[patch V4 02/12] uaccess: Provide ASM GOTO safe wrappers for unsafe_*_user()

Thomas Gleixner
2025-10-22T12:49:07Z

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 <torvalds@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
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_get_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