[PATCH v3 1/2] arm64: add an MTE support check to the top of mte_thread_switch()

[PATCH v3 1/2] arm64: add an MTE support check to the top of mte_thread_switch()

[Peter Collingbourne]

This lets us avoid doing unnecessary work on hardware that does
not support MTE, and will allow us to freely use MTE instructions
in the code called by mte_thread_switch().

Since this would mean that we do a redundant check in
mte_check_tfsr_el1(), remove it and add two checks now required in
its callers.

Signed-off-by: Peter Collingbourne <pcc@google.com>
Link: https://linux-review.googlesource.com/id/I02fd000d1ef2c86c7d2952a7f099b254ec227a5d
---
v3:
- remove check from mte_check_tfsr_el1()

 arch/arm64/include/asm/mte.h |  6 ++++++
 arch/arm64/kernel/mte.c      | 10 ++++------
 2 files changed, 10 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/include/asm/mte.h b/arch/arm64/include/asm/mte.h
index 3f93b9e0b339..02511650cffe 100644
--- a/arch/arm64/include/asm/mte.h
+++ b/arch/arm64/include/asm/mte.h
@@ -99,11 +99,17 @@ void mte_check_tfsr_el1(void);
 
 static inline void mte_check_tfsr_entry(void)
 {
+	if (!system_supports_mte())
+		return;
+
 	mte_check_tfsr_el1();
 }
 
 static inline void mte_check_tfsr_exit(void)
 {
+	if (!system_supports_mte())
+		return;
+
 	/*
 	 * The asynchronous faults are sync'ed automatically with
 	 * TFSR_EL1 on kernel entry but for exit an explicit dsb()
diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
index 9d314a3bad3b..e5e801bc5312 100644
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c
@@ -142,12 +142,7 @@ void mte_enable_kernel_async(void)
 #ifdef CONFIG_KASAN_HW_TAGS
 void mte_check_tfsr_el1(void)
 {
-	u64 tfsr_el1;
-
-	if (!system_supports_mte())
-		return;
-
-	tfsr_el1 = read_sysreg_s(SYS_TFSR_EL1);
+	u64 tfsr_el1 = read_sysreg_s(SYS_TFSR_EL1);
 
 	if (unlikely(tfsr_el1 & SYS_TFSR_EL1_TF1)) {
 		/*
@@ -199,6 +194,9 @@ void mte_thread_init_user(void)
 
 void mte_thread_switch(struct task_struct *next)
 {
+	if (!system_supports_mte())
+		return;
+
 	mte_update_sctlr_user(next);
 
 	/*
-- 
2.33.0.309.g3052b89438-goog


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[PATCH v3 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@google.com>
Acked-by: Andrey Konovalov <andreyknvl@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 e5e801bc5312..bb8204d4c5d3 100644
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c
@@ -157,7 +157,7 @@ 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
@@ -177,6 +177,24 @@ 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)
@@ -197,7 +215,7 @@ 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.
@@ -242,7 +260,7 @@ 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


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Re: [PATCH v3 2/2] arm64: kasan: mte: move GCR_EL1 switch to task switch when KASAN disabled

[Catalin Marinas]

On Wed, Sep 15, 2021 at 12:03:36PM -0700, Peter Collingbourne wrote:
> -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
> @@ -177,6 +177,24 @@ 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);
> +}

The only nitpick I have is that I'd like to keep the GCR_EL1 updating in
a separate function, mte_update_gcr_user() or something (it can be
static inline) that you'd explicitly call from mte_thread_switch() and
set_mte_ctrl(). The original mte_update_sctlr_user() was not writing any
system registers.

Maybe a better alternative would be to move the update_sctlr_el1() call
in mte_update_sctlr_user() as well (and rename it more generically to
mte_update_user_ctrl()) but with a new bool update_sctlr argument that's
false on the thread switch path.

Either way:

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

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Re: (subset) [PATCH v3 1/2] arm64: add an MTE support check to the top of mte_thread_switch()

[Catalin Marinas]

On Wed, 15 Sep 2021 12:03:35 -0700, Peter Collingbourne wrote:
> This lets us avoid doing unnecessary work on hardware that does
> not support MTE, and will allow us to freely use MTE instructions
> in the code called by mte_thread_switch().
> 
> Since this would mean that we do a redundant check in
> mte_check_tfsr_el1(), remove it and add two checks now required in
> its callers.

I applied the first patch to arm64 (for-next/fixes), it should appear in
5.15-rc3. It looks to me more like a performance regression with the
additional dsb+isb on the syscall path for hw not supporting MTE (and I
added a Fixes tag). I'll leave the second patch to Will for 5.16.

Thanks!

[1/2] arm64: add an MTE support check to the top of mte_thread_switch()
      https://git.kernel.org/arm64/c/8c8a3b5bd960

-- 
Catalin


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Re: [PATCH v3 2/2] arm64: kasan: mte: move GCR_EL1 switch to task switch when KASAN disabled

[Peter Collingbourne]

On Tue, Sep 21, 2021 at 5:40 AM Catalin Marinas <catalin.marinas@arm.com> wrote:
>
> On Wed, Sep 15, 2021 at 12:03:36PM -0700, Peter Collingbourne wrote:
> > -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
> > @@ -177,6 +177,24 @@ 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);
> > +}
>
> The only nitpick I have is that I'd like to keep the GCR_EL1 updating in
> a separate function, mte_update_gcr_user() or something (it can be
> static inline) that you'd explicitly call from mte_thread_switch() and
> set_mte_ctrl(). The original mte_update_sctlr_user() was not writing any
> system registers.

Done in v4. (I mildly prefer the way I originally wrote it as I think
it's less error prone, but I don't mind that much.) Sorry, I forgot to
add your Reviewed-by.

Peter

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