linux-kernel - [RFC PATCH v5 08/18] arm64: Context-switch POR

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Message-ID: <20250815085512.2182322-9-kevin.brodsky@arm.com>
Date: Fri, 15 Aug 2025 09:55:02 +0100
From: Kevin Brodsky <kevin.brodsky@....com>
To: linux-hardening@...r.kernel.org
Cc: linux-kernel@...r.kernel.org,
	Kevin Brodsky <kevin.brodsky@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andy Lutomirski <luto@...nel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	David Hildenbrand <david@...hat.com>,
	Ira Weiny <ira.weiny@...el.com>,
	Jann Horn <jannh@...gle.com>,
	Jeff Xu <jeffxu@...omium.org>,
	Joey Gouly <joey.gouly@....com>,
	Kees Cook <kees@...nel.org>,
	Linus Walleij <linus.walleij@...aro.org>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Marc Zyngier <maz@...nel.org>,
	Mark Brown <broonie@...nel.org>,
	Matthew Wilcox <willy@...radead.org>,
	Maxwell Bland <mbland@...orola.com>,
	"Mike Rapoport (IBM)" <rppt@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Pierre Langlois <pierre.langlois@....com>,
	Quentin Perret <qperret@...gle.com>,
	Rick Edgecombe <rick.p.edgecombe@...el.com>,
	Ryan Roberts <ryan.roberts@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Vlastimil Babka <vbabka@...e.cz>,
	Will Deacon <will@...nel.org>,
	linux-arm-kernel@...ts.infradead.org,
	linux-mm@...ck.org,
	x86@...nel.org
Subject: [RFC PATCH v5 08/18] arm64: Context-switch POR_EL1

POR_EL1 is about to be used by the kpkeys framework, modifying it
for (typically small) sections of code. If an exception occurs
during that window and scheduling occurs, we must ensure that
POR_EL1 is context-switched as needed (saving the old value and
restoring the new one). An ISB is needed to ensure the write takes
effect, so we skip it if the new value is the same as the old, like
for POR_EL0.

Signed-off-by: Kevin Brodsky <kevin.brodsky@....com>
---
 arch/arm64/include/asm/processor.h | 1 +
 arch/arm64/kernel/process.c        | 9 +++++++++
 2 files changed, 10 insertions(+)

diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
index 61d62bfd5a7b..9340e94a27f6 100644
--- a/arch/arm64/include/asm/processor.h
+++ b/arch/arm64/include/asm/processor.h
@@ -187,6 +187,7 @@ struct thread_struct {
 	u64			svcr;
 	u64			tpidr2_el0;
 	u64			por_el0;
+	u64			por_el1;
 #ifdef CONFIG_ARM64_GCS
 	unsigned int		gcs_el0_mode;
 	unsigned int		gcs_el0_locked;
diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index 96482a1412c6..f698839f018f 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -428,6 +428,9 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 
 	ptrauth_thread_init_kernel(p);
 
+	if (system_supports_poe())
+		p->thread.por_el1 = read_sysreg_s(SYS_POR_EL1);
+
 	if (likely(!args->fn)) {
 		*childregs = *current_pt_regs();
 		childregs->regs[0] = 0;
@@ -678,6 +681,12 @@ static void permission_overlay_switch(struct task_struct *next)
 		 * of POR_EL0.
 		 */
 	}
+
+	current->thread.por_el1 = read_sysreg_s(SYS_POR_EL1);
+	if (current->thread.por_el1 != next->thread.por_el1) {
+		write_sysreg_s(next->thread.por_el1, SYS_POR_EL1);
+		isb();
+	}
 }
 
 /*
-- 
2.47.0