Re: [PATCH v11 01/11] x86/hw_breakpoints: Make DR7 updates NMI safe

[Google]

On Thu, 6 Aug 2026 09:07:35 +0900
Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx> wrote:

> On Tue, 4 Aug 2026 01:07:23 +0200
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> > On Tue, Aug 04, 2026 at 07:55:17AM +0900, Masami Hiramatsu wrote:
> > > On Mon, 3 Aug 2026 08:57:44 +0200
> > > Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > > 
> > > > On Sun, Aug 02, 2026 at 05:18:37PM +0900, Masami Hiramatsu (Google) wrote:
> > > > > From: Jinchao Wang <wangjinchao600@xxxxxxxxx>
> > > > > 
> > > > > Hardware breakpoint installation and removal run with IRQs disabled, but
> > > > > an NMI can still enter the same code through KGDB. The interrupted
> > > > > operation and the NMI can consequently claim the same slot or overwrite
> > > > > each other's DR7 state.
> > > > 
> > > > Is KGDB really the only way to trip this? Mostly I think we let KGDB
> > > > have the pieces if it does something 'funny'.
> > > 
> > > Hmm, I think even if so, this detection is better to be handled in
> > > hw_breakpoint layer. And I plan to use this from kprobe events,
> > > which is also a kind of NMI.
> > > 
> > > > 
> > > > > diff --git a/arch/x86/kernel/hw_breakpoint.c b/arch/x86/kernel/hw_breakpoint.c
> > > > > index f846c15f21ca..9ef24b55737f 100644
> > > > > --- a/arch/x86/kernel/hw_breakpoint.c
> > > > > +++ b/arch/x86/kernel/hw_breakpoint.c
> > > > > @@ -40,6 +40,9 @@
> > > > >  DEFINE_PER_CPU(unsigned long, cpu_dr7);
> > > > >  EXPORT_PER_CPU_SYMBOL(cpu_dr7);
> > > > >  
> > > > > +/* Sequence number of the per-CPU DR7 state. */
> > > > > +DEFINE_PER_CPU(unsigned int, cpu_dr7_seq);

This is actual sequence number.

> > > > 
> > > > > diff --git a/arch/x86/kernel/nmi.c b/arch/x86/kernel/nmi.c
> > > > > index 3c9f60d6ca5a..f55a0cbd5927 100644
> > > > > --- a/arch/x86/kernel/nmi.c
> > > > > +++ b/arch/x86/kernel/nmi.c
> > > > > @@ -532,10 +532,13 @@ enum nmi_states {
> > > > >  static DEFINE_PER_CPU(enum nmi_states, nmi_state);
> > > > >  static DEFINE_PER_CPU(unsigned long, nmi_cr2);
> > > > >  static DEFINE_PER_CPU(unsigned long, nmi_dr7);
> > > > > +static DEFINE_PER_CPU(unsigned int, nmi_dr7_seq);

and this is a backup.

> > > > 
> > > > This is weird, why have two distinct sequence numbers for dr7?
> > > 
> > > nmi_dr7_seq is for sequence number of operation, which is for
> > > detecting dr7 overwrite in NMI.
> > > nmi_dr7 is for saving the DR7.
> > 
> > I mean cpu_dr7_seq and nmi_dr7_seq.
> 
> Oops, Indeed. Let me fix it. Thanks!

BTW, do we really need nmi_dr7 as a per-cpu variable?

Since this patch changes local_db_save()/restore() to handle both
DR7 and its sequence number, we need to save/restore both.
I thought it is used for handling nested NMI but in exc_nmi, we have
this gate check before using the nmi_dr7.

	if (this_cpu_read(nmi_state) != NMI_NOT_RUNNING) {
		this_cpu_write(nmi_state, NMI_LATCHED);
		return;
	}

Thus the nested NMI is handled serial, not stacked. So we can use
a local variable instead of per-cpu variable.

Thank you,

> 
> 
> 
> -- 
> Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>


-- 
Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>