Re: [RFC PATCH V2 4/7] x86/hw_breakpoint: Prevent data breakpoints on user_pcid_flush_mask

From: Andy Lutomirski
Date: Tue May 26 2020 - 00:39:07 EST

On Mon, May 25, 2020 at 9:31 PM Lai Jiangshan
<jiangshanlai+lkml@xxxxxxxxx> wrote:
> On Tue, May 26, 2020 at 12:21 PM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> >
> > On Mon, May 25, 2020 at 6:42 PM Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx> wrote:
> > >
> > > The percpu user_pcid_flush_mask is used for CPU entry
> > > If a data breakpoint on it, it will cause an unwanted #DB.
> > > Protect the full cpu_tlbstate structure to be sure.
> > >
> > > There are some other percpu data used in CPU entry, but they are
> > > either in already-protected cpu_tss_rw or are safe to trigger #DB
> > > (espfix_waddr, espfix_stack).
> >
> > How hard would it be to rework this to have DECLARE_PERCPU_NODEBUG()
> > and DEFINE_PERCPU_NODEBUG() or similar?
> I don't know, but it is an excellent idea. Although the patchset
> protects only 2 or 3 portions of percpu data, but there is many
> percpu data used in tracing or kprobe code. They are needed to be
> protected too.
> Adds CC:
> Steven Rostedt <rostedt@xxxxxxxxxxx>
> Masami Hiramatsu <mhiramat@xxxxxxxxxx>

PeterZ is moving things in the direction of more aggressively
disabling hardware breakpoints in the nasty paths where we won't
survive a hardware breakpoint. Does the tracing code have portions
that won't survive a limited amount of recursion?

I'm hoping that we can keep the number of no-breakpoint-here percpu
variables low. Maybe we could recruit objtool to help make sure we
got all of them, but that would be a much larger project.

Would we currently survive a breakpoint on the thread stack? I don't
see any extremely obvious reason that we wouldn't. Blocking such a
breakpoint would be annoying.