Re: [PATCH bpf-next v3 5/9] bpf: Allow LSM programs to use bpf spin locks

[Martin KaFai Lau]

On Wed, Nov 04, 2020 at 05:44:49PM +0100, KP Singh wrote:
> From: KP Singh <kpsingh@xxxxxxxxxx>
> 
> Usage of spin locks was not allowed for tracing programs due to
> insufficient preemption checks. The verifier does not currently prevent
> LSM programs from using spin locks, but the helpers are not exposed
> via bpf_lsm_func_proto.
This could be the first patch but don't feel strongly about it.

> 
> Based on the discussion in [1], non-sleepable LSM programs should be
> able to use bpf_spin_{lock, unlock}.
> 
> Sleepable LSM programs can be preempted which means that allowng spin
> locks will need more work (disabling preemption and the verifier
> ensuring that no sleepable helpers are called when a spin lock is held).
> 
> [1]: https://lore.kernel.org/bpf/20201103153132.2717326-1-kpsingh@xxxxxxxxxxxx/T/#md601a053229287659071600d3483523f752cd2fb
> 
> Signed-off-by: KP Singh <kpsingh@xxxxxxxxxx>
> ---
>  kernel/bpf/bpf_lsm.c  |  4 ++++
>  kernel/bpf/verifier.c | 17 +++++++++++++++++
>  2 files changed, 21 insertions(+)
> 
> diff --git a/kernel/bpf/bpf_lsm.c b/kernel/bpf/bpf_lsm.c
> index 61f8cc52fd5b..93383df2140b 100644
> --- a/kernel/bpf/bpf_lsm.c
> +++ b/kernel/bpf/bpf_lsm.c
> @@ -63,6 +63,10 @@ bpf_lsm_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
>  		return &bpf_task_storage_get_proto;
>  	case BPF_FUNC_task_storage_delete:
>  		return &bpf_task_storage_delete_proto;
> +	case BPF_FUNC_spin_lock:
> +		return &bpf_spin_lock_proto;
> +	case BPF_FUNC_spin_unlock:
> +		return &bpf_spin_unlock_proto;
>  	default:
>  		return tracing_prog_func_proto(func_id, prog);
>  	}
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 314018e8fc12..7c6c246077cf 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -9739,6 +9739,23 @@ static int check_map_prog_compatibility(struct bpf_verifier_env *env,
>  		return -EINVAL;
>  	}
>  
> +	if (map_value_has_spin_lock(map)) {
> +		if (prog_type == BPF_PROG_TYPE_SOCKET_FILTER) {
> +			verbose(env, "socket filter progs cannot use bpf_spin_lock yet\n");
> +			return -EINVAL;
> +		}
> +
> +		if (is_tracing_prog_type(prog_type)) {
> +			verbose(env, "tracing progs cannot use bpf_spin_lock yet\n");
> +			return -EINVAL;
> +		}
It is good to have a more specific verifier log.  However,
these are duplicated checks (a few lines above in the same function).
They should at least be removed.

> +
> +		if (prog->aux->sleepable) {
> +			verbose(env, "sleepable progs cannot use bpf_spin_lock yet\n");
> +			return -EINVAL;
> +		}
> +	}
> +
>  	if ((bpf_prog_is_dev_bound(prog->aux) || bpf_map_is_dev_bound(map)) &&
>  	    !bpf_offload_prog_map_match(prog, map)) {
>  		verbose(env, "offload device mismatch between prog and map\n");
> -- 
> 2.29.1.341.ge80a0c044ae-goog
>