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

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Message-ID: <c068193b-75fb-49d2-9104-775051ffd941@citrix.com>
Date: Thu, 30 May 2024 01:24:56 +0100
From: Andrew Cooper <andrew.cooper3@...rix.com>
To: Gatlin Newhouse <gatlin.newhouse@...il.com>,
 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 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.

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.

~Andrew