linux-kernel - Re: [PATCH v10 07/14] unwind_user/deferred: Make unwind deferral requests NMI-safe

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 for Android: free password hash cracker in your pocket

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

Message-ID: <20250619090417.20d37c7c@batman.local.home>
Date: Thu, 19 Jun 2025 09:04:17 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
 bpf@...r.kernel.org, x86@...nel.org, Masami Hiramatsu
 <mhiramat@...nel.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Josh Poimboeuf <jpoimboe@...nel.org>, Ingo Molnar <mingo@...nel.org>, Jiri
 Olsa <jolsa@...nel.org>, Namhyung Kim <namhyung@...nel.org>, Thomas
 Gleixner <tglx@...utronix.de>, Andrii Nakryiko <andrii@...nel.org>, Indu
 Bhagat <indu.bhagat@...cle.com>, "Jose E. Marchesi" <jemarch@....org>, Beau
 Belgrave <beaub@...ux.microsoft.com>, Jens Remus <jremus@...ux.ibm.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>, Andrew Morton
 <akpm@...ux-foundation.org>
Subject: Re: [PATCH v10 07/14] unwind_user/deferred: Make unwind deferral
 requests NMI-safe

On Thu, 19 Jun 2025 12:39:51 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> On Thu, Jun 19, 2025 at 06:19:28AM -0400, Steven Rostedt wrote:
> 
> > We currently care about x86-64, arm64, ppc 64 and s390. I'm assuming
> > they all have a proper 64 bit cmpxchg.  
> 
> They do. The only 64bit architecture that does not is HPPA IIRC.

It appears that I refactored the ring buffer code to get rid of all 64
bit cmpxchg(), but I do have this in the code:

        if ((!IS_ENABLED(CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG) || 
             IS_ENABLED(CONFIG_GENERIC_ATOMIC64)) &&
            (unlikely(in_nmi()))) {
                return NULL;
        }

We could do something similar, in the function that asks for a deferred
stack trace:

	/* NMI requires having safe 64 bit cmpxchg operations */
	if ((!IS_ENABLED(CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG) || !IS_ENABLED(CONFIG_64BIT)) && in_nmi())
		return -EINVAL;

As the only thing not supported is requesting a deferred stack trace
from NMI context when 64 bit cmpxchg is not available. No reason to not
support the rest of the functionality.

I'll have to wrap the cmpxchg too to not be performed and just do the
update, as for these archs, NMI is not an issue and interrupts will be
disabled, so no cmpxchg is needed.

-- Steve