lkml.org 
[lkml]   [2020]   [Jan]   [6]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: BPF tracing trampoline synchronization between update/freeing and execution?
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.

\
 
 \ /
  Last update: 2020-01-06 23:29    [W:0.057 / U:2.552 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site