Subject: Re: [PATCH v4 2/9] perf/core: open access for CAP_SYS_PERFMON privileged process

From: Alexey Budankov <[email-masked]>
Date: Fri, 10 Jan 2020 19:41:41 +0300

On 10.01.2020 17:02, Peter Zijlstra wrote:
> On Thu, Jan 09, 2020 at 02:36:50PM +0300, Alexey Budankov wrote:
>> On 08.01.2020 19:07, Peter Zijlstra wrote:
>>> On Wed, Dec 18, 2019 at 12:25:35PM +0300, Alexey Budankov wrote:
> 
>>>> diff --git a/kernel/events/core.c b/kernel/events/core.c
>>>> index 059ee7116008..d9db414f2197 100644
>>>> --- a/kernel/events/core.c
>>>> +++ b/kernel/events/core.c
>>>> @@ -9056,7 +9056,7 @@ static int perf_kprobe_event_init(struct perf_event *event)
>>>>  	if (event->attr.type != perf_kprobe.type)
>>>>  		return -ENOENT;
>>>>  
>>>> -	if (!capable(CAP_SYS_ADMIN))
>>>> +	if (!perfmon_capable())
>>>>  		return -EACCES;
>>>>  
>>>>  	/*
>>>
>>> This one only allows attaching to already extant kprobes, right? It does
>>> not allow creation of kprobes.
>>
>> This unblocks creation of local trace kprobes and uprobes by CAP_SYS_PERFMON 
>> privileged process, exactly the same as for CAP_SYS_ADMIN privileged process.
> 
> I've no idea what you just said; it's just words.
> 
> Again, this only allows attaching to previously created kprobes, it does
> not allow creating kprobes, right?

Not really, this allows creating a kprobe using perf_event_open syscall that
associates file descriptor with the kprobe [1].

Lifetime of that kprobe is equal to the lifetime of the file descriptor and 
the kprobe is not visible in tracefs: /sys/kernel/debug/tracing/kprobe_events

> 
> That is; I don't think CAP_SYS_PERFMON should be allowed to create
> kprobes.
> 
> As might be clear; I don't actually know what the user-ABI is for
> creating kprobes.

~Alexey

---

[1] https://lore.kernel.org/lkml/[email-masked]/