On 8/27/20 11:38 AM, Catalin Marinas wrote:

On Fri, Aug 14, 2020 at 07:27:06PM +0200, Andrey Konovalov wrote:

quoted

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index cde127508e38..a17fefb0571b 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S

@@ -172,6 +172,29 @@ alternative_else_nop_endif
 #endif
 	.endm
 
+	/* Note: tmp should always be a callee-saved register */

Why callee-saved? Do you preserve it anywhere here?

Aargh, this is an old comment, I forgot to remove it after the last refactor.
Thank you for pointing this out.

quoted

+	.macro mte_restore_gcr, el, tsk, tmp, tmp2
+#ifdef CONFIG_ARM64_MTE
+alternative_if_not ARM64_MTE
+	b	1f
+alternative_else_nop_endif
+	.if	\el == 0
+	ldr	\tmp, [\tsk, #THREAD_GCR_EL1_USER]
+	.else
+	ldr_l	\tmp, gcr_kernel_excl
+	.endif
+	/*
+	 * Calculate and set the exclude mask preserving
+	 * the RRND (bit[16]) setting.
+	 */
+	mrs_s	\tmp2, SYS_GCR_EL1
+	bfi	\tmp2, \tmp, #0, #16
+	msr_s	SYS_GCR_EL1, \tmp2
+	isb
+1:
+#endif
+	.endm
+
 	.macro	kernel_entry, el, regsize = 64
 	.if	\regsize == 32
 	mov	w0, w0				// zero upper 32 bits of x0

@@ -209,6 +232,8 @@ alternative_else_nop_endif
 
 	ptrauth_keys_install_kernel tsk, x20, x22, x23
 
+	mte_restore_gcr 1, tsk, x22, x23
+
 	scs_load tsk, x20
 	.else
 	add	x21, sp, #S_FRAME_SIZE

@@ -386,6 +411,8 @@ alternative_else_nop_endif
 	/* No kernel C function calls after this as user keys are set. */
 	ptrauth_keys_install_user tsk, x0, x1, x2
 
+	mte_restore_gcr 0, tsk, x0, x1
+
 	apply_ssbd 0, x0, x1
 	.endif
 

@@ -957,6 +984,7 @@ SYM_FUNC_START(cpu_switch_to)
 	mov	sp, x9
 	msr	sp_el0, x1
 	ptrauth_keys_install_kernel x1, x8, x9, x10
+	mte_restore_gcr 1, x1, x8, x9
 	scs_save x0, x8
 	scs_load x1, x8
 	ret

Since we set GCR_EL1 on exception entry and return, why is this needed?
We don't have a per-kernel thread GCR_EL1, it's global to all threads,
so I think cpu_switch_to() should not be touched.

I agree, we can remove it. We only require the kernel entry and the kernel exit
ones.

quoted

diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
index 7717ea9bc2a7..cfac7d02f032 100644
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c

@@ -18,10 +18,14 @@
 
 #include <asm/barrier.h>
 #include <asm/cpufeature.h>
+#include <asm/kasan.h>
+#include <asm/kprobes.h>
 #include <asm/mte.h>
 #include <asm/ptrace.h>
 #include <asm/sysreg.h>
 
+u64 gcr_kernel_excl __read_mostly;

Could we make this __ro_after_init?

Yes, it makes sense, it should be updated only once through mte_init_tags().

Something to consider though here is that this might not be the right approach
if in future we want to add stack tagging. In such a case we need to know the
kernel exclude mask before any C code is executed. Initializing the mask via
mte_init_tags() it is too late.

I was thinking to add a compilation define instead of having gcr_kernel_excl in
place. This might not work if the kernel excl mask is meant to change during the
execution.

Thoughts?

quoted

+
 static void mte_sync_page_tags(struct page *page, pte_t *ptep, bool check_swap)
 {
 	pte_t old_pte = READ_ONCE(*ptep);

@@ -115,6 +119,13 @@ void * __must_check mte_set_mem_tag_range(void *addr, size_t size, u8 tag)
 	return ptr;
 }
 
+void mte_init_tags(u64 max_tag)
+{
+	u64 incl = ((1ULL << ((max_tag & MTE_TAG_MAX) + 1)) - 1);

I'd rather use GENMASK here, it is more readable.

Agree, we can change it.

-- 
Regards,
Vincenzo

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