uaccess: Provide scoped user access regions

KAGA-KOKO · ingomolnar · commit e497310b4ffb · 2025-11-04T08:27:52.000+01:00
User space access regions are tedious and require similar code patterns all over the place: if (!user_read_access_begin(from, sizeof(*from))) return -EFAULT; unsafe_get_user(val, from, Efault); user_read_access_end(); return 0; Efault: user_read_access_end(); return -EFAULT; This got worse with the recent addition of masked user access, which optimizes the speculation prevention: if (can_do_masked_user_access()) from = masked_user_read_access_begin((from)); else if (!user_read_access_begin(from, sizeof(*from))) return -EFAULT; unsafe_get_user(val, from, Efault); user_read_access_end(); return 0; Efault: user_read_access_end(); return -EFAULT; There have been issues with using the wrong user_*_access_end() variant in the error path and other typical Copy&Pasta problems, e.g. using the wrong fault label in the user accessor which ends up using the wrong accesss end variant. These patterns beg for scopes with automatic cleanup. The resulting outcome is: scoped_user_read_access(from, Efault) unsafe_get_user(val, from, Efault); return 0; Efault: return -EFAULT; The scope guarantees the proper cleanup for the access mode is invoked both in the success and the failure (fault) path. The scoped_user_$MODE_access() macros are implemented as self terminating nested for() loops. Thanks to Andrew Cooper for pointing me at them. The scope can therefore be left with 'break', 'goto' and 'return'. Even 'continue' "works" due to the self termination mechanism. Both GCC and clang optimize all the convoluted macro maze out and the above results with clang in: b80: f3 0f 1e fa endbr64 b84: 48 b8 ef cd ab 89 67 45 23 01 movabs $0x123456789abcdef,%rax b8e: 48 39 c7 cmp %rax,%rdi b91: 48 0f 47 f8 cmova %rax,%rdi b95: 90 nop b96: 90 nop b97: 90 nop b98: 31 c9 xor %ecx,%ecx b9a: 8b 07 mov (%rdi),%eax b9c: 89 06 mov %eax,(%rsi) b9e: 85 c9 test %ecx,%ecx ba0: 0f 94 c0 sete %al ba3: 90 nop ba4: 90 nop ba5: 90 nop ba6: c3 ret Which looks as compact as it gets. The NOPs are placeholder for STAC/CLAC. GCC emits the fault path seperately: bf0: f3 0f 1e fa endbr64 bf4: 48 b8 ef cd ab 89 67 45 23 01 movabs $0x123456789abcdef,%rax bfe: 48 39 c7 cmp %rax,%rdi c01: 48 0f 47 f8 cmova %rax,%rdi c05: 90 nop c06: 90 nop c07: 90 nop c08: 31 d2 xor %edx,%edx c0a: 8b 07 mov (%rdi),%eax c0c: 89 06 mov %eax,(%rsi) c0e: 85 d2 test %edx,%edx c10: 75 09 jne c1b <afoo+0x2b> c12: 90 nop c13: 90 nop c14: 90 nop c15: b8 01 00 00 00 mov $0x1,%eax c1a: c3 ret c1b: 90 nop c1c: 90 nop c1d: 90 nop c1e: 31 c0 xor %eax,%eax c20: c3 ret The fault labels for the scoped*() macros and the fault labels for the actual user space accessors can be shared and must be placed outside of the scope. If masked user access is enabled on an architecture, then the pointer handed in to scoped_user_$MODE_access() can be modified to point to a guaranteed faulting user address. This modification is only scope local as the pointer is aliased inside the scope. When the scope is left the alias is not longer in effect. IOW the original pointer value is preserved so it can be used e.g. for fixup or diagnostic purposes in the fault path. Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Signed-off-by: Ingo Molnar <mingo@kernel.org> Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> Link: https://patch.msgid.link/20251027083745.546420421@linutronix.de
diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
@@ -2,6 +2,7 @@
 #ifndef __LINUX_UACCESS_H__
 #define __LINUX_UACCESS_H__
 
+#include <linux/cleanup.h>
 #include <linux/fault-inject-usercopy.h>
 #include <linux/instrumented.h>
 #include <linux/minmax.h>
@@ -35,9 +36,17 @@
 
 #ifdef masked_user_access_begin
  #define can_do_masked_user_access() 1
+# ifndef masked_user_write_access_begin
+#  define masked_user_write_access_begin masked_user_access_begin
+# endif
+# ifndef masked_user_read_access_begin
+#  define masked_user_read_access_begin masked_user_access_begin
+#endif
 #else
  #define can_do_masked_user_access() 0
  #define masked_user_access_begin(src) NULL
+ #define masked_user_read_access_begin(src) NULL
+ #define masked_user_write_access_begin(src) NULL
  #define mask_user_address(src) (src)
 #endif
 
@@ -633,6 +642,189 @@ static inline void user_access_restore(unsigned long flags) { }
 #define user_read_access_end user_access_end
 #endif
 
+/* Define RW variant so the below _mode macro expansion works */
+#define masked_user_rw_access_begin(u)	masked_user_access_begin(u)
+#define user_rw_access_begin(u, s)	user_access_begin(u, s)
+#define user_rw_access_end()		user_access_end()
+
+/* Scoped user access */
+#define USER_ACCESS_GUARD(_mode)				\
+static __always_inline void __user *				\
+class_user_##_mode##_begin(void __user *ptr)			\
+{								\
+	return ptr;						\
+}								\
+								\
+static __always_inline void					\
+class_user_##_mode##_end(void __user *ptr)			\
+{								\
+	user_##_mode##_access_end();				\
+}								\
+								\
+DEFINE_CLASS(user_ ##_mode## _access, void __user *,		\
+	     class_user_##_mode##_end(_T),			\
+	     class_user_##_mode##_begin(ptr), void __user *ptr)	\
+								\
+static __always_inline class_user_##_mode##_access_t		\
+class_user_##_mode##_access_ptr(void __user *scope)		\
+{								\
+	return scope;						\
+}
+
+USER_ACCESS_GUARD(read)
+USER_ACCESS_GUARD(write)
+USER_ACCESS_GUARD(rw)
+#undef USER_ACCESS_GUARD
+
+/**
+ * __scoped_user_access_begin - Start a scoped user access
+ * @mode:	The mode of the access class (read, write, rw)
+ * @uptr:	The pointer to access user space memory
+ * @size:	Size of the access
+ * @elbl:	Error label to goto when the access region is rejected
+ *
+ * Internal helper for __scoped_user_access(). Don't use directly.
+ */
+#define __scoped_user_access_begin(mode, uptr, size, elbl)		\
+({									\
+	typeof(uptr) __retptr;						\
+									\
+	if (can_do_masked_user_access()) {				\
+		__retptr = masked_user_##mode##_access_begin(uptr);	\
+	} else {							\
+		__retptr = uptr;					\
+		if (!user_##mode##_access_begin(uptr, size))		\
+			goto elbl;					\
+	}								\
+	__retptr;							\
+})
+
+/**
+ * __scoped_user_access - Open a scope for user access
+ * @mode:	The mode of the access class (read, write, rw)
+ * @uptr:	The pointer to access user space memory
+ * @size:	Size of the access
+ * @elbl:	Error label to goto when the access region is rejected. It
+ *		must be placed outside the scope
+ *
+ * If the user access function inside the scope requires a fault label, it
+ * can use @elbl or a different label outside the scope, which requires
+ * that user access which is implemented with ASM GOTO has been properly
+ * wrapped. See unsafe_get_user() for reference.
+ *
+ *	scoped_user_rw_access(ptr, efault) {
+ *		unsafe_get_user(rval, &ptr->rval, efault);
+ *		unsafe_put_user(wval, &ptr->wval, efault);
+ *	}
+ *	return 0;
+ *  efault:
+ *	return -EFAULT;
+ *
+ * The scope is internally implemented as a autoterminating nested for()
+ * loop, which can be left with 'return', 'break' and 'goto' at any
+ * point.
+ *
+ * When the scope is left user_##@_mode##_access_end() is automatically
+ * invoked.
+ *
+ * When the architecture supports masked user access and the access region
+ * which is determined by @uptr and @size is not a valid user space
+ * address, i.e. < TASK_SIZE, the scope sets the pointer to a faulting user
+ * space address and does not terminate early. This optimizes for the good
+ * case and lets the performance uncritical bad case go through the fault.
+ *
+ * The eventual modification of the pointer is limited to the scope.
+ * Outside of the scope the original pointer value is unmodified, so that
+ * the original pointer value is available for diagnostic purposes in an
+ * out of scope fault path.
+ *
+ * Nesting scoped user access into a user access scope is invalid and fails
+ * the build. Nesting into other guards, e.g. pagefault is safe.
+ *
+ * The masked variant does not check the size of the access and relies on a
+ * mapping hole (e.g. guard page) to catch an out of range pointer, the
+ * first access to user memory inside the scope has to be within
+ * @uptr ... @uptr + PAGE_SIZE - 1
+ *
+ * Don't use directly. Use scoped_masked_user_$MODE_access() instead.
+ */
+#define __scoped_user_access(mode, uptr, size, elbl)					\
+for (bool done = false; !done; done = true)						\
+	for (void __user *_tmpptr = __scoped_user_access_begin(mode, uptr, size, elbl); \
+	     !done; done = true)							\
+		for (CLASS(user_##mode##_access, scope)(_tmpptr); !done; done = true)	\
+			/* Force modified pointer usage within the scope */		\
+			for (const typeof(uptr) uptr = _tmpptr; !done; done = true)
+
+/**
+ * scoped_user_read_access_size - Start a scoped user read access with given size
+ * @usrc:	Pointer to the user space address to read from
+ * @size:	Size of the access starting from @usrc
+ * @elbl:	Error label to goto when the access region is rejected
+ *
+ * For further information see __scoped_user_access() above.
+ */
+#define scoped_user_read_access_size(usrc, size, elbl)		\
+	__scoped_user_access(read, usrc, size, elbl)
+
+/**
+ * scoped_user_read_access - Start a scoped user read access
+ * @usrc:	Pointer to the user space address to read from
+ * @elbl:	Error label to goto when the access region is rejected
+ *
+ * The size of the access starting from @usrc is determined via sizeof(*@usrc)).
+ *
+ * For further information see __scoped_user_access() above.
+ */
+#define scoped_user_read_access(usrc, elbl)				\
+	scoped_user_read_access_size(usrc, sizeof(*(usrc)), elbl)
+
+/**
+ * scoped_user_write_access_size - Start a scoped user write access with given size
+ * @udst:	Pointer to the user space address to write to
+ * @size:	Size of the access starting from @udst
+ * @elbl:	Error label to goto when the access region is rejected
+ *
+ * For further information see __scoped_user_access() above.
+ */
+#define scoped_user_write_access_size(udst, size, elbl)			\
+	__scoped_user_access(write, udst, size, elbl)
+
+/**
+ * scoped_user_write_access - Start a scoped user write access
+ * @udst:	Pointer to the user space address to write to
+ * @elbl:	Error label to goto when the access region is rejected
+ *
+ * The size of the access starting from @udst is determined via sizeof(*@udst)).
+ *
+ * For further information see __scoped_user_access() above.
+ */
+#define scoped_user_write_access(udst, elbl)				\
+	scoped_user_write_access_size(udst, sizeof(*(udst)), elbl)
+
+/**
+ * scoped_user_rw_access_size - Start a scoped user read/write access with given size
+ * @uptr	Pointer to the user space address to read from and write to
+ * @size:	Size of the access starting from @uptr
+ * @elbl:	Error label to goto when the access region is rejected
+ *
+ * For further information see __scoped_user_access() above.
+ */
+#define scoped_user_rw_access_size(uptr, size, elbl)			\
+	__scoped_user_access(rw, uptr, size, elbl)
+
+/**
+ * scoped_user_rw_access - Start a scoped user read/write access
+ * @uptr	Pointer to the user space address to read from and write to
+ * @elbl:	Error label to goto when the access region is rejected
+ *
+ * The size of the access starting from @uptr is determined via sizeof(*@uptr)).
+ *
+ * For further information see __scoped_user_access() above.
+ */
+#define scoped_user_rw_access(uptr, elbl)				\
+	scoped_user_rw_access_size(uptr, sizeof(*(uptr)), elbl)
+
 #ifdef CONFIG_HARDENED_USERCOPY
 void __noreturn usercopy_abort(const char *name, const char *detail,
 			       bool to_user, unsigned long offset,
