linux-kernel - Re: [PATCH v6 19/29] KVM: VMX: Mark vmx_l1d_should flush and vmx_l1d_flush

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: <aO2S3oZwOW_UgAci@google.com>
Date: Mon, 13 Oct 2025 17:01:34 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Valentin Schneider <vschneid@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, rcu@...r.kernel.org, 
	x86@...nel.org, linux-arm-kernel@...ts.infradead.org, 
	loongarch@...ts.linux.dev, linux-riscv@...ts.infradead.org, 
	linux-arch@...r.kernel.org, linux-trace-kernel@...r.kernel.org, 
	Josh Poimboeuf <jpoimboe@...nel.org>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, 
	"H. Peter Anvin" <hpa@...or.com>, Andy Lutomirski <luto@...nel.org>, Peter Zijlstra <peterz@...radead.org>, 
	Arnaldo Carvalho de Melo <acme@...nel.org>, Paolo Bonzini <pbonzini@...hat.com>, Arnd Bergmann <arnd@...db.de>, 
	Frederic Weisbecker <frederic@...nel.org>, "Paul E. McKenney" <paulmck@...nel.org>, 
	Jason Baron <jbaron@...mai.com>, Steven Rostedt <rostedt@...dmis.org>, 
	Ard Biesheuvel <ardb@...nel.org>, Sami Tolvanen <samitolvanen@...gle.com>, 
	"David S. Miller" <davem@...emloft.net>, Neeraj Upadhyay <neeraj.upadhyay@...nel.org>, 
	Joel Fernandes <joelagnelf@...dia.com>, Josh Triplett <josh@...htriplett.org>, 
	Boqun Feng <boqun.feng@...il.com>, Uladzislau Rezki <urezki@...il.com>, 
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Mel Gorman <mgorman@...e.de>, 
	Andrew Morton <akpm@...ux-foundation.org>, Masahiro Yamada <masahiroy@...nel.org>, 
	Han Shen <shenhan@...gle.com>, Rik van Riel <riel@...riel.com>, Jann Horn <jannh@...gle.com>, 
	Dan Carpenter <dan.carpenter@...aro.org>, Oleg Nesterov <oleg@...hat.com>, 
	Juri Lelli <juri.lelli@...hat.com>, Clark Williams <williams@...hat.com>, 
	Yair Podemsky <ypodemsk@...hat.com>, Marcelo Tosatti <mtosatti@...hat.com>, 
	Daniel Wagner <dwagner@...e.de>, Petr Tesarik <ptesarik@...e.com>
Subject: Re: [PATCH v6 19/29] KVM: VMX: Mark vmx_l1d_should flush and
 vmx_l1d_flush_cond keys as allowed in .noinstr

On Fri, Oct 10, 2025, Valentin Schneider wrote:
> Later commits will cause objtool to warn about static keys being used in
> .noinstr sections in order to safely defer instruction patching IPIs
> targeted at NOHZ_FULL CPUs.
> 
> These keys are used in .noinstr code, and can be modified at runtime
> (/proc/kernel/vmx* write). However it is not expected that they will be
> flipped during latency-sensitive operations, and thus shouldn't be a source
> of interference wrt the text patching IPI.
>
> Mark it to let objtool know not to warn about it.

Can you elaborate in the changelog on what will happen if the key is toggle?
IIUC, smp_text_poke_batch_finish() will force IPIs if noinstr code is being
patched.  Even just a small footnote like this:

  Note, smp_text_poke_batch_finish() never defers IPIs if noinstr code is
  being patched, i.e. this is purely about silencing objtool warnings.

to make it clear that there's no bug/race being introduced.

> Reported-by: Josh Poimboeuf <jpoimboe@...nel.org>
> Signed-off-by: Valentin Schneider <vschneid@...hat.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index aa157fe5b7b31..dce2bd7375ec8 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -204,8 +204,15 @@ module_param(pt_mode, int, S_IRUGO);
>  
>  struct x86_pmu_lbr __ro_after_init vmx_lbr_caps;
>  
> -static DEFINE_STATIC_KEY_FALSE(vmx_l1d_should_flush);
> -static DEFINE_STATIC_KEY_FALSE(vmx_l1d_flush_cond);
> +/*
> + * Both of these static keys end up being used in .noinstr sections, however
> + * they are only modified:
> + * - at init
> + * - from a /proc/kernel/vmx* write
> + * thus during latency-sensitive operations they should remain stable.
> + */
> +static DEFINE_STATIC_KEY_FALSE_NOINSTR(vmx_l1d_should_flush);
> +static DEFINE_STATIC_KEY_FALSE_NOINSTR(vmx_l1d_flush_cond);
>  static DEFINE_MUTEX(vmx_l1d_flush_mutex);
>  
>  /* Storage for pre module init parameter parsing */
> -- 
> 2.51.0
>