Subject: Re: [PATCH v3 net-next] bpf: fix bpf_perf_event_read() helper

From: "Wangnan (F)" <[email-masked]>
Date: Fri, 23 Oct 2015 10:21:06 +0800

On 2015/10/23 8:10, Alexei Starovoitov wrote:
> Fix safety checks for bpf_perf_event_read():
> - only non-inherited events can be added to perf_event_array map
>    (do this check statically at map insertion time)
> - dynamically check that event is local and !pmu->count
> Otherwise buggy bpf program can cause kernel splat.
>
> Also fix error path after perf_event_attrs()
> and remove redundant 'extern'.
>
> Fixes: 35578d798400 ("bpf: Implement function bpf_perf_event_read() that get the selected hardware PMU conuter")
> Signed-off-by: Alexei Starovoitov <[email-masked]>
> ---
> v2->v3:
> . refactor checks based on Wangnan's and Peter's feedback
> while refactoring realized that these two issues need fixes as well:
> . fix perf_event_attrs() error path
> . remove redundant extern
>
> v1->v2: fix compile in case of !CONFIG_PERF_EVENTS
>
> Even in the worst case the crash is not possible.
> Only warn_on_once, so imo net-next is ok.
>
>   include/linux/bpf.h      |    1 -
>   kernel/bpf/arraymap.c    |   25 ++++++++++++++++---------
>   kernel/trace/bpf_trace.c |    7 ++++++-
>   3 files changed, 22 insertions(+), 11 deletions(-)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index e3a51b74e275..75718fa28260 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -194,7 +194,6 @@ extern const struct bpf_func_proto bpf_map_lookup_elem_proto;
>   extern const struct bpf_func_proto bpf_map_update_elem_proto;
>   extern const struct bpf_func_proto bpf_map_delete_elem_proto;
>   
> -extern const struct bpf_func_proto bpf_perf_event_read_proto;
>   extern const struct bpf_func_proto bpf_get_prandom_u32_proto;
>   extern const struct bpf_func_proto bpf_get_smp_processor_id_proto;
>   extern const struct bpf_func_proto bpf_tail_call_proto;
> diff --git a/kernel/bpf/arraymap.c b/kernel/bpf/arraymap.c
> index e3cfe46b074f..3f4c99e06c6b 100644
> --- a/kernel/bpf/arraymap.c
> +++ b/kernel/bpf/arraymap.c
> @@ -292,16 +292,23 @@ static void *perf_event_fd_array_get_ptr(struct bpf_map *map, int fd)
>   
>   	attr = perf_event_attrs(event);
>   	if (IS_ERR(attr))
> -		return (void *)attr;
> +		goto err;
>   
> -	if (attr->type != PERF_TYPE_RAW &&
> -	    !(attr->type == PERF_TYPE_SOFTWARE &&
> -	      attr->config == PERF_COUNT_SW_BPF_OUTPUT) &&
> -	    attr->type != PERF_TYPE_HARDWARE) {
> -		perf_event_release_kernel(event);
> -		return ERR_PTR(-EINVAL);
> -	}
> -	return event;
> +	if (attr->inherit)
> +		goto err;
> +

Since Peter suggest it is pointless for a system-wide perf_event
has inherit bit set [1], I think it should be safe to enable
system-wide perf_event pass this check?

I'll check code to make sure.

[1] 
http://lkml.kernel.org/r/[email-masked]

> +	if (attr->type == PERF_TYPE_RAW)
> +		return event;
> +
> +	if (attr->type == PERF_TYPE_HARDWARE)
> +		return event;
> +
> +	if (attr->type == PERF_TYPE_SOFTWARE &&
> +	    attr->config == PERF_COUNT_SW_BPF_OUTPUT)
> +		return event;
> +err:
> +	perf_event_release_kernel(event);
> +	return ERR_PTR(-EINVAL);
>   }
>   
>   static void perf_event_fd_array_put_ptr(void *ptr)
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index 47febbe7998e..003df3887287 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -199,6 +199,11 @@ static u64 bpf_perf_event_read(u64 r1, u64 index, u64 r3, u64 r4, u64 r5)
>   	if (!event)
>   		return -ENOENT;
>   
> +	/* make sure event is local and doesn't have pmu::count */
> +	if (event->oncpu != smp_processor_id() ||
> +	    event->pmu->count)
> +		return -EINVAL;
> +
>   	/*
>   	 * we don't know if the function is run successfully by the
>   	 * return value. It can be judged in other places, such as
> @@ -207,7 +212,7 @@ static u64 bpf_perf_event_read(u64 r1, u64 index, u64 r3, u64 r4, u64 r5)
>   	return perf_event_read_local(event);
>   }
>   
> -const struct bpf_func_proto bpf_perf_event_read_proto = {
> +static const struct bpf_func_proto bpf_perf_event_read_proto = {
>   	.func		= bpf_perf_event_read,
>   	.gpl_only	= false,
>   	.ret_type	= RET_INTEGER,