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

Vegard Nossum
2025-06-26T09:36:27Z

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).
>
> 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, 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;
}
@@ -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 ca94828def624..77cfd75718623 100644
--- a/arch/x86/lib/clear_page_64.S
+++ b/arch/x86/lib/clear_page_64.S
@@ -64,6 +64,7 @@ EXPORT_SYMBOL_GPL(clear_page_erms)
*
* Output:
* rcx: uncleared bytes or 0 if successful.
+ * rdx: clobbered
*/
SYM_FUNC_START(rep_stos_alternative)
ANNOTATE_NOENDBR

Thanks,

Vegard