arm64: start using 'asm goto' for get_user() when available

This generates noticeably better code with compilers that support it,
since we don't need to test the error register etc, the exception just
jumps to the error handling directly.

Note that this also marks SW_TTBR0_PAN incompatible with KCSAN support,
since KCSAN wants to save and restore the user access state.

KCSAN and SW_TTBR0_PAN were probably always incompatible, but it became
obvious only when implementing the unsafe user access functions.  At
that point the default empty user_access_save/restore() functions
weren't provided by the default fallback functions.

Signed-off-by: Linus Torvalds <[email protected]>

Author: torvalds

arch/arm64/Kconfig

@@ -1649,6 +1649,7 @@ config RODATA_FULL_DEFAULT_ENABLED
 
 config ARM64_SW_TTBR0_PAN
 	bool "Emulate Privileged Access Never using TTBR0_EL1 switching"
+	depends on !KCSAN
 	help
 	  Enabling this option prevents the kernel from accessing
 	  user-space memory directly by pointing TTBR0_EL1 to a reserved

arch/arm64/include/asm/uaccess.h

@@ -184,29 +184,40 @@ static inline void __user *__uaccess_mask_ptr(const void __user *ptr)
  * The "__xxx_error" versions set the third argument to -EFAULT if an error
  * occurs, and leave it unchanged on success.
  */
-#define __get_mem_asm(load, reg, x, addr, err, type)			\
+#ifdef CONFIG_CC_HAS_ASM_GOTO_OUTPUT
+#define __get_mem_asm(load, reg, x, addr, label, type)			\
+	asm_goto_output(						\
+	"1:	" load "	" reg "0, [%1]\n"			\
+	_ASM_EXTABLE_##type##ACCESS_ERR(1b, %l2, %w0)			\
+	: "=r" (x)							\
+	: "r" (addr) : : label)
+#else
+#define __get_mem_asm(load, reg, x, addr, label, type) do {		\
+	int __gma_err = 0;						\
 	asm volatile(							\
 	"1:	" load "	" reg "1, [%2]\n"			\
 	"2:\n"								\
 	_ASM_EXTABLE_##type##ACCESS_ERR_ZERO(1b, 2b, %w0, %w1)		\
-	: "+r" (err), "=r" (x)						\
-	: "r" (addr))
+	: "+r" (__gma_err), "=r" (x)					\
+	: "r" (addr));							\
+	if (__gma_err) goto label; } while (0)
+#endif
 
-#define __raw_get_mem(ldr, x, ptr, err, type)					\
+#define __raw_get_mem(ldr, x, ptr, label, type)					\
 do {										\
 	unsigned long __gu_val;							\
 	switch (sizeof(*(ptr))) {						\
 	case 1:									\
-		__get_mem_asm(ldr "b", "%w", __gu_val, (ptr), (err), type);	\
+		__get_mem_asm(ldr "b", "%w", __gu_val, (ptr), label, type);	\
 		break;								\
 	case 2:									\
-		__get_mem_asm(ldr "h", "%w", __gu_val, (ptr), (err), type);	\
+		__get_mem_asm(ldr "h", "%w", __gu_val, (ptr), label, type);	\
 		break;								\
 	case 4:									\
-		__get_mem_asm(ldr, "%w", __gu_val, (ptr), (err), type);		\
+		__get_mem_asm(ldr, "%w", __gu_val, (ptr), label, type);		\
 		break;								\
 	case 8:									\
-		__get_mem_asm(ldr, "%x",  __gu_val, (ptr), (err), type);	\
+		__get_mem_asm(ldr, "%x",  __gu_val, (ptr), label, type);	\
 		break;								\
 	default:								\
 		BUILD_BUG();							\
@@ -219,27 +230,34 @@ do {										\
  * uaccess_ttbr0_disable(). As `x` and `ptr` could contain blocking functions,
  * we must evaluate these outside of the critical section.
  */
-#define __raw_get_user(x, ptr, err)					\
+#define __raw_get_user(x, ptr, label)					\
 do {									\
 	__typeof__(*(ptr)) __user *__rgu_ptr = (ptr);			\
 	__typeof__(x) __rgu_val;					\
 	__chk_user_ptr(ptr);						\
-									\
-	uaccess_ttbr0_enable();						\
-	__raw_get_mem("ldtr", __rgu_val, __rgu_ptr, err, U);		\
-	uaccess_ttbr0_disable();					\
-									\
-	(x) = __rgu_val;						\
+	do {								\
+		__label__ __rgu_failed;					\
+		uaccess_ttbr0_enable();					\
+		__raw_get_mem("ldtr", __rgu_val, __rgu_ptr, __rgu_failed, U);	\
+		uaccess_ttbr0_disable();				\
+		(x) = __rgu_val;					\
+		break;							\
+	__rgu_failed:							\
+		uaccess_ttbr0_disable();				\
+		goto label;						\
+	} while (0);							\
 } while (0)
 
 #define __get_user_error(x, ptr, err)					\
 do {									\
+	__label__ __gu_failed;						\
 	__typeof__(*(ptr)) __user *__p = (ptr);				\
 	might_fault();							\
 	if (access_ok(__p, sizeof(*__p))) {				\
 		__p = uaccess_mask_ptr(__p);				\
-		__raw_get_user((x), __p, (err));			\
+		__raw_get_user((x), __p, __gu_failed);			\
 	} else {							\
+	__gu_failed:							\
 		(x) = (__force __typeof__(x))0; (err) = -EFAULT;	\
 	}								\
 } while (0)
@@ -262,15 +280,18 @@ do {									\
 do {									\
 	__typeof__(dst) __gkn_dst = (dst);				\
 	__typeof__(src) __gkn_src = (src);				\
-	int __gkn_err = 0;						\
-									\
-	__mte_enable_tco_async();					\
-	__raw_get_mem("ldr", *((type *)(__gkn_dst)),			\
-		      (__force type *)(__gkn_src), __gkn_err, K);	\
-	__mte_disable_tco_async();					\
+	do { 								\
+		__label__ __gkn_label;					\
 									\
-	if (unlikely(__gkn_err))					\
+		__mte_enable_tco_async();				\
+		__raw_get_mem("ldr", *((type *)(__gkn_dst)),		\
+		      (__force type *)(__gkn_src), __gkn_label, K);	\
+		__mte_disable_tco_async();				\
+		break;							\
+	__gkn_label:							\
+		__mte_disable_tco_async();				\
 		goto err_label;						\
+	} while (0);							\
 } while (0)
 
 #define __put_mem_asm(store, reg, x, addr, err, type)			\
@@ -381,6 +402,60 @@ extern unsigned long __must_check __arch_copy_to_user(void __user *to, const voi
 	__actu_ret;							\
 })
 
+static __must_check __always_inline bool user_access_begin(const void __user *ptr, size_t len)
+{
+	if (unlikely(!access_ok(ptr,len)))
+		return 0;
+	uaccess_ttbr0_enable();
+	return 1;
+}
+#define user_access_begin(a,b)	user_access_begin(a,b)
+#define user_access_end()	uaccess_ttbr0_disable()
+
+/*
+ * The arm64 inline asms should learn abut asm goto, and we should
+ * teach user_access_begin() about address masking.
+ */
+#define unsafe_put_user(x, ptr, label)	do {				\
+	int __upu_err = 0;						\
+	__raw_put_mem("sttr", x, uaccess_mask_ptr(ptr), __upu_err, U);	\
+	if (__upu_err) goto label;				\
+} while (0)
+
+#define unsafe_get_user(x, ptr, label) \
+	__raw_get_mem("ldtr", x, uaccess_mask_ptr(ptr), label, U)
+
+/*
+ * KCSAN uses these to save and restore ttbr state.
+ * We do not support KCSAN with ARM64_SW_TTBR0_PAN, so
+ * they are no-ops.
+ */
+static inline unsigned long user_access_save(void) { return 0; }
+static inline void user_access_restore(unsigned long enabled) { }
+
+/*
+ * We want the unsafe accessors to always be inlined and use
+ * the error labels - thus the macro games.
+ */
+#define unsafe_copy_loop(dst, src, len, type, label)				\
+	while (len >= sizeof(type)) {						\
+		unsafe_put_user(*(type *)(src),(type __user *)(dst),label);	\
+		dst += sizeof(type);						\
+		src += sizeof(type);						\
+		len -= sizeof(type);						\
+	}
+
+#define unsafe_copy_to_user(_dst,_src,_len,label)			\
+do {									\
+	char __user *__ucu_dst = (_dst);				\
+	const char *__ucu_src = (_src);					\
+	size_t __ucu_len = (_len);					\
+	unsafe_copy_loop(__ucu_dst, __ucu_src, __ucu_len, u64, label);	\
+	unsafe_copy_loop(__ucu_dst, __ucu_src, __ucu_len, u32, label);	\
+	unsafe_copy_loop(__ucu_dst, __ucu_src, __ucu_len, u16, label);	\
+	unsafe_copy_loop(__ucu_dst, __ucu_src, __ucu_len, u8, label);	\
+} while (0)
+
 #define INLINE_COPY_TO_USER
 #define INLINE_COPY_FROM_USER
 

arch/arm64/kernel/mte.c

@@ -582,25 +582,23 @@ subsys_initcall(register_mte_tcf_preferred_sysctl);
 size_t mte_probe_user_range(const char __user *uaddr, size_t size)
 {
 	const char __user *end = uaddr + size;
-	int err = 0;
 	char val;
 
-	__raw_get_user(val, uaddr, err);
-	if (err)
-		return size;
+	__raw_get_user(val, uaddr, efault);
 
 	uaddr = PTR_ALIGN(uaddr, MTE_GRANULE_SIZE);
 	while (uaddr < end) {
 		/*
 		 * A read is sufficient for mte, the caller should have probed
 		 * for the pte write permission if required.
 		 */
-		__raw_get_user(val, uaddr, err);
-		if (err)
-			return end - uaddr;
+		__raw_get_user(val, uaddr, efault);
 		uaddr += MTE_GRANULE_SIZE;
 	}
 	(void)val;
 
 	return 0;
+
+efault:
+	return end - uaddr;
 }