linux-kernel - Re: [PATCH v4 22/30] context_tracking: Exit CT_STATE

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] [day] [month] [year] [list]
Message-ID: <Z5BEBCWVWP_fq2zY@google.com>
Date: Tue, 21 Jan 2025 17:04:04 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: Valentin Schneider <vschneid@...hat.com>, linux-kernel@...r.kernel.org, x86@...nel.org, 
	virtualization@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org, 
	loongarch@...ts.linux.dev, linux-riscv@...ts.infradead.org, 
	linux-perf-users@...r.kernel.org, xen-devel@...ts.xenproject.org, 
	kvm@...r.kernel.org, linux-arch@...r.kernel.org, rcu@...r.kernel.org, 
	linux-hardening@...r.kernel.org, linux-mm@...ck.org, 
	linux-kselftest@...r.kernel.org, bpf@...r.kernel.org, 
	bcm-kernel-feedback-list@...adcom.com, Juergen Gross <jgross@...e.com>, 
	Ajay Kaher <ajay.kaher@...adcom.com>, Alexey Makhalov <alexey.amakhalov@...adcom.com>, 
	Russell King <linux@...linux.org.uk>, Catalin Marinas <catalin.marinas@....com>, 
	Will Deacon <will@...nel.org>, Huacai Chen <chenhuacai@...nel.org>, WANG Xuerui <kernel@...0n.name>, 
	Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>, 
	Albert Ou <aou@...s.berkeley.edu>, 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>, 
	Peter Zijlstra <peterz@...radead.org>, Arnaldo Carvalho de Melo <acme@...nel.org>, 
	Namhyung Kim <namhyung@...nel.org>, Mark Rutland <mark.rutland@....com>, 
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>, 
	Ian Rogers <irogers@...gle.com>, Adrian Hunter <adrian.hunter@...el.com>, 
	Kan Liang <kan.liang@...ux.intel.com>, Boris Ostrovsky <boris.ostrovsky@...cle.com>, 
	Josh Poimboeuf <jpoimboe@...nel.org>, Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, 
	Paolo Bonzini <pbonzini@...hat.com>, Andy Lutomirski <luto@...nel.org>, Arnd Bergmann <arnd@...db.de>, 
	"Paul E. McKenney" <paulmck@...nel.org>, Jason Baron <jbaron@...mai.com>, 
	Steven Rostedt <rostedt@...dmis.org>, Ard Biesheuvel <ardb@...nel.org>, 
	Neeraj Upadhyay <neeraj.upadhyay@...nel.org>, Joel Fernandes <joel@...lfernandes.org>, 
	Josh Triplett <josh@...htriplett.org>, Boqun Feng <boqun.feng@...il.com>, 
	Uladzislau Rezki <urezki@...il.com>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, 
	Lai Jiangshan <jiangshanlai@...il.com>, Zqiang <qiang.zhang1211@...il.com>, 
	Juri Lelli <juri.lelli@...hat.com>, Clark Williams <williams@...hat.com>, 
	Yair Podemsky <ypodemsk@...hat.com>, Tomas Glozar <tglozar@...hat.com>, 
	Vincent Guittot <vincent.guittot@...aro.org>, Dietmar Eggemann <dietmar.eggemann@....com>, 
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, Kees Cook <kees@...nel.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, Christoph Hellwig <hch@...radead.org>, 
	Shuah Khan <shuah@...nel.org>, Sami Tolvanen <samitolvanen@...gle.com>, 
	Miguel Ojeda <ojeda@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>, 
	"Mike Rapoport (Microsoft)" <rppt@...nel.org>, Samuel Holland <samuel.holland@...ive.com>, Rong Xu <xur@...gle.com>, 
	Nicolas Saenz Julienne <nsaenzju@...hat.com>, Geert Uytterhoeven <geert@...ux-m68k.org>, 
	Yosry Ahmed <yosryahmed@...gle.com>, 
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, 
	"Masami Hiramatsu (Google)" <mhiramat@...nel.org>, Jinghao Jia <jinghao7@...inois.edu>, 
	Luis Chamberlain <mcgrof@...nel.org>, Randy Dunlap <rdunlap@...radead.org>, 
	Tiezhu Yang <yangtiezhu@...ngson.cn>
Subject: Re: [PATCH v4 22/30] context_tracking: Exit CT_STATE_IDLE upon
 irq/nmi entry

On Wed, Jan 22, 2025, Frederic Weisbecker wrote:
> Le Tue, Jan 14, 2025 at 06:51:35PM +0100, Valentin Schneider a écrit :
> > ct_nmi_{enter, exit}() only touches the RCU watching counter and doesn't
> > modify the actual CT state part context_tracking.state. This means that
> > upon receiving an IRQ when idle, the CT_STATE_IDLE->CT_STATE_KERNEL
> > transition only happens in ct_idle_exit().
> > 
> > One can note that ct_nmi_enter() can only ever be entered with the CT state
> > as either CT_STATE_KERNEL or CT_STATE_IDLE, as an IRQ/NMI happenning in the
> > CT_STATE_USER or CT_STATE_GUEST states will be routed down to ct_user_exit().
> 
> Are you sure? An NMI can fire between guest_state_enter_irqoff() and
> __svm_vcpu_run().

Heh, technically, they can't.  On SVM, KVM clears GIF prior to svm_vcpu_enter_exit(),
and restores GIF=1 only after it returns.  I.e. NMIs are fully blocked _on SVM_.

VMX unfortunately doesn't provide GIF, and so NMIs can arrive at any time.  It's
infeasible for software to prevent them, so we're stuck with that.  [In theory,
KVM could deliberately generate an NMI and not do IRET so that NMIs are blocked,
but that would be beyond crazy].

> And NMIs interrupting userspace don't call enter_from_user_mode(). In fact
> they don't call irqentry_enter_from_user_mode() like regular IRQs but
> irqentry_nmi_enter() instead. Well that's for archs implementing common entry
> code, I can't speak for the others.
> 
> Unifying the behaviour between user and idle such that the IRQs/NMIs exit the
> CT_STATE can be interesting but I fear this may not come for free. You would
> need to save the old state on IRQ/NMI entry and restore it on exit.
> 
> Do we really need it?
> 
> Thanks.