didn't have preemption disabled:
> unix_dgram_sendmsg->sk_filter->sk_filter_trim_cap->bpf_prog_run_save_cb->BPF_PROG_RUN
> 
> and a combination of af_packet with tun device didn't disable either:
> tpacket_snd->packet_direct_xmit->packet_pick_tx_queue->ndo_select_queue->
>  tun_select_queue->tun_ebpf_select_queue->bpf_prog_run_clear_cb->BPF_PROG_RUN
> 
> Disable preemption before executing BPF programs (both classic and extended).
> 
> Reported-by: Jann Horn <jannh@...gle.com>
> Signed-off-by: Alexei Starovoitov <ast@...nel.org>

Acked-by: Song Liu <songliubraving@...com>


> ---
> include/linux/filter.h | 21 ++++++++++++++++++---
> kernel/bpf/cgroup.c    |  2 +-
> 2 files changed, 19 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/filter.h b/include/linux/filter.h
> index ad106d845b22..e532fcc6e4b5 100644
> --- a/include/linux/filter.h
> +++ b/include/linux/filter.h
> @@ -591,8 +591,8 @@ static inline u8 *bpf_skb_cb(struct sk_buff *skb)
> 	return qdisc_skb_cb(skb)->data;
> }
> 
> -static inline u32 bpf_prog_run_save_cb(const struct bpf_prog *prog,
> -				       struct sk_buff *skb)
> +static inline u32 __bpf_prog_run_save_cb(const struct bpf_prog *prog,
> +					 struct sk_buff *skb)
> {
> 	u8 *cb_data = bpf_skb_cb(skb);
> 	u8 cb_saved[BPF_SKB_CB_LEN];
> @@ -611,15 +611,30 @@ static inline u32 bpf_prog_run_save_cb(const struct bpf_prog *prog,
> 	return res;
> }
> 
> +static inline u32 bpf_prog_run_save_cb(const struct bpf_prog *prog,
> +				       struct sk_buff *skb)
> +{
> +	u32 res;
> +
> +	preempt_disable();
> +	res = __bpf_prog_run_save_cb(prog, skb);
> +	preempt_enable();
> +	return res;
> +}
> +
> static inline u32 bpf_prog_run_clear_cb(const struct bpf_prog *prog,
> 					struct sk_buff *skb)
> {
> 	u8 *cb_data = bpf_skb_cb(skb);
> +	u32 res;
> 
> 	if (unlikely(prog->cb_access))
> 		memset(cb_data, 0, BPF_SKB_CB_LEN);
> 
> -	return BPF_PROG_RUN(prog, skb);
> +	preempt_disable();
> +	res = BPF_PROG_RUN(prog, skb);
> +	preempt_enable();
> +	return res;
> }
> 
> static __always_inline u32 bpf_prog_run_xdp(const struct bpf_prog *prog,
> diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
> index ab612fe9862f..d17d05570a3f 100644
> --- a/kernel/bpf/cgroup.c
> +++ b/kernel/bpf/cgroup.c
> @@ -572,7 +572,7 @@ int __cgroup_bpf_run_filter_skb(struct sock *sk,
> 	bpf_compute_and_save_data_end(skb, &saved_data_end);
> 
> 	ret = BPF_PROG_RUN_ARRAY(cgrp->bpf.effective[type], skb,
> -				 bpf_prog_run_save_cb);
> +				 __bpf_prog_run_save_cb);
> 	bpf_restore_data_end(skb, saved_data_end);
> 	__skb_pull(skb, offset);
> 	skb->sk = save_sk;
> -- 
> 2.20.0
>