linux-kernel - Re: [PATCH] x86/traps: Enable UBSAN traps on x86

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Date: Fri, 31 May 2024 13:36:35 -0700
From: Gatlin Newhouse <gatlin.newhouse@...il.com>
To: Andrew Cooper <andrew.cooper3@...rix.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, Kees Cook <keescook@...omium.org>, 
	Marco Elver <elver@...gle.com>, Andrey Konovalov <andreyknvl@...il.com>, 
	Andrey Ryabinin <ryabinin.a.a@...il.com>, Nathan Chancellor <nathan@...nel.org>, 
	Nick Desaulniers <ndesaulniers@...gle.com>, Bill Wendling <morbo@...gle.com>, 
	Justin Stitt <justinstitt@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Baoquan He <bhe@...hat.com>, Rick Edgecombe <rick.p.edgecombe@...el.com>, 
	Changbin Du <changbin.du@...wei.com>, Pengfei Xu <pengfei.xu@...el.com>, 
	Josh Poimboeuf <jpoimboe@...nel.org>, Xin Li <xin3.li@...el.com>, Jason Gunthorpe <jgg@...pe.ca>, 
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, linux-kernel@...r.kernel.org, kasan-dev@...glegroups.com, 
	linux-hardening@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH] x86/traps: Enable UBSAN traps on x86

On Thu, May 30, 2024 at 01:24:56AM UTC, Andrew Cooper wrote:
> On 29/05/2024 3:20 am, Gatlin Newhouse wrote:
> > diff --git a/arch/x86/include/asm/bug.h b/arch/x86/include/asm/bug.h
> > index a3ec87d198ac..e3fbed9073f8 100644
> > --- a/arch/x86/include/asm/bug.h
> > +++ b/arch/x86/include/asm/bug.h
> > @@ -13,6 +13,14 @@
> >  #define INSN_UD2	0x0b0f
> >  #define LEN_UD2		2
> >  
> > +/*
> > + * In clang we have UD1s reporting UBSAN failures on X86, 64 and 32bit.
> > + */
> > +#define INSN_UD1	0xb90f
> > +#define LEN_UD1		2
> > +#define INSN_REX	0x67
> > +#define LEN_REX		1
> 
> That's an address size override prefix, not a REX prefix.

Good to know, thanks.

> What information is actually encoded in this UD1 instruction?  I can't
> find anything any documentation which actually discusses how the ModRM
> byte is encoded.

lib/ubsan.h has a comment before the ubsan_checks enum which links to line 113
in LLVM's clang/lib/CodeGen/CodeGenFunction.h which defines the values for the
ModRM byte. I think the Undefined Behavior Sanitizer pass does the actual
encoding of UB type to values but I'm not an expert in LLVM.

> ~Andrew