From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 8D59BCD6E40 for ; Tue, 26 May 2026 11:19:19 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:Cc:To:In-Reply-To:References :Message-Id:Content-Transfer-Encoding:Content-Type:MIME-Version:Subject:Date: From:Reply-To:Content-ID:Content-Description:Resent-Date:Resent-From: Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=emygvYxhkWsqMuM5q9K6kqLmMjPTQyXSCWgwyuJaPJY=; b=rlJx3lu4G7cZSsQSpHZWbMkv7M i/q/FO/vo9yPD2t92+nfprVrKjYkTJKiI8rP8ZGkKWcBttwbs8ioA8N2anpXCc2PeStk1p+ZWRGIr TzN0Wa7p1G//ZjpGfzTBJbj3EmYxCO3Iym71/5Ny2coAjdhO4CdCRbrNcrVNZIZifTsSzHMtmZYWa OZSb7JX+Nic2pDdI5krSKWTjRGDw0fKJ9jhJiVwGlAACCKgOBHcFdqXPR8Y3tAtJh4gtbdKeRY7oz 6ckSgC+dSSCYvWzK8lHQKGYobbwnnNFheb9wVEfHyLNFBfIwIlLkpBDrP69TdceGPDqvpJWx71olZ 1WXdmzSg==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.99.1 #2 (Red Hat Linux)) id 1wRpoV-00000001lkY-3FaU; Tue, 26 May 2026 11:19:12 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.99.1 #2 (Red Hat Linux)) id 1wRpoC-00000001lXB-2xlz for linux-arm-kernel@lists.infradead.org; Tue, 26 May 2026 11:18:56 +0000 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id E6DEF16F2; Tue, 26 May 2026 04:18:46 -0700 (PDT) Received: from localhost.localdomain (e123572-lin.cambridge.arm.com [10.1.194.54]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 27F6D3F7D8; Tue, 26 May 2026 04:18:47 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1779794332; bh=f+o3e9jzGdoDfuO+adnmSdYfcmQjcKxMxDU6OWrV3wo=; h=From:Date:Subject:References:In-Reply-To:To:Cc:From; b=Z2yhCO+0glanNMpcjEtQJ/w7SiPK2pLeNqNIszMDIQnLC7FyNWkJKh+0FaNJkADVw oCUknh15J4XZ9damwbiBICy8lVbhyZChQsIkvhTagh/dkWDPNAFBeqdkP6i+Q2yad/ QTx3llxmEHu+AFJIJ5qDvltcn5e6H3IiEsWOqkbo= From: Kevin Brodsky Date: Tue, 26 May 2026 12:16:11 +0100 Subject: [PATCH RFC v8 22/24] arm64: kpkeys: Batch KPKEYS_CTX_PGTABLES switches MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Message-Id: <20260526-kpkeys-v8-22-eaaacdacc67c@arm.com> References: <20260526-kpkeys-v8-0-eaaacdacc67c@arm.com> In-Reply-To: <20260526-kpkeys-v8-0-eaaacdacc67c@arm.com> To: linux-hardening@vger.kernel.org Cc: Kevin Brodsky , Andrew Morton , Andy Lutomirski , Catalin Marinas , Dave Hansen , "David Hildenbrand (Arm)" , Ira Weiny , Jann Horn , Jeff Xu , Joey Gouly , Kees Cook , Linus Walleij , Marc Zyngier , Mark Brown , Matthew Wilcox , Maxwell Bland , "Mike Rapoport (IBM)" , Peter Zijlstra , Pierre Langlois , Quentin Perret , Rick Edgecombe , Ryan Roberts , Vlastimil Babka , Will Deacon , Yang Shi , Yeoreum Yun , linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org, x86@kernel.org, Lorenzo Stoakes , Thomas Gleixner X-Mailer: b4 0.15.2 X-Developer-Signature: v=1; a=ed25519-sha256; t=1779794212; l=4465; i=kevin.brodsky@arm.com; s=20260427; h=from:subject:message-id; bh=f+o3e9jzGdoDfuO+adnmSdYfcmQjcKxMxDU6OWrV3wo=; b=0J/grukgqVXGPI4sxKZSOOHsNw1TH6qH9g4Z8OmoH5fl1qwIvtsWZMkKtU79kvfE+eqTuxetR Ve2LsAYtKj3DDxCRTqrZDhEB/3G9K61ztemgjXsqj0PTVBHZkbOFaTd X-Developer-Key: i=kevin.brodsky@arm.com; a=ed25519; pk=N2QG+eJKrvkNovwhhwJhnJ4+ScVfsGCHldmqLfcMTFs= X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260526_041853_717059_6D93562F X-CRM114-Status: GOOD ( 21.39 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org The kpkeys_hardened_pgtables feature currently switches kpkeys context in every helper that writes to page tables, such as set_pte(). With kpkeys implemented using POE, this entails a pair of ISBs whenever such helper is called. A simple way to reduce this overhead is to make use of the lazy MMU mode. We amend the kpkeys_hardened_pgtables guard so that no kpkeys context switch (i.e. POR_EL1 update) is issued while the lazy MMU mode is active. Instead, we switch to KPKEYS_CTX_PGTABLES when entering the lazy MMU mode, and restore the previous context when exiting it. Restoring the previous kpkeys context requires storing the original value of POR_EL1 somewhere. This is a full 64-bit value so we cannot simply use a TIF flag. There is no straightforward way to reuse current->thread.por_el1 for that purpose - this is where the current value of POR_EL1 is stored on a context switch, i.e. the value corresponding to KPKEYS_CTX_PGTABLES inside a lazy_mmu section. Instead, we add a new member to thread_struct to hold that value temporarily. This isn't optimal as that member is unused outside of lazy MMU sections, but it is the simplest option. Nesting of sections is not a concern as arch_{enter,leave}_lazy_mmu_mode() are not called in inner sections (nor do we need to do anything there). A further optimisation this patch makes is to merge the ISBs when exiting lazy_mmu mode. That is, if an ISB is going to be issued by emit_pte_barriers() because kernel pgtables were modified in the lazy MMU section, we skip the ISB after restoring POR_EL1. This is done by checking TIF_LAZY_MMU_PENDING and ensuring that POR_EL1 is restored before emit_pte_barriers() is called. Signed-off-by: Kevin Brodsky --- arch/arm64/include/asm/pgtable.h | 50 +++++++++++++++++++++++++++++++++++--- arch/arm64/include/asm/processor.h | 1 + 2 files changed, 47 insertions(+), 4 deletions(-) diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h index 20072f32677d..1c0dcfd14678 100644 --- a/arch/arm64/include/asm/pgtable.h +++ b/arch/arm64/include/asm/pgtable.h @@ -43,10 +43,44 @@ #ifdef CONFIG_KPKEYS_HARDENED_PGTABLES KPKEYS_GUARD_COND(kpkeys_hardened_pgtables, KPKEYS_CTX_PGTABLES, - kpkeys_hardened_pgtables_enabled()) -#else + kpkeys_hardened_pgtables_enabled() && + !is_lazy_mmu_mode_active()) + +static void kpkeys_lazy_mmu_enter(void) +{ + if (!kpkeys_hardened_pgtables_enabled()) + return; + + current->thread.por_el1_lazy_mmu = kpkeys_set_context(KPKEYS_CTX_PGTABLES); +} + +static void kpkeys_lazy_mmu_exit(void) +{ + u64 saved_por_el1; + + if (!kpkeys_hardened_pgtables_enabled()) + return; + + saved_por_el1 = current->thread.por_el1_lazy_mmu; + + /* + * We skip any barrier if TIF_LAZY_MMU_PENDING is set: + * emit_pte_barriers() will issue an ISB just after this function + * returns. + */ + if (test_thread_flag(TIF_LAZY_MMU_PENDING)) + __kpkeys_set_pkey_reg_nosync(saved_por_el1); + else + arch_kpkeys_restore_pkey_reg(saved_por_el1); +} +#else /* CONFIG_KPKEYS_HARDENED_PGTABLES */ KPKEYS_GUARD_NOOP(kpkeys_hardened_pgtables) -#endif + +static void kpkeys_lazy_mmu_enter(void) {} +static void kpkeys_lazy_mmu_exit(void) {} +#endif /* CONFIG_KPKEYS_HARDENED_PGTABLES */ + + static inline void emit_pte_barriers(void) { @@ -79,7 +113,10 @@ static inline void queue_pte_barriers(void) } } -static inline void arch_enter_lazy_mmu_mode(void) {} +static inline void arch_enter_lazy_mmu_mode(void) +{ + kpkeys_lazy_mmu_enter(); +} static inline void arch_flush_lazy_mmu_mode(void) { @@ -89,6 +126,11 @@ static inline void arch_flush_lazy_mmu_mode(void) static inline void arch_leave_lazy_mmu_mode(void) { + /* + * The ordering should be preserved to allow kpkeys_lazy_mmu_exit() + * to skip any barrier when TIF_LAZY_MMU_PENDING is set. + */ + kpkeys_lazy_mmu_exit(); arch_flush_lazy_mmu_mode(); } diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h index 6095322343fc..c3a86ddce637 100644 --- a/arch/arm64/include/asm/processor.h +++ b/arch/arm64/include/asm/processor.h @@ -193,6 +193,7 @@ struct thread_struct { u64 tpidr2_el0; u64 por_el0; u64 por_el1; + u64 por_el1_lazy_mmu; #ifdef CONFIG_ARM64_GCS unsigned int gcs_el0_mode; unsigned int gcs_el0_locked; -- 2.51.2