linux-kernel - Re: [PATCH] x86/nmi: Fix "in NMI handler" check

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Date: Thu, 8 Feb 2024 06:59:25 -0800
From: Breno Leitao <leitao@...ian.org>
To: Xin Li <xin@...or.com>
Cc: paulmck@...nel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>, leit@...a.com,
	Mark Rutland <mark.rutland@....com>, Ingo Molnar <mingo@...nel.org>,
	"open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/nmi: Fix "in NMI handler" check

On Wed, Feb 07, 2024 at 10:44:57AM -0800, Xin Li wrote:
> On 2/7/2024 10:30 AM, Paul E. McKenney wrote:
> > On Wed, Feb 07, 2024 at 08:52:35AM -0800, Breno Leitao wrote:
> > > Commit 344da544f177 ("x86/nmi: Print reasons why backtrace NMIs are
> > > ignored") creates a super nice framework to diagnose NMIs.
> > > 
> > > Every time nmi_exc() is called, it increments a per_cpu counter
> > > (nsp->idt_nmi_seq). At its exit, it also increments the same counter.
> > > Looking at this counter, you can see how many times that function was
> > > called (dividing by 2), and, if the function is still being executed, by
> > > checking the idt_nmi_seq's last bit.
> > > 
> > > On the check side (nmi_backtrace_stall_check()), that variable is
> > > queried to check if the NMI is still being executed, but, there is a
> > > mistake in the bitwise operation. That code wants to check if the last
> > > bit of the idt_nmi_seq is set or not, but, does the opposite, and check
> > > for all the other bits, which will always be true after the first
> > > exc_nmi() executed successfully.
> > > 
> > > This appends the misleading string to the dump "(CPU currently in NMI
> > > handler function)"
> > > 
> > > Fix it by checking the last bit, and if it is set, append the string.
> > > 
> > > Fixes: 344da544f177 ("x86/nmi: Print reasons why backtrace NMIs are ignored")
> > > Signed-off-by: Breno Leitao <leitao@...ian.org>
> > 
> > If someone else is taking this:
> > 
> > Reviewed-by: Paul E. McKenney <paulmck@...nel.org>
> > 
> > (I am queueing it for testing in any case.)
> 
> Does this fix need to be backported?
> 
> Commit 344da544f177 has a date on Dec 16 2022.

I would say so, if users are using this detection mechanism and want to
trust the messages being displayed by the kernel.