linux-kernel - Re: [PATCH] tracing/user_events: Run BPF program if attached

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Date:   Wed, 17 May 2023 20:14:31 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Beau Belgrave <beaub@...ux.microsoft.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-trace-kernel@...r.kernel.org,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, bpf <bpf@...r.kernel.org>,
        David Vernet <void@...ifault.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Dave Thaler <dthaler@...rosoft.com>,
        Christian Brauner <brauner@...nel.org>,
        Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH] tracing/user_events: Run BPF program if attached

On Wed, May 17, 2023 at 7:08 PM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Wed, 17 May 2023 18:18:14 -0700
> Beau Belgrave <beaub@...ux.microsoft.com> wrote:
>
> > We should tell folks to use a group and give access to the group like
> > Steven said earlier.
>
> Could we possibly add an "owner" attribute to the event. That is, the
> creator of the event is the only one that can write to that event. Or at
> least by the user that created it (not actually the process).
>
> So if the user "rostedt" creates an event, only "rostedt" can write to or
> delete the event.

That can work, but why name is global in the first place?
Just make everything per-fd.

> The delete IOCTL is different than reg/unreg. I don't see a problem with
> adding a CAP_SYSADMIN check on the delete IOCTL (and other delete paths)
> to prevent this. It shouldn't affect anything we are doing to add this
> and it makes it so non-admins cannot delete any events if they are given
> write access to the user_events_data file.

sysadmin for delete is a pointless.
user_events_ioctl_reg() has the same issue.
Two different processes using you fancy TRACELOGGING_DEFINE_PROVIDER()
macro and picking the same name will race.

TRACELOGGING_DEFINE_PROVIDER( // defines the MyProvider symbol
    MyProvider, // Name of the provider symbol to define
    "MyCompany_MyComponent_MyProvider", // Human-readable provider
name, no ' ' or ':' chars.
    // {d5b90669-1aad-5db8-16c9-6286a7fcfe33} // Provider guid
(ignored on Linux)
    (0xd5b90669,0x1aad,0x5db8,0x16,0xc9,0x62,0x86,0xa7,0xfc,0xfe,0x33));

I totally get it that Beau is copy pasting these ideas from windows,
but windows is likely similarly broken if it's registering names
globally.

FD should be the isolation boundary.
fd = open("/sys/kernel/tracing/user_event")
and make sure all events are bound to that file.
when file is closed the events _should be auto deleted_.

That's another issue I just spotted.
Looks like user_events_release() is leaking memory.
user_event_refs are just lost.

tbh the more I look into the code the more I want to suggest to mark it
depends on BROKEN
and go back to redesign.