[PATCH v3 2/6] x86/uaccess: Avoid barrier_nospec() in 64-bit __get_user()

[Josh Poimboeuf]

The barrier_nospec() in 64-bit __get_user() is slow.  Instead use
pointer masking to force the user pointer to all 1's if a previous
access_ok() mispredicted true for an invalid address.

Note that for safety on some AMD CPUs, this relies on recent commit
86e6b1547b3d ("x86: fix user address masking non-canonical speculation
issue").

Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
---
 arch/x86/lib/getuser.S | 24 ++++++++++++++++++++----
 1 file changed, 20 insertions(+), 4 deletions(-)

diff --git a/arch/x86/lib/getuser.S b/arch/x86/lib/getuser.S
index 4357ec2a0bfc..998d5be6b794 100644
--- a/arch/x86/lib/getuser.S
+++ b/arch/x86/lib/getuser.S
@@ -112,8 +112,12 @@ EXPORT_SYMBOL(__get_user_8)
 
 /* .. and the same for __get_user, just without the range checks */
 SYM_FUNC_START(__get_user_nocheck_1)
-	ASM_STAC
+#ifdef CONFIG_X86_64
+	check_range size=1
+#else
 	ASM_BARRIER_NOSPEC
+#endif
+	ASM_STAC
 	UACCESS movzbl (%_ASM_AX),%edx
 	xor %eax,%eax
 	ASM_CLAC
@@ -122,8 +126,12 @@ SYM_FUNC_END(__get_user_nocheck_1)
 EXPORT_SYMBOL(__get_user_nocheck_1)
 
 SYM_FUNC_START(__get_user_nocheck_2)
-	ASM_STAC
+#ifdef CONFIG_X86_64
+	check_range size=2
+#else
 	ASM_BARRIER_NOSPEC
+#endif
+	ASM_STAC
 	UACCESS movzwl (%_ASM_AX),%edx
 	xor %eax,%eax
 	ASM_CLAC
@@ -132,8 +140,12 @@ SYM_FUNC_END(__get_user_nocheck_2)
 EXPORT_SYMBOL(__get_user_nocheck_2)
 
 SYM_FUNC_START(__get_user_nocheck_4)
-	ASM_STAC
+#ifdef CONFIG_X86_64
+	check_range size=4
+#else
 	ASM_BARRIER_NOSPEC
+#endif
+	ASM_STAC
 	UACCESS movl (%_ASM_AX),%edx
 	xor %eax,%eax
 	ASM_CLAC
@@ -142,8 +154,12 @@ SYM_FUNC_END(__get_user_nocheck_4)
 EXPORT_SYMBOL(__get_user_nocheck_4)
 
 SYM_FUNC_START(__get_user_nocheck_8)
-	ASM_STAC
+#ifdef CONFIG_X86_64
+	check_range size=8
+#else
 	ASM_BARRIER_NOSPEC
+#endif
+	ASM_STAC
 #ifdef CONFIG_X86_64
 	UACCESS movq (%_ASM_AX),%rdx
 #else
-- 
2.47.0