BACKPORT: arm64: Introduce uaccess_{disable,enable} functionality based on TTBR0_EL1

This patch adds the uaccess macros/functions to disable access to user
space by setting TTBR0_EL1 to a reserved zeroed page. Since the value
written to TTBR0_EL1 must be a physical address, for simplicity this
patch introduces a reserved_ttbr0 page at a constant offset from
swapper_pg_dir. The uaccess_disable code uses the ttbr1_el1 value
adjusted by the reserved_ttbr0 offset.

Enabling access to user is done by restoring TTBR0_EL1 with the value
from the struct thread_info ttbr0 variable. Interrupts must be disabled
during the uaccess_ttbr0_enable code to ensure the atomicity of the
thread_info.ttbr0 read and TTBR0_EL1 write. This patch also moves the
get_thread_info asm macro from entry.S to assembler.h for reuse in the
uaccess_ttbr0_* macros.

Cc: Will Deacon <will.deacon@arm.com>
Cc: James Morse <james.morse@arm.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Bug: 31432001
Change-Id: I54ada623160cb47f5762e0e39a5e84a75252dbfd
(cherry picked from commit 4b65a5db362783ab4b04ca1c1d2ad70ed9b0ba2a)
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>

arch/arm64/include/asm/assembler.h

@@ -53,6 +53,15 @@
 	msr	daifclr, #2
 	.endm
 
+	.macro	save_and_disable_irq, flags
+	mrs	\flags, daif
+	msr	daifset, #2
+	.endm
+
+	.macro	restore_irq, flags
+	msr	daif, \flags
+	.endm
+
 /*
  * Enable and disable debug exceptions.
  */
@@ -371,6 +380,13 @@ alternative_endif
 	movk	\reg, :abs_g0_nc:\val
 	.endm
 
+/*
+ * Return the current thread_info.
+ */
+	.macro	get_thread_info, rd
+	mrs	\rd, sp_el0
+	.endm
+
 /*
  * Errata workaround post TTBR0_EL1 update.
  */

arch/arm64/include/asm/cpufeature.h

@@ -189,6 +189,12 @@ static inline bool system_supports_mixed_endian_el0(void)
 	return id_aa64mmfr0_mixed_endian_el0(read_system_reg(SYS_ID_AA64MMFR0_EL1));
 }
 
+static inline bool system_uses_ttbr0_pan(void)
+{
+	return IS_ENABLED(CONFIG_ARM64_SW_TTBR0_PAN) &&
+		!cpus_have_cap(ARM64_HAS_PAN);
+}
+
 #endif /* __ASSEMBLY__ */
 
 #endif

arch/arm64/include/asm/kernel-pgtable.h

@@ -19,6 +19,7 @@
 #ifndef __ASM_KERNEL_PGTABLE_H
 #define __ASM_KERNEL_PGTABLE_H
 
+#include <asm/pgtable.h>
 #include <asm/sparsemem.h>
 
 /*
@@ -54,6 +55,12 @@
 #define SWAPPER_DIR_SIZE	(SWAPPER_PGTABLE_LEVELS * PAGE_SIZE)
 #define IDMAP_DIR_SIZE		(IDMAP_PGTABLE_LEVELS * PAGE_SIZE)
 
+#ifdef CONFIG_ARM64_SW_TTBR0_PAN
+#define RESERVED_TTBR0_SIZE	(PAGE_SIZE)
+#else
+#define RESERVED_TTBR0_SIZE	(0)
+#endif
+
 /* Initial memory map size */
 #if ARM64_SWAPPER_USES_SECTION_MAPS
 #define SWAPPER_BLOCK_SHIFT	SECTION_SHIFT

arch/arm64/include/asm/thread_info.h

@@ -48,6 +48,9 @@ struct thread_info {
 	unsigned long		flags;		/* low level flags */
 	mm_segment_t		addr_limit;	/* address limit */
 	struct task_struct	*task;		/* main task structure */
+#ifdef CONFIG_ARM64_SW_TTBR0_PAN
+	u64			ttbr0;		/* saved TTBR0_EL1 */
+#endif
 	int			preempt_count;	/* 0 => preemptable, <0 => bug */
 	int			cpu;		/* cpu */
 };

arch/arm64/include/asm/uaccess.h

@@ -19,6 +19,7 @@
 #define __ASM_UACCESS_H
 
 #include <asm/alternative.h>
+#include <asm/kernel-pgtable.h>
 #include <asm/sysreg.h>
 
 #ifndef __ASSEMBLY__
@@ -129,16 +130,71 @@ static inline void set_fs(mm_segment_t fs)
 /*
  * User access enabling/disabling.
  */
+#ifdef CONFIG_ARM64_SW_TTBR0_PAN
+static inline void __uaccess_ttbr0_disable(void)
+{
+	unsigned long ttbr;
+
+	/* reserved_ttbr0 placed at the end of swapper_pg_dir */
+	ttbr = read_sysreg(ttbr1_el1) + SWAPPER_DIR_SIZE;
+	write_sysreg(ttbr, ttbr0_el1);
+	isb();
+}
+
+static inline void __uaccess_ttbr0_enable(void)
+{
+	unsigned long flags;
+
+	/*
+	 * Disable interrupts to avoid preemption between reading the 'ttbr0'
+	 * variable and the MSR. A context switch could trigger an ASID
+	 * roll-over and an update of 'ttbr0'.
+	 */
+	local_irq_save(flags);
+	write_sysreg(current_thread_info()->ttbr0, ttbr0_el1);
+	isb();
+	local_irq_restore(flags);
+}
+
+static inline bool uaccess_ttbr0_disable(void)
+{
+	if (!system_uses_ttbr0_pan())
+		return false;
+	__uaccess_ttbr0_disable();
+	return true;
+}
+
+static inline bool uaccess_ttbr0_enable(void)
+{
+	if (!system_uses_ttbr0_pan())
+		return false;
+	__uaccess_ttbr0_enable();
+	return true;
+}
+#else
+static inline bool uaccess_ttbr0_disable(void)
+{
+	return false;
+}
+
+static inline bool uaccess_ttbr0_enable(void)
+{
+	return false;
+}
+#endif
+
 #define __uaccess_disable(alt)						\
 do {									\
-	asm(ALTERNATIVE("nop", SET_PSTATE_PAN(1), alt,			\
-			CONFIG_ARM64_PAN));				\
+	if (!uaccess_ttbr0_disable())					\
+		asm(ALTERNATIVE("nop", SET_PSTATE_PAN(1), alt,		\
+				CONFIG_ARM64_PAN));			\
 } while (0)
 
 #define __uaccess_enable(alt)						\
 do {									\
-	asm(ALTERNATIVE("nop", SET_PSTATE_PAN(0), alt,			\
-			CONFIG_ARM64_PAN));				\
+	if (uaccess_ttbr0_enable())					\
+		asm(ALTERNATIVE("nop", SET_PSTATE_PAN(0), alt,		\
+				CONFIG_ARM64_PAN));			\
 } while (0)
 
 static inline void uaccess_disable(void)
@@ -369,16 +425,56 @@ extern __must_check long strnlen_user(const char __user *str, long n);
 #include <asm/assembler.h>
 
 /*
- * User access enabling/disabling macros. These are no-ops when UAO is
- * present.
+ * User access enabling/disabling macros.
+ */
+#ifdef CONFIG_ARM64_SW_TTBR0_PAN
+	.macro	__uaccess_ttbr0_disable, tmp1
+	mrs	\tmp1, ttbr1_el1		// swapper_pg_dir
+	add	\tmp1, \tmp1, #SWAPPER_DIR_SIZE	// reserved_ttbr0 at the end of swapper_pg_dir
+	msr	ttbr0_el1, \tmp1		// set reserved TTBR0_EL1
+	isb
+	.endm
+
+	.macro	__uaccess_ttbr0_enable, tmp1
+	get_thread_info \tmp1
+	ldr	\tmp1, [\tmp1, #TSK_TI_TTBR0]	// load saved TTBR0_EL1
+	msr	ttbr0_el1, \tmp1		// set the non-PAN TTBR0_EL1
+	isb
+	.endm
+
+	.macro	uaccess_ttbr0_disable, tmp1
+alternative_if_not ARM64_HAS_PAN
+	__uaccess_ttbr0_disable \tmp1
+alternative_else_nop_endif
+	.endm
+
+	.macro	uaccess_ttbr0_enable, tmp1, tmp2
+alternative_if_not ARM64_HAS_PAN
+	save_and_disable_irq \tmp2		// avoid preemption
+	__uaccess_ttbr0_enable \tmp1
+	restore_irq \tmp2
+alternative_else_nop_endif
+	.endm
+#else
+	.macro	uaccess_ttbr0_disable, tmp1
+	.endm
+
+	.macro	uaccess_ttbr0_enable, tmp1, tmp2
+	.endm
+#endif
+
+/*
+ * These macros are no-ops when UAO is present.
  */
 	.macro	uaccess_disable_not_uao, tmp1
+	uaccess_ttbr0_disable \tmp1
 alternative_if ARM64_ALT_PAN_NOT_UAO
 	SET_PSTATE_PAN(1)
 alternative_else_nop_endif
 	.endm
 
 	.macro	uaccess_enable_not_uao, tmp1, tmp2
+	uaccess_ttbr0_enable \tmp1, \tmp2
 alternative_if ARM64_ALT_PAN_NOT_UAO
 	SET_PSTATE_PAN(0)
 alternative_else_nop_endif

arch/arm64/kernel/asm-offsets.c

@@ -40,6 +40,9 @@ int main(void)
   DEFINE(TI_ADDR_LIMIT,		offsetof(struct thread_info, addr_limit));
   DEFINE(TI_TASK,		offsetof(struct thread_info, task));
   DEFINE(TI_CPU,		offsetof(struct thread_info, cpu));
+#ifdef CONFIG_ARM64_SW_TTBR0_PAN
+  DEFINE(TSK_TI_TTBR0,		offsetof(struct thread_info, ttbr0));
+#endif
   BLANK();
   DEFINE(THREAD_CPU_CONTEXT,	offsetof(struct task_struct, thread.cpu_context));
   BLANK();

arch/arm64/kernel/cpufeature.c

@@ -46,6 +46,7 @@ unsigned int compat_elf_hwcap2 __read_mostly;
 #endif
 
 DECLARE_BITMAP(cpu_hwcaps, ARM64_NCAPS);
+EXPORT_SYMBOL(cpu_hwcaps);
 
 #define __ARM64_FTR_BITS(SIGNED, STRICT, TYPE, SHIFT, WIDTH, SAFE_VAL) \
 	{						\

arch/arm64/kernel/entry.S

@@ -187,10 +187,6 @@ alternative_endif
 	eret					// return to kernel
 	.endm
 
-	.macro	get_thread_info, rd
-	mrs	\rd, sp_el0
-	.endm
-
 	.macro	irq_stack_entry
 	mov	x19, sp			// preserve the original sp
 

arch/arm64/kernel/head.S

@@ -318,14 +318,14 @@ __create_page_tables:
 	 * dirty cache lines being evicted.
 	 */
 	mov	x0, x25
-	add	x1, x26, #SWAPPER_DIR_SIZE
+	add	x1, x26, #SWAPPER_DIR_SIZE + RESERVED_TTBR0_SIZE
 	bl	__inval_cache_range
 
 	/*
 	 * Clear the idmap and swapper page tables.
 	 */
 	mov	x0, x25
-	add	x6, x26, #SWAPPER_DIR_SIZE
+	add	x6, x26, #SWAPPER_DIR_SIZE + RESERVED_TTBR0_SIZE
 1:	stp	xzr, xzr, [x0], #16
 	stp	xzr, xzr, [x0], #16
 	stp	xzr, xzr, [x0], #16
@@ -404,7 +404,7 @@ __create_page_tables:
 	 * tables again to remove any speculatively loaded cache lines.
 	 */
 	mov	x0, x25
-	add	x1, x26, #SWAPPER_DIR_SIZE
+	add	x1, x26, #SWAPPER_DIR_SIZE + RESERVED_TTBR0_SIZE
 	dmb	sy
 	bl	__inval_cache_range
 

arch/arm64/kernel/vmlinux.lds.S

@@ -194,6 +194,11 @@ SECTIONS
 	swapper_pg_dir = .;
 	. += SWAPPER_DIR_SIZE;
 
+#ifdef CONFIG_ARM64_SW_TTBR0_PAN
+	reserved_ttbr0 = .;
+	. += RESERVED_TTBR0_SIZE;
+#endif
+
 	_end = .;
 
 	STABS_DEBUG