Re: [PATCHv7 00/16] x86: Enable Linear Address Space Separation support

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

On Thu, Jun 26, 2025 at 11:35:21AM +0200, Vegard Nossum wrote:
> 
> On 26/06/2025 11:22, Vegard Nossum wrote:
> > 
> > On 25/06/2025 14:50, Kirill A. Shutemov wrote:
> > > Linear Address Space Separation (LASS) is a security feature that
> > > intends to
> > > prevent malicious virtual address space accesses across user/kernel mode.
> > 
> > I applied these patches on top of tip/master and when I try to boot it
> > fails with errno 12 (ENOMEM - Cannot allocate memory):
> > 
> > [    1.517526] Kernel panic - not syncing: Requested init /bin/bash
> > failed (error -12).

For some reason, I failed to reproduce it. What is your toolchain?

> > Just using standard defconfig and booting in qemu/KVM with 2G RAM.
> > 
> > Bisect lands on "x86/asm: Introduce inline memcpy and memset".
> 
> I think the newly added mulq to rep_stos_alternative clobbers %rdx,

Yes, it makes sense.

> at
> least this patch fixed it for me:
> 
> diff --git a/arch/x86/include/asm/string.h b/arch/x86/include/asm/string.h
> index 5cd0f18a431fe..bc096526432a1 100644
> --- a/arch/x86/include/asm/string.h
> +++ b/arch/x86/include/asm/string.h
> @@ -28,7 +28,7 @@ static __always_inline void *__inline_memcpy(void *to,
> const void *from, size_t
>                      "2:\n\t"
>                      _ASM_EXTABLE_UA(1b, 2b)
>                      :"+c" (len), "+D" (to), "+S" (from),
> ASM_CALL_CONSTRAINT
> -                    : : "memory", _ASM_AX);
> +                    : : "memory", _ASM_AX, _ASM_DX);
> 
>         return ret + len;
>  }

This part is not needed. rep_movs_alternative() doesn't touch RDX.

I will fold the patch below.

Or maybe some asm guru can suggest a better way to fix it without
clobbering RDX?

diff --git a/arch/x86/include/asm/string.h b/arch/x86/include/asm/string.h
index 5cd0f18a431f..b0a26a3f11e0 100644
--- a/arch/x86/include/asm/string.h
+++ b/arch/x86/include/asm/string.h
@@ -44,7 +44,7 @@ static __always_inline void *__inline_memset(void *addr, int v, size_t len)
 		     _ASM_EXTABLE_UA(1b, 2b)
 		     : "+c" (len), "+D" (addr), ASM_CALL_CONSTRAINT
 		     : "a" ((uint8_t)v)
-		     : "memory", _ASM_SI);
+		     : "memory", _ASM_SI, _ASM_DX);
 
 	return ret + len;
 }
diff --git a/arch/x86/lib/clear_page_64.S b/arch/x86/lib/clear_page_64.S
index ca94828def62..d904c781fa3f 100644
--- a/arch/x86/lib/clear_page_64.S
+++ b/arch/x86/lib/clear_page_64.S
@@ -64,12 +64,15 @@ EXPORT_SYMBOL_GPL(clear_page_erms)
  *
  * Output:
  * rcx: uncleared bytes or 0 if successful.
+ * rdx: clobbered
  */
 SYM_FUNC_START(rep_stos_alternative)
 	ANNOTATE_NOENDBR
 
 	movzbq %al, %rsi
 	movabs $0x0101010101010101, %rax
+
+	/* %rdx:%rax = %rax * %rsi */
 	mulq %rsi
 
 	cmpq $64,%rcx
-- 
  Kiryl Shutsemau / Kirill A. Shutemov