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t=1787062396; bh=xA0rE/FT+Rw5RceGcBEctNQLfvdtoipNdnlahnAV9wI=; h=From:Date:Subject:References:In-Reply-To:To:Cc:From; b=PaEKXgo2a1atscVqobA5We4qNxf3yVi5ZG5FeobOlPIHto7A3/fW8fx7aV9/rhYPm NnNNbvcMY+3WSn6Qqk7VMrXlQvptFqtKrh2B8Lohqzu2+kdIK9sX20SjiqrJACpnSB FG3ey81Jp9eVVlBZfnZtPXW3Hrv7qjmIK5L04TYg= From: Kevin Brodsky Date: Tue, 18 Aug 2026 15:09:05 +0100 Subject: [PATCH RFC v9 23/25] arm64: kpkeys: Batch KPKEYS_CTX_PGTABLES switches MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Message-Id: <20260818-kpkeys-v9-23-743ad31b2c8f@arm.com> References: <20260818-kpkeys-v9-0-743ad31b2c8f@arm.com> In-Reply-To: <20260818-kpkeys-v9-0-743ad31b2c8f@arm.com> To: linux-hardening@vger.kernel.org Cc: Kevin Brodsky , Andrew Morton , Andy Lutomirski , Catalin Marinas , Dave Hansen , "David Hildenbrand (Arm)" , Jann Horn , Jeff Xu , Joey Gouly , Kees Cook , Linu Cherian , Linus Walleij , Marc Zyngier , Mark Brown , Matthew Wilcox , Maxwell Bland , "Mike Rapoport (IBM)" , Peter Zijlstra , Pierre Langlois , =?utf-8?q?Pierre-Cl=C3=A9ment_Tosi?= , 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, Ira Weiny , Lorenzo Stoakes , Thomas Gleixner X-Mailer: b4 0.15.2 X-Developer-Signature: v=1; a=ed25519-sha256; t=1787062277; l=4826; i=kevin.brodsky@arm.com; s=20260427; h=from:subject:message-id; bh=xA0rE/FT+Rw5RceGcBEctNQLfvdtoipNdnlahnAV9wI=; b=Lj0nrpoaI7njErVNUrG7scYHM7h+9q61FBZRj/CpRoG9vuWSBD3xy7JUED6CAcJU18tF1mcjK Cv+Trxvp1aLBxgvRj358Rq8G9Q3EbaKAkgUl60F7XAlpBgJI9m5Qhzn X-Developer-Key: i=kevin.brodsky@arm.com; a=ed25519; pk=N2QG+eJKrvkNovwhhwJhnJ4+ScVfsGCHldmqLfcMTFs= X-Rspam-User: X-Stat-Signature: yeyx8w43aicnwnm39tt1bqem4mtsr75a X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 51DD31C0009 X-HE-Tag: 1787062397-122965 X-HE-Meta: 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 UEtQVhO7 wtcEP6gfoTbczlYYde3+dq/4WmSmWKTM5B1+EoLX1zzuOYtRk3bDnzK7RP4Tkt+yIw39cS6QIaODGbG9uUHj+JXuw1KE36ADyZnLbaeqghc0EBnpUwbxBVmWFJOXGjScBtkY9IGM+ktkkCLBM6Lf6fbTQZJx91rggg2NZkoHK8zVkpPiGSV324QzQUBY3BYW0tnJo/xscptIfb+KllRktgrYis+4hXvJtu6LwAGWEkYa0sLVJ69OA0GfcgVNxRLQRjJFlmPYGzZLFJJtoXsl01P4E/bRT4YF/qXujshFNGq8HMesrIM5C6d64eQvJCPpYxCNrkpgVx+ofIj1U2Nld/nN3rg== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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 struct kpkeys_state somewhere. POR_EL1 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 state 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 lazy MMU 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 | 4 +++ 2 files changed, 50 insertions(+), 4 deletions(-) diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h index b4e52d6bb6ba..6e5d14c7e8b8 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 inline void kpkeys_lazy_mmu_enter(void) +{ + if (!kpkeys_hardened_pgtables_enabled()) + return; + + current->thread.kpkeys_state_lazy_mmu = + kpkeys_enter_context(KPKEYS_CTX_PGTABLES); +} + +static inline void kpkeys_lazy_mmu_exit(void) +{ + const struct kpkeys_state *state = + ¤t->thread.kpkeys_state_lazy_mmu; + + if (!kpkeys_hardened_pgtables_enabled()) + return; + + /* + * 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) && state->entered_context) + __kpkeys_set_pkey_reg_nosync(state->arch_state.por); + else + kpkeys_leave_context(state); +} +#else /* CONFIG_KPKEYS_HARDENED_PGTABLES */ KPKEYS_GUARD_NOOP(kpkeys_hardened_pgtables) -#endif + +static inline void kpkeys_lazy_mmu_enter(void) {} +static inline 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 8d7fa479cd36..23660a233da2 100644 --- a/arch/arm64/include/asm/processor.h +++ b/arch/arm64/include/asm/processor.h @@ -30,6 +30,7 @@ #include #include #include +#include #include #include #include @@ -196,6 +197,9 @@ struct thread_struct { u64 tpidr2_el0; u64 por_el0; u64 por_el1; +#ifdef CONFIG_KPKEYS_HARDENED_PGTABLES + struct kpkeys_state kpkeys_state_lazy_mmu; +#endif #ifdef CONFIG_ARM64_GCS unsigned int gcs_el0_mode; unsigned int gcs_el0_locked; -- 2.51.2