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From:   Andy Lutomirski <luto@kernel.org>
Date:   Mon, 25 May 2020 21:38:51 -0700
Message-ID: <CALCETrW6mazV1JVXYtGW7tUXveNvMKnFwi4zqWUgwMXax_Ea_Q@mail.gmail.com>
Subject: Re: [RFC PATCH V2 4/7] x86/hw_breakpoint: Prevent data breakpoints on user_pcid_flush_mask
To:     Lai Jiangshan <jiangshanlai+lkml@gmail.com>
Cc:     Andy Lutomirski <luto@kernel.org>,
        Lai Jiangshan <laijs@linux.alibaba.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>, X86 ML <x86@kernel.org>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Alexandre Chartre <alexandre.chartre@oracle.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Masami Hiramatsu <mhiramat@kernel.org>
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On Mon, May 25, 2020 at 9:31 PM Lai Jiangshan
<jiangshanlai+lkml@gmail.com> wrote:
>
> On Tue, May 26, 2020 at 12:21 PM Andy Lutomirski <luto@kernel.org> wrote:
> >
> > On Mon, May 25, 2020 at 6:42 PM Lai Jiangshan <laijs@linux.alibaba.com> 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@goodmis.org>
> Masami Hiramatsu <mhiramat@kernel.org>

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.