linux-kernel - Re: [PATCH v13 01/17] preempt: Track NMI nesting to separate per-CPU counter

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20251014194349.GC1206438@noisy.programming.kicks-ass.net>
Date: Tue, 14 Oct 2025 21:43:49 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Joel Fernandes <joelagnelf@...dia.com>
Cc: Lyude Paul <lyude@...hat.com>, rust-for-linux@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Boqun Feng <boqun.feng@...il.com>, linux-kernel@...r.kernel.org,
	Daniel Almeida <daniel.almeida@...labora.com>,
	Danilo Krummrich <dakr@...nel.org>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Uladzislau Rezki <urezki@...il.com>,
	Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>, Gary Guo <gary@...yguo.net>,
	Bj??rn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <lossin@...nel.org>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>, Ryo Takakura <ryotkkr98@...il.com>,
	K Prateek Nayak <kprateek.nayak@....com>,
	"open list:CPU FREQUENCY SCALING FRAMEWORK" <linux-pm@...r.kernel.org>
Subject: Re: [PATCH v13 01/17] preempt: Track NMI nesting to separate per-CPU
 counter

On Tue, Oct 14, 2025 at 01:55:47PM -0400, Joel Fernandes wrote:
> 
> 
> On 10/14/2025 6:48 AM, Peter Zijlstra wrote:
> > On Mon, Oct 13, 2025 at 11:48:03AM -0400, Lyude Paul wrote:
> > 
> >>  #define __nmi_enter()						\
> >>  	do {							\
> >>  		lockdep_off();					\
> >>  		arch_nmi_enter();				\
> >> -		BUG_ON(in_nmi() == NMI_MASK);			\
> >> -		__preempt_count_add(NMI_OFFSET + HARDIRQ_OFFSET);	\
> >> +		BUG_ON(__this_cpu_read(nmi_nesting) == UINT_MAX);	\
> >> +		__this_cpu_inc(nmi_nesting);			\
> > 
> > An NMI that nests from here..
> > 
> >> +		__preempt_count_add(HARDIRQ_OFFSET);		\
> >> +		if (__this_cpu_read(nmi_nesting) == 1)		\
> > 
> > .. until here, will see nmi_nesting > 1 and not set NMI_OFFSET.
> 
> This is true, I can cure it by setting NMI_OFFSET unconditionally when
> nmi_nesting >= 1. Then the outer most NMI will then reset it. I think that will
> work. Do you see any other issue with doing so?

unconditionally set NMI_FFSET, regardless of nmi_nesting
and only clear on exit when nmi_nesting == 0.

Notably, when you use u64 __preempt_count, you can limit this to 32bit
only. The NMI nesting can happen in the single instruction window
between ADD and ADC. But on 64bit you don't have that gap and so don't
need to fix it.