[PATCH RFC v9 23/25] arm64: kpkeys: Batch KPKEYS_CTX_PGTABLES switches

[Kevin Brodsky]

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 <kevin.brodsky at arm.com>
---
 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 =
+		&current->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 <linux/build_bug.h>
 #include <linux/cache.h>
 #include <linux/init.h>
+#include <linux/kpkeys_types.h>
 #include <linux/stddef.h>
 #include <linux/string.h>
 #include <linux/thread_info.h>
@@ -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