[PATCH v2 2/2] arm64: kasan: mte: move GCR_EL1 switch to task switch when KASAN disabled

[Peter Collingbourne]

It is not necessary to write to GCR_EL1 on every kernel entry and
exit when HW tag-based KASAN is disabled because the kernel will not
execute any IRG instructions in that mode. Since accessing GCR_EL1
can be expensive on some microarchitectures, avoid doing so by moving
the access to task switch when HW tag-based KASAN is disabled.

Signed-off-by: Peter Collingbourne <pcc at google.com>
Acked-by: Andrey Konovalov <andreyknvl at gmail.com>
Link: https://linux-review.googlesource.com/id/I78e90d60612a94c24344526f476ac4ff216e10d2
---
v2:
- add prerequisite patch to fix booting on non-MTE hardware

 arch/arm64/kernel/entry.S | 10 +++++-----
 arch/arm64/kernel/mte.c   | 24 +++++++++++++++++++++---
 include/linux/kasan.h     |  9 +++++++--
 3 files changed, 33 insertions(+), 10 deletions(-)

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index bc6d5a970a13..2f69ae43941d 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -168,9 +168,9 @@ alternative_else_nop_endif
 
 	.macro mte_set_kernel_gcr, tmp, tmp2
 #ifdef CONFIG_KASAN_HW_TAGS
-alternative_if_not ARM64_MTE
+alternative_cb	kasan_hw_tags_enable
 	b	1f
-alternative_else_nop_endif
+alternative_cb_end
 	mov	\tmp, KERNEL_GCR_EL1
 	msr_s	SYS_GCR_EL1, \tmp
 1:
@@ -178,10 +178,10 @@ alternative_else_nop_endif
 	.endm
 
 	.macro mte_set_user_gcr, tsk, tmp, tmp2
-#ifdef CONFIG_ARM64_MTE
-alternative_if_not ARM64_MTE
+#ifdef CONFIG_KASAN_HW_TAGS
+alternative_cb	kasan_hw_tags_enable
 	b	1f
-alternative_else_nop_endif
+alternative_cb_end
 	ldr	\tmp, [\tsk, #THREAD_MTE_CTRL]
 
 	mte_set_gcr \tmp, \tmp2
diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
index 8fc2d90eeb00..5a026a91b4bf 100644
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c
@@ -162,7 +162,7 @@ void mte_check_tfsr_el1(void)
 }
 #endif
 
-static void mte_update_sctlr_user(struct task_struct *task)
+static void mte_update_sctlr_user_and_gcr_excl(struct task_struct *task)
 {
 	/*
 	 * This must be called with preemption disabled and can only be called
@@ -182,6 +182,24 @@ static void mte_update_sctlr_user(struct task_struct *task)
 	else if (resolved_mte_tcf & MTE_CTRL_TCF_SYNC)
 		sctlr |= SCTLR_EL1_TCF0_SYNC;
 	task->thread.sctlr_user = sctlr;
+
+	/*
+	 * SYS_GCR_EL1 will be set to current->thread.mte_ctrl value by
+	 * mte_set_user_gcr() in kernel_exit, but only if KASAN is enabled.
+	 */
+	if (!kasan_hw_tags_enabled())
+		write_sysreg_s(((mte_ctrl >> MTE_CTRL_GCR_USER_EXCL_SHIFT) &
+				SYS_GCR_EL1_EXCL_MASK) | SYS_GCR_EL1_RRND,
+			       SYS_GCR_EL1);
+}
+
+void __init kasan_hw_tags_enable(struct alt_instr *alt, __le32 *origptr,
+				 __le32 *updptr, int nr_inst)
+{
+	BUG_ON(nr_inst != 1); /* Branch -> NOP */
+
+	if (kasan_hw_tags_enabled())
+		*updptr = cpu_to_le32(aarch64_insn_gen_nop());
 }
 
 void mte_thread_init_user(void)
@@ -202,7 +220,7 @@ void mte_thread_switch(struct task_struct *next)
 	if (!system_supports_mte())
 		return;
 
-	mte_update_sctlr_user(next);
+	mte_update_sctlr_user_and_gcr_excl(next);
 
 	/*
 	 * Check if an async tag exception occurred at EL1.
@@ -247,7 +265,7 @@ long set_mte_ctrl(struct task_struct *task, unsigned long arg)
 	task->thread.mte_ctrl = mte_ctrl;
 	if (task == current) {
 		preempt_disable();
-		mte_update_sctlr_user(task);
+		mte_update_sctlr_user_and_gcr_excl(task);
 		update_sctlr_el1(task->thread.sctlr_user);
 		preempt_enable();
 	}
diff --git a/include/linux/kasan.h b/include/linux/kasan.h
index dd874a1ee862..de5f5913374d 100644
--- a/include/linux/kasan.h
+++ b/include/linux/kasan.h
@@ -89,7 +89,7 @@ static __always_inline bool kasan_enabled(void)
 	return static_branch_likely(&kasan_flag_enabled);
 }
 
-static inline bool kasan_has_integrated_init(void)
+static inline bool kasan_hw_tags_enabled(void)
 {
 	return kasan_enabled();
 }
@@ -104,7 +104,7 @@ static inline bool kasan_enabled(void)
 	return IS_ENABLED(CONFIG_KASAN);
 }
 
-static inline bool kasan_has_integrated_init(void)
+static inline bool kasan_hw_tags_enabled(void)
 {
 	return false;
 }
@@ -125,6 +125,11 @@ static __always_inline void kasan_free_pages(struct page *page,
 
 #endif /* CONFIG_KASAN_HW_TAGS */
 
+static inline bool kasan_has_integrated_init(void)
+{
+	return kasan_hw_tags_enabled();
+}
+
 #ifdef CONFIG_KASAN
 
 struct kasan_cache {
-- 
2.33.0.309.g3052b89438-goog