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

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Message-ID: <20250629124041.04a30869@pumpkin>
Date: Sun, 29 Jun 2025 12:40:41 +0100
From: David Laight <david.laight.linux@...il.com>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc: Vegard Nossum <vegard.nossum@...cle.com>, Andy Lutomirski
 <luto@...nel.org>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar
 <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, Dave Hansen
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Subject: Re: [PATCHv7 00/16] x86: Enable Linear Address Space Separation
 support

On Thu, 26 Jun 2025 15:47:36 +0300
"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com> wrote:

> 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?

Or separate out the code where the value is a compile-time zero.
That is pretty much 99% of the calls.

	David