linux-hardening - [PATCH v5 20/25] stackleack: Mark stack_erasing

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Message-ID: <20250429113242.998312-21-vschneid@redhat.com>
Date: Tue, 29 Apr 2025 13:32:37 +0200
From: Valentin Schneider <vschneid@...hat.com>
To: linux-kernel@...r.kernel.org,
	virtualization@...ts.linux.dev,
	linux-arm-kernel@...ts.infradead.org,
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Cc: Josh Poimboeuf <jpoimboe@...nel.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Yair Podemsky <ypodemsk@...hat.com>,
	Daniel Wagner <dwagner@...e.de>,
	Petr Tesarik <ptesarik@...e.com>,
	Nicolas Saenz Julienne <nsaenz@...zon.com>,
	Frederic Weisbecker <frederic@...nel.org>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Sean Christopherson <seanjc@...gle.com>,
	Juergen Gross <jgross@...e.com>,
	Ajay Kaher <ajay.kaher@...adcom.com>,
	Alexey Makhalov <alexey.amakhalov@...adcom.com>,
	Broadcom internal kernel review list <bcm-kernel-feedback-list@...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>,
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	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>,
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Subject: [PATCH v5 20/25] stackleack: Mark stack_erasing_bypass key as allowed in .noinstr

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.

stack_erasing_bypass is used in .noinstr code, and can be modified at runtime
(proc/sys/kernel/stack_erasing write). However it is not expected that it
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.

Reported-by: Josh Poimboeuf <jpoimboe@...nel.org>
Signed-off-by: Valentin Schneider <vschneid@...hat.com>
---
 kernel/stackleak.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/stackleak.c b/kernel/stackleak.c
index bb65321761b43..51b24d1e04ba2 100644
--- a/kernel/stackleak.c
+++ b/kernel/stackleak.c
@@ -19,7 +19,11 @@
 #include <linux/sysctl.h>
 #include <linux/init.h>
 
-static DEFINE_STATIC_KEY_FALSE(stack_erasing_bypass);
+/*
+ * This static key can only be modified via its sysctl interface. It is
+ * expected it will remain stable during latency-senstive operations.
+ */
+static DEFINE_STATIC_KEY_FALSE_NOINSTR(stack_erasing_bypass);
 
 #ifdef CONFIG_SYSCTL
 static int stack_erasing_sysctl(const struct ctl_table *table, int write,
-- 
2.49.0