From 34bfeea4a9e9cdae713637541f240c3adfdfede3 Mon Sep 17 00:00:00 2001
From: Catalin Marinas <catalin.marinas@arm.com>
Date: Mon, 4 May 2020 14:42:36 +0100
Subject: [PATCH] arm64: mte: Clear the tags when a page is mapped in
 user-space with PROT_MTE

Pages allocated by the kernel are not guaranteed to have the tags
zeroed, especially as the kernel does not (yet) use MTE itself. To
ensure the user can still access such pages when mapped into its address
space, clear the tags via set_pte_at(). A new page flag - PG_mte_tagged
(PG_arch_2) - is used to track pages with valid allocation tags.

Since the zero page is mapped as pte_special(), it won't be covered by
the above set_pte_at() mechanism. Clear its tags during early MTE
initialisation.

Co-developed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
---
 arch/arm64/include/asm/mte.h     | 16 +++++++++++++++
 arch/arm64/include/asm/pgtable.h |  7 +++++++
 arch/arm64/kernel/cpufeature.c   | 18 +++++++++++++++++
 arch/arm64/kernel/mte.c          | 14 +++++++++++++
 arch/arm64/lib/Makefile          |  2 ++
 arch/arm64/lib/mte.S             | 34 ++++++++++++++++++++++++++++++++
 6 files changed, 91 insertions(+)
 create mode 100644 arch/arm64/lib/mte.S

diff --git a/arch/arm64/include/asm/mte.h b/arch/arm64/include/asm/mte.h
index a0bf310da74b42..1716b3d0248907 100644
--- a/arch/arm64/include/asm/mte.h
+++ b/arch/arm64/include/asm/mte.h
@@ -7,12 +7,28 @@
 
 #ifndef __ASSEMBLY__
 
+#include <linux/page-flags.h>
+
+#include <asm/pgtable-types.h>
+
+void mte_clear_page_tags(void *addr);
+
 #ifdef CONFIG_ARM64_MTE
 
+/* track which pages have valid allocation tags */
+#define PG_mte_tagged	PG_arch_2
+
+void mte_sync_tags(pte_t *ptep, pte_t pte);
 void flush_mte_state(void);
 
 #else
 
+/* unused if !CONFIG_ARM64_MTE, silence the compiler */
+#define PG_mte_tagged	0
+
+static inline void mte_sync_tags(pte_t *ptep, pte_t pte)
+{
+}
 static inline void flush_mte_state(void)
 {
 }
diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index d5d3fbe739534f..0a205a8e91b2e2 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -9,6 +9,7 @@
 #include <asm/proc-fns.h>
 
 #include <asm/memory.h>
+#include <asm/mte.h>
 #include <asm/pgtable-hwdef.h>
 #include <asm/pgtable-prot.h>
 #include <asm/tlbflush.h>
@@ -90,6 +91,8 @@ extern unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)];
 #define pte_user_exec(pte)	(!(pte_val(pte) & PTE_UXN))
 #define pte_cont(pte)		(!!(pte_val(pte) & PTE_CONT))
 #define pte_devmap(pte)		(!!(pte_val(pte) & PTE_DEVMAP))
+#define pte_tagged(pte)		((pte_val(pte) & PTE_ATTRINDX_MASK) == \
+				 PTE_ATTRINDX(MT_NORMAL_TAGGED))
 
 #define pte_cont_addr_end(addr, end)						\
 ({	unsigned long __boundary = ((addr) + CONT_PTE_SIZE) & CONT_PTE_MASK;	\
@@ -284,6 +287,10 @@ static inline void set_pte_at(struct mm_struct *mm, unsigned long addr,
 	if (pte_present(pte) && pte_user_exec(pte) && !pte_special(pte))
 		__sync_icache_dcache(pte);
 
+	if (system_supports_mte() &&
+	    pte_present(pte) && pte_tagged(pte) && !pte_special(pte))
+		mte_sync_tags(ptep, pte);
+
 	__check_racy_pte_update(mm, ptep, pte);
 
 	set_pte(ptep, pte);
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index fabc8a23722360..add9da5d8ea340 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -75,6 +75,7 @@
 #include <asm/cpu_ops.h>
 #include <asm/fpsimd.h>
 #include <asm/mmu_context.h>
+#include <asm/mte.h>
 #include <asm/processor.h>
 #include <asm/sysreg.h>
 #include <asm/traps.h>
@@ -1704,6 +1705,22 @@ static void bti_enable(const struct arm64_cpu_capabilities *__unused)
 }
 #endif /* CONFIG_ARM64_BTI */
 
+#ifdef CONFIG_ARM64_MTE
+static void cpu_enable_mte(struct arm64_cpu_capabilities const *cap)
+{
+	static bool cleared_zero_page = false;
+
+	/*
+	 * Clear the tags in the zero page. This needs to be done via the
+	 * linear map which has the Tagged attribute.
+	 */
+	if (!cleared_zero_page) {
+		cleared_zero_page = true;
+		mte_clear_page_tags(lm_alias(empty_zero_page));
+	}
+}
+#endif /* CONFIG_ARM64_MTE */
+
 /* Internal helper functions to match cpu capability type */
 static bool
 cpucap_late_cpu_optional(const struct arm64_cpu_capabilities *cap)
@@ -2133,6 +2150,7 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
 		.field_pos = ID_AA64PFR1_MTE_SHIFT,
 		.min_field_value = ID_AA64PFR1_MTE,
 		.sign = FTR_UNSIGNED,
+		.cpu_enable = cpu_enable_mte,
 	},
 #endif /* CONFIG_ARM64_MTE */
 	{},
diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
index 032016823957ba..5bf9bbed5a252e 100644
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c
@@ -3,12 +3,26 @@
  * Copyright (C) 2020 ARM Ltd.
  */
 
+#include <linux/bitops.h>
+#include <linux/mm.h>
 #include <linux/thread_info.h>
 
 #include <asm/cpufeature.h>
 #include <asm/mte.h>
 #include <asm/sysreg.h>
 
+void mte_sync_tags(pte_t *ptep, pte_t pte)
+{
+	struct page *page = pte_page(pte);
+	long i, nr_pages = compound_nr(page);
+
+	/* if PG_mte_tagged is set, tags have already been initialised */
+	for (i = 0; i < nr_pages; i++, page++) {
+		if (!test_and_set_bit(PG_mte_tagged, &page->flags))
+			mte_clear_page_tags(page_address(page));
+	}
+}
+
 void flush_mte_state(void)
 {
 	if (!system_supports_mte())
diff --git a/arch/arm64/lib/Makefile b/arch/arm64/lib/Makefile
index 2fc253466dbf8a..d31e1169d9b8e9 100644
--- a/arch/arm64/lib/Makefile
+++ b/arch/arm64/lib/Makefile
@@ -16,3 +16,5 @@ lib-$(CONFIG_ARCH_HAS_UACCESS_FLUSHCACHE) += uaccess_flushcache.o
 obj-$(CONFIG_CRC32) += crc32.o
 
 obj-$(CONFIG_FUNCTION_ERROR_INJECTION) += error-inject.o
+
+obj-$(CONFIG_ARM64_MTE) += mte.o
diff --git a/arch/arm64/lib/mte.S b/arch/arm64/lib/mte.S
new file mode 100644
index 00000000000000..a367056400862a
--- /dev/null
+++ b/arch/arm64/lib/mte.S
@@ -0,0 +1,34 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Copyright (C) 2020 ARM Ltd.
+ */
+#include <linux/linkage.h>
+
+#include <asm/assembler.h>
+#include <asm/sysreg.h>
+
+	.arch	armv8.5-a+memtag
+
+/*
+ * multitag_transfer_size - set \reg to the block size that is accessed by the
+ * LDGM/STGM instructions.
+ */
+	.macro	multitag_transfer_size, reg, tmp
+	mrs_s	\reg, SYS_GMID_EL1
+	ubfx	\reg, \reg, #SYS_GMID_EL1_BS_SHIFT, #SYS_GMID_EL1_BS_SIZE
+	mov	\tmp, #4
+	lsl	\reg, \tmp, \reg
+	.endm
+
+/*
+ * Clear the tags in a page
+ *   x0 - address of the page to be cleared
+ */
+SYM_FUNC_START(mte_clear_page_tags)
+	multitag_transfer_size x1, x2
+1:	stgm	xzr, [x0]
+	add	x0, x0, x1
+	tst	x0, #(PAGE_SIZE - 1)
+	b.ne	1b
+	ret
+SYM_FUNC_END(mte_clear_page_tags)
-- 
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