[PATCH v8 32/43] arm64: mte: Switch GCR_EL1 in kernel entry and exit

Andrey Konovalov andreyknvl at google.com
Wed Nov 4 18:18:47 EST 2020


From: Vincenzo Frascino <vincenzo.frascino at arm.com>

When MTE is present, the GCR_EL1 register contains the tags mask that
allows to exclude tags from the random generation via the IRG instruction.

With the introduction of the new Tag-Based KASAN API that provides a
mechanism to reserve tags for special reasons, the MTE implementation
has to make sure that the GCR_EL1 setting for the kernel does not affect
the userspace processes and viceversa.

Save and restore the kernel/user mask in GCR_EL1 in kernel entry and exit.

Signed-off-by: Vincenzo Frascino <vincenzo.frascino at arm.com>
Co-developed-by: Andrey Konovalov <andreyknvl at google.com>
Signed-off-by: Andrey Konovalov <andreyknvl at google.com>
---
Change-Id: I0081cba5ace27a9111bebb239075c9a466af4c84
---
 arch/arm64/include/asm/mte-def.h |  1 -
 arch/arm64/include/asm/mte.h     |  2 ++
 arch/arm64/kernel/asm-offsets.c  |  3 +++
 arch/arm64/kernel/entry.S        | 41 ++++++++++++++++++++++++++++++++
 arch/arm64/kernel/mte.c          | 28 +++++++++++++++++++---
 5 files changed, 71 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/include/asm/mte-def.h b/arch/arm64/include/asm/mte-def.h
index 8401ac5840c7..2d73a1612f09 100644
--- a/arch/arm64/include/asm/mte-def.h
+++ b/arch/arm64/include/asm/mte-def.h
@@ -10,6 +10,5 @@
 #define MTE_TAG_SHIFT		56
 #define MTE_TAG_SIZE		4
 #define MTE_TAG_MASK		GENMASK((MTE_TAG_SHIFT + (MTE_TAG_SIZE - 1)), MTE_TAG_SHIFT)
-#define MTE_TAG_MAX		(MTE_TAG_MASK >> MTE_TAG_SHIFT)
 
 #endif /* __ASM_MTE_DEF_H  */
diff --git a/arch/arm64/include/asm/mte.h b/arch/arm64/include/asm/mte.h
index cf1cd181dcb2..d02aff9f493d 100644
--- a/arch/arm64/include/asm/mte.h
+++ b/arch/arm64/include/asm/mte.h
@@ -18,6 +18,8 @@
 
 #include <asm/pgtable-types.h>
 
+extern u64 gcr_kernel_excl;
+
 void mte_clear_page_tags(void *addr);
 unsigned long mte_copy_tags_from_user(void *to, const void __user *from,
 				      unsigned long n);
diff --git a/arch/arm64/kernel/asm-offsets.c b/arch/arm64/kernel/asm-offsets.c
index 7d32fc959b1a..dfe6ed8446ac 100644
--- a/arch/arm64/kernel/asm-offsets.c
+++ b/arch/arm64/kernel/asm-offsets.c
@@ -47,6 +47,9 @@ int main(void)
 #ifdef CONFIG_ARM64_PTR_AUTH
   DEFINE(THREAD_KEYS_USER,	offsetof(struct task_struct, thread.keys_user));
   DEFINE(THREAD_KEYS_KERNEL,	offsetof(struct task_struct, thread.keys_kernel));
+#endif
+#ifdef CONFIG_ARM64_MTE
+  DEFINE(THREAD_GCR_EL1_USER,	offsetof(struct task_struct, thread.gcr_user_excl));
 #endif
   BLANK();
   DEFINE(S_X0,			offsetof(struct pt_regs, regs[0]));
diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index b295fb912b12..07646ef4f184 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -173,6 +173,43 @@ alternative_else_nop_endif
 #endif
 	.endm
 
+	.macro mte_set_gcr, tmp, tmp2
+#ifdef CONFIG_ARM64_MTE
+	/*
+	 * 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
+#endif
+	.endm
+
+	.macro mte_set_kernel_gcr, tmp, tmp2
+#ifdef CONFIG_KASAN_HW_TAGS
+alternative_if_not ARM64_MTE
+	b	1f
+alternative_else_nop_endif
+	ldr_l	\tmp, gcr_kernel_excl
+
+	mte_set_gcr \tmp, \tmp2
+1:
+#endif
+	.endm
+
+	.macro mte_set_user_gcr, tsk, tmp, tmp2
+#ifdef CONFIG_ARM64_MTE
+alternative_if_not ARM64_MTE
+	b	1f
+alternative_else_nop_endif
+	ldr	\tmp, [\tsk, #THREAD_GCR_EL1_USER]
+
+	mte_set_gcr \tmp, \tmp2
+1:
+#endif
+	.endm
+
 	.macro	kernel_entry, el, regsize = 64
 	.if	\regsize == 32
 	mov	w0, w0				// zero upper 32 bits of x0
@@ -212,6 +249,8 @@ alternative_else_nop_endif
 
 	ptrauth_keys_install_kernel tsk, x20, x22, x23
 
+	mte_set_kernel_gcr x22, x23
+
 	scs_load tsk, x20
 	.else
 	add	x21, sp, #S_FRAME_SIZE
@@ -330,6 +369,8 @@ 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_set_user_gcr tsk, x0, x1
+
 	apply_ssbd 0, x0, x1
 	.endif
 
diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
index 14b0c19a33e3..cc7e0f8707f7 100644
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c
@@ -23,6 +23,8 @@
 #include <asm/ptrace.h>
 #include <asm/sysreg.h>
 
+u64 gcr_kernel_excl __ro_after_init;
+
 static void mte_sync_page_tags(struct page *page, pte_t *ptep, bool check_swap)
 {
 	pte_t old_pte = READ_ONCE(*ptep);
@@ -123,6 +125,23 @@ void *mte_set_mem_tag_range(void *addr, size_t size, u8 tag)
 
 void __init mte_init_tags(u64 max_tag)
 {
+	static bool gcr_kernel_excl_initialized = false;
+
+	if (!gcr_kernel_excl_initialized) {
+		/*
+		 * The format of the tags in KASAN is 0xFF and in MTE is 0xF.
+		 * This conversion extracts an MTE tag from a KASAN tag.
+		 */
+		u64 incl = GENMASK(FIELD_GET(MTE_TAG_MASK >> MTE_TAG_SHIFT,
+					     max_tag), 0);
+
+		gcr_kernel_excl = ~incl & SYS_GCR_EL1_EXCL_MASK;
+		gcr_kernel_excl_initialized = true;
+	}
+
+	/* Enable the kernel exclude mask for random tags generation. */
+	write_sysreg_s(SYS_GCR_EL1_RRND | gcr_kernel_excl, SYS_GCR_EL1);
+
 	/* Enable MTE Sync Mode for EL1. */
 	sysreg_clear_set(sctlr_el1, SCTLR_ELx_TCF_MASK, SCTLR_ELx_TCF_SYNC);
 	isb();
@@ -163,7 +182,11 @@ static void update_gcr_el1_excl(u64 excl)
 static void set_gcr_el1_excl(u64 excl)
 {
 	current->thread.gcr_user_excl = excl;
-	update_gcr_el1_excl(excl);
+
+	/*
+	 * SYS_GCR_EL1 will be set to current->thread.gcr_user_excl value
+	 * by mte_set_user_gcr() in kernel_exit,
+	 */
 }
 
 void flush_mte_state(void)
@@ -189,7 +212,6 @@ void mte_thread_switch(struct task_struct *next)
 	/* avoid expensive SCTLR_EL1 accesses if no change */
 	if (current->thread.sctlr_tcf0 != next->thread.sctlr_tcf0)
 		update_sctlr_el1_tcf0(next->thread.sctlr_tcf0);
-	update_gcr_el1_excl(next->thread.gcr_user_excl);
 }
 
 void mte_suspend_exit(void)
@@ -197,7 +219,7 @@ void mte_suspend_exit(void)
 	if (!system_supports_mte())
 		return;
 
-	update_gcr_el1_excl(current->thread.gcr_user_excl);
+	update_gcr_el1_excl(gcr_kernel_excl);
 }
 
 long set_mte_ctrl(struct task_struct *task, unsigned long arg)
-- 
2.29.1.341.ge80a0c044ae-goog




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