Subject: [patch V6 02/12] uaccess: Provide ASM GOTO safe wrappers for unsafe_*_user()

From: Thomas Gleixner <[email-masked]>
Date: Wed, 29 Oct 2025 10:40:52 +0100

ASM GOTO is miscompiled by GCC when it is used inside a auto cleanup scope:

bool foo(u32 __user *p, u32 val)
{
	scoped_guard(pagefault)
		unsafe_put_user(val, p, efault);
	return true;
efault:
	return false;
}

 e80:	e8 00 00 00 00       	call   e85 <foo+0x5>
 e85:	65 48 8b 05 00 00 00 00 mov    %gs:0x0(%rip),%rax
 e8d:	83 80 04 14 00 00 01 	addl   $0x1,0x1404(%rax)   // pf_disable++
 e94:	89 37                	mov    %esi,(%rdi)
 e96:	83 a8 04 14 00 00 01 	subl   $0x1,0x1404(%rax)   // pf_disable--
 e9d:	b8 01 00 00 00       	mov    $0x1,%eax           // success
 ea2:	e9 00 00 00 00       	jmp    ea7 <foo+0x27>      // ret
 ea7:	31 c0                	xor    %eax,%eax           // fail
 ea9:	e9 00 00 00 00       	jmp    eae <foo+0x2e>      // ret

which is broken as it leaks the pagefault disable counter on failure.

Clang at least fails the build.

Linus suggested to add a local label into the macro scope and let that
jump to the actual caller supplied error label.

       	__label__ local_label;                                  \
        arch_unsafe_get_user(x, ptr, local_label);              \
	if (0) {                                                \
	local_label:                                            \
		goto label;                                     \

That works for both GCC and clang.

clang:

 c80:	0f 1f 44 00 00       	   nopl   0x0(%rax,%rax,1)	
 c85:	65 48 8b 0c 25 00 00 00 00 mov    %gs:0x0,%rcx
 c8e:	ff 81 04 14 00 00    	   incl   0x1404(%rcx)	   // pf_disable++
 c94:	31 c0                	   xor    %eax,%eax        // set retval to false
 c96:	89 37                      mov    %esi,(%rdi)      // write
 c98:	b0 01                	   mov    $0x1,%al         // set retval to true
 c9a:	ff 89 04 14 00 00    	   decl   0x1404(%rcx)     // pf_disable--
 ca0:	2e e9 00 00 00 00    	   cs jmp ca6 <foo+0x26>   // ret

The exception table entry points correctly to c9a

GCC:

 f70:   e8 00 00 00 00          call   f75 <baz+0x5>
 f75:   65 48 8b 05 00 00 00 00 mov    %gs:0x0(%rip),%rax
 f7d:   83 80 04 14 00 00 01    addl   $0x1,0x1404(%rax)  // pf_disable++
 f84:   8b 17                   mov    (%rdi),%edx
 f86:   89 16                   mov    %edx,(%rsi)
 f88:   83 a8 04 14 00 00 01    subl   $0x1,0x1404(%rax) // pf_disable--
 f8f:   b8 01 00 00 00          mov    $0x1,%eax         // success
 f94:   e9 00 00 00 00          jmp    f99 <baz+0x29>    // ret
 f99:   83 a8 04 14 00 00 01    subl   $0x1,0x1404(%rax) // pf_disable--
 fa0:   31 c0                   xor    %eax,%eax         // fail
 fa2:   e9 00 00 00 00          jmp    fa7 <baz+0x37>    // ret

The exception table entry points correctly to f99

So both compilers optimize out the extra goto and emit correct and
efficient code.

Provide a generic wrapper to do that to avoid modifying all the affected
architecture specific implementation with that workaround.

The only change required for architectures is to rename unsafe_*_user() to
arch_unsafe_*_user(). That's done in subsequent changes.

Suggested-by: Linus Torvalds <[email-masked]>
Signed-off-by: Thomas Gleixner <[email-masked]>
---
V5a: Use put in __put_kernel_nofault() - Yann
---
 include/linux/uaccess.h |   72 +++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 68 insertions(+), 4 deletions(-)

--- a/include/linux/uaccess.h
+++ b/include/linux/uaccess.h
@@ -518,7 +518,34 @@ long strncpy_from_user_nofault(char *dst
 		long count);
 long strnlen_user_nofault(const void __user *unsafe_addr, long count);
 
-#ifndef __get_kernel_nofault
+#ifdef arch_get_kernel_nofault
+/*
+ * Wrap the architecture implementation so that @label can be outside of a
+ * cleanup() scope. A regular C goto works correctly, but ASM goto does
+ * not. Clang rejects such an attempt, but GCC silently emits buggy code.
+ */
+#define __get_kernel_nofault(dst, src, type, label)		\
+do {								\
+	__label__ local_label;					\
+	arch_get_kernel_nofault(dst, src, type, local_label);	\
+	if (0) {						\
+	local_label:						\
+		goto label;					\
+	}							\
+} while (0)
+
+#define __put_kernel_nofault(dst, src, type, label)		\
+do {								\
+	__label__ local_label;					\
+	arch_put_kernel_nofault(dst, src, type, local_label);	\
+	if (0) {						\
+	local_label:						\
+		goto label;					\
+	}							\
+} while (0)
+
+#elif !defined(__get_kernel_nofault) /* arch_get_kernel_nofault */
+
 #define __get_kernel_nofault(dst, src, type, label)	\
 do {							\
 	type __user *p = (type __force __user *)(src);	\
@@ -535,7 +562,8 @@ do {							\
 	if (__put_user(data, p))			\
 		goto label;				\
 } while (0)
-#endif
+
+#endif  /* !__get_kernel_nofault */
 
 /**
  * get_kernel_nofault(): safely attempt to read from a location
@@ -549,7 +577,42 @@ do {							\
 	copy_from_kernel_nofault(&(val), __gk_ptr, sizeof(val));\
 })
 
-#ifndef user_access_begin
+#ifdef user_access_begin
+
+#ifdef arch_unsafe_get_user
+/*
+ * Wrap the architecture implementation so that @label can be outside of a
+ * cleanup() scope. A regular C goto works correctly, but ASM goto does
+ * not. Clang rejects such an attempt, but GCC silently emits buggy code.
+ *
+ * Some architectures use internal local labels already, but this extra
+ * indirection here is harmless because the compiler optimizes it out
+ * completely in any case. This construct just ensures that the ASM GOTO
+ * target is always in the local scope. The C goto 'label' works correct
+ * when leaving a cleanup() scope.
+ */
+#define unsafe_get_user(x, ptr, label)			\
+do {							\
+	__label__ local_label;				\
+	arch_unsafe_get_user(x, ptr, local_label);	\
+	if (0) {					\
+	local_label:					\
+		goto label;				\
+	}						\
+} while (0)
+
+#define unsafe_put_user(x, ptr, label)			\
+do {							\
+	__label__ local_label;				\
+	arch_unsafe_put_user(x, ptr, local_label);	\
+	if (0) {					\
+	local_label:					\
+		goto label;				\
+	}						\
+} while (0)
+#endif /* arch_unsafe_get_user */
+
+#else /* user_access_begin */
 #define user_access_begin(ptr,len) access_ok(ptr, len)
 #define user_access_end() do { } while (0)
 #define unsafe_op_wrap(op, err) do { if (unlikely(op)) goto err; } while (0)
@@ -559,7 +622,8 @@ do {							\
 #define unsafe_copy_from_user(d,s,l,e) unsafe_op_wrap(__copy_from_user(d,s,l),e)
 static inline unsigned long user_access_save(void) { return 0UL; }
 static inline void user_access_restore(unsigned long flags) { }
-#endif
+#endif /* !user_access_begin */
+
 #ifndef user_write_access_begin
 #define user_write_access_begin user_access_begin
 #define user_write_access_end user_access_end