From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Jann Horn <jannh@google.com>,
bpf@vger.kernel.org, live-patching@vger.kernel.org,
Alexei Starovoitov <ast@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
Steven Rostedt <rostedt@goodmis.org>,
KP Singh <kpsingh@chromium.org>,
Andy Lutomirski <luto@amacapital.net>,
kernel list <linux-kernel@vger.kernel.org>,
the arch/x86 maintainers <x86@kernel.org>,
Josh Poimboeuf <jpoimboe@redhat.com>,
Ingo Molnar <mingo@redhat.com>
Subject: Re: BPF tracing trampoline synchronization between update/freeing and execution?
Date: Mon, 6 Jan 2020 14:29:08 -0800 [thread overview]
Message-ID: <20200106222907.yjoranutzjdersty@ast-mbp> (raw)
In-Reply-To: <20200106165654.GP2844@hirez.programming.kicks-ass.net>
On Mon, Jan 06, 2020 at 05:56:54PM +0100, Peter Zijlstra wrote:
> On Mon, Jan 06, 2020 at 05:39:30PM +0100, Jann Horn wrote:
> > Hi!
> >
> > I was chatting with kpsingh about BPF trampolines, and I noticed that
> > it looks like BPF trampolines (as of current bpf-next/master) seem to
> > be missing synchronization between trampoline code updates and
> > trampoline execution. Or maybe I'm missing something?
> >
> > If I understand correctly, trampolines are executed directly from the
> > fentry placeholders at the start of arbitrary kernel functions, so
> > they can run without any locks held. So for example, if task A starts
> > executing a trampoline on entry to sys_open(), then gets preempted in
> > the middle of the trampoline, and then task B quickly calls
> > BPF_RAW_TRACEPOINT_OPEN twice, and then task A continues execution,
> > task A will end up executing the middle of newly-written machine code,
> > which can probably end up crashing the kernel somehow?
> >
> > I think that at least to synchronize trampoline text freeing with
> > concurrent trampoline execution, it is necessary to do something
> > similar to what the livepatching code does with klp_check_stack(), and
> > then either use a callback from the scheduler to periodically re-check
> > tasks that were in the trampoline or let the trampoline tail-call into
> > a cleanup helper that is part of normal kernel text. And you'd
> > probably have to gate BPF trampolines on
> > CONFIG_HAVE_RELIABLE_STACKTRACE.
>
> ftrace uses synchronize_rcu_tasks() to flip between trampolines iirc.
good catch and good suggestion. synchronize_rcu_tasks() is needed here too.
next prev parent reply other threads:[~2020-01-06 22:29 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-06 16:39 BPF tracing trampoline synchronization between update/freeing and execution? Jann Horn
2020-01-06 16:56 ` Peter Zijlstra
2020-01-06 22:29 ` Alexei Starovoitov [this message]
2020-01-07 8:28 ` Petr Mladek
2020-01-07 9:03 ` Petr Mladek
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20200106222907.yjoranutzjdersty@ast-mbp \
--to=alexei.starovoitov@gmail.com \
--cc=ast@kernel.org \
--cc=bpf@vger.kernel.org \
--cc=daniel@iogearbox.net \
--cc=jannh@google.com \
--cc=jpoimboe@redhat.com \
--cc=kpsingh@chromium.org \
--cc=linux-kernel@vger.kernel.org \
--cc=live-patching@vger.kernel.org \
--cc=luto@amacapital.net \
--cc=mingo@redhat.com \
--cc=peterz@infradead.org \
--cc=rostedt@goodmis.org \
--cc=x86@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).