[PATCH v2 2/6] x86/uaccess: Avoid barrier_nospec() in __get_user()

[Josh Poimboeuf]

On 64-bit, the barrier_nospec() in __get_user() is overkill and
painfully slow.  Instead, use pointer masking to force the user pointer
to a non-kernel value in speculative paths.

Doing so makes get_user() and __get_user() identical in behavior, so
converge their implementations.

Signed-off-by: Josh Poimboeuf <jpoimboe at kernel.org>
---
 arch/x86/lib/getuser.S | 25 +++++++++++++++++++++----
 1 file changed, 21 insertions(+), 4 deletions(-)

diff --git a/arch/x86/lib/getuser.S b/arch/x86/lib/getuser.S
index 094224ec9dca..7c9bf8f0b3ac 100644
--- a/arch/x86/lib/getuser.S
+++ b/arch/x86/lib/getuser.S
@@ -105,6 +105,26 @@ SYM_FUNC_START(__get_user_8)
 SYM_FUNC_END(__get_user_8)
 EXPORT_SYMBOL(__get_user_8)
 
+#ifdef CONFIG_X86_64
+
+/*
+ * On x86-64, get_user() does address masking rather than a conditional
+ * bounds check so there's no functional difference with __get_user().
+ */
+SYM_FUNC_ALIAS(__get_user_nocheck_1, __get_user_1);
+EXPORT_SYMBOL(__get_user_nocheck_1);
+
+SYM_FUNC_ALIAS(__get_user_nocheck_2, __get_user_2);
+EXPORT_SYMBOL(__get_user_nocheck_2);
+
+SYM_FUNC_ALIAS(__get_user_nocheck_4, __get_user_4);
+EXPORT_SYMBOL(__get_user_nocheck_4);
+
+SYM_FUNC_ALIAS(__get_user_nocheck_8, __get_user_8);
+EXPORT_SYMBOL(__get_user_nocheck_8);
+
+#else /* CONFIG_X86_32 */
+
 /* .. and the same for __get_user, just without the range checks */
 SYM_FUNC_START(__get_user_nocheck_1)
 	ASM_STAC
@@ -139,19 +159,16 @@ EXPORT_SYMBOL(__get_user_nocheck_4)
 SYM_FUNC_START(__get_user_nocheck_8)
 	ASM_STAC
 	ASM_BARRIER_NOSPEC
-#ifdef CONFIG_X86_64
-	UACCESS movq (%_ASM_AX),%rdx
-#else
 	xor %ecx,%ecx
 	UACCESS movl (%_ASM_AX),%edx
 	UACCESS movl 4(%_ASM_AX),%ecx
-#endif
 	xor %eax,%eax
 	ASM_CLAC
 	RET
 SYM_FUNC_END(__get_user_nocheck_8)
 EXPORT_SYMBOL(__get_user_nocheck_8)
 
+#endif /* CONFIG_X86_32 */
 
 SYM_CODE_START_LOCAL(__get_user_handle_exception)
 	ASM_CLAC
-- 
2.47.0