[PATCH] x86/uaccess: Avoid barrier_nospec() in copy_from_user()

[Kirill A. Shutemov]

On Sun, Oct 13, 2024 at 11:50:55PM -0700, Linus Torvalds wrote:
> Anyway, the attached patch
> 
> diff --git a/arch/x86/lib/getuser.S b/arch/x86/lib/getuser.S
> index d066aecf8aeb..7d5730aa18b8 100644
> --- a/arch/x86/lib/getuser.S
> +++ b/arch/x86/lib/getuser.S
> @@ -37,11 +37,17 @@
>  
>  #define ASM_BARRIER_NOSPEC ALTERNATIVE "", "lfence", X86_FEATURE_LFENCE_RDTSC
>  
> +#define X86_CANONICAL_MASK ALTERNATIVE \
> +	"movq $0x80007fffffffffff,%rdx", \
> +	"movq $0x80ffffffffffffff,%rdx", X86_FEATURE_LA57
> +
>  .macro check_range size:req
>  .if IS_ENABLED(CONFIG_X86_64)
>  	mov %rax, %rdx
>  	sar $63, %rdx
>  	or %rdx, %rax
> +	X86_CANONICAL_MASK
> +	and %rdx,%rax
>  .else
>  	cmp $TASK_SIZE_MAX-\size+1, %eax
>  	jae .Lbad_get_user

Given that LAM enforces bit 47/56 to be equal to bit 63 I think we can do
this unconditionally instead of masking:

diff --git a/arch/x86/lib/getuser.S b/arch/x86/lib/getuser.S
index d066aecf8aeb..86d4511520b1 100644
--- a/arch/x86/lib/getuser.S
+++ b/arch/x86/lib/getuser.S
@@ -37,9 +37,14 @@
 
 #define ASM_BARRIER_NOSPEC ALTERNATIVE "", "lfence", X86_FEATURE_LFENCE_RDTSC
 
+#define SHIFT_LEFT_TO_MSB ALTERNATIVE \
+	"shl $(64 - 48), %rdx", \
+	"shl $(64 - 57), %rdx", X86_FEATURE_LA57
+
 .macro check_range size:req
 .if IS_ENABLED(CONFIG_X86_64)
 	mov %rax, %rdx
+	SHIFT_LEFT_TO_MSB
 	sar $63, %rdx
 	or %rdx, %rax
 .else
-- 
  Kiryl Shutsemau / Kirill A. Shutemov