linux-kernel - Re: Why do kprobes and uprobes singlestep?

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Date:   Mon, 1 Mar 2021 18:48:43 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Oleg Nesterov <oleg@...hat.com>, Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
        "David S. Miller" <davem@...emloft.net>, X86 ML <x86@...nel.org>,
        Andrew Cooper <andrew.cooper3@...rix.com>
Subject: Re: Why do kprobes and uprobes singlestep?

On Mon, Mar 1, 2021 at 6:22 PM Masami Hiramatsu <mhiramat@...nel.org> wrote:
>
> Hi Oleg and Andy,
>
> On Mon, 1 Mar 2021 17:51:31 +0100
> Oleg Nesterov <oleg@...hat.com> wrote:
>
> > Hi Andy,
> >
> > sorry for delay.
> >
> > On 02/23, Andy Lutomirski wrote:
> > >
> > > A while back, I let myself be convinced that kprobes genuinely need to
> > > single-step the kernel on occasion, and I decided that this sucked but
> > > I could live with it.  it would, however, be Really Really Nice (tm)
> > > if we could have a rule that anyone running x86 Linux who single-steps
> > > the kernel (e.g. kgdb and nothing else) gets to keep all the pieces
> > > when the system falls apart around them.  Specifically, if we don't
> > > allow kernel single-stepping and if we suitably limit kernel
> > > instruction breakpoints (the latter isn't actually a major problem),
> > > then we don't really really need to use IRET to return to the kernel,
> > > and that means we can avoid some massive NMI nastiness.
> >
> > Not sure I understand you correctly, I know almost nothing about low-level
> > x86  magic.
>
> x86 has normal interrupt and NMI. When an NMI occurs the CPU masks NMI
> (the mask itself is hidden status) and IRET releases the mask. The problem
> is that if an INT3 is hit in the NMI handler and does a single-stepping,
> it has to use IRET for atomically setting TF and return.
>
> >
> > But I guess this has nothing to do with uprobes, they do not single-step
> > in kernel mode, right?
>
> Agreed, if the problematic case is IRET from NMI handler, uprobes doesn't
> hit because it only invoked from user-space.
> Andy, what would you think?

Indeed, this isn't a problem for uprobes.  The problem for uprobes is
that all the notifiers from #DB are kind of messy, and I would like to
get rid of them if possible.