linux-kernel - Re: [PATCH v3] x86/traps: Enable UBSAN traps on x86

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Date: Wed, 26 Jun 2024 12:07:52 -0700
From: Kees Cook <kees@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: 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>, 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>,
	"Mike Rapoport (IBM)" <rppt@...nel.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>, Xin Li <xin3.li@...el.com>,
	Jason Gunthorpe <jgg@...pe.ca>, Uros Bizjak <ubizjak@...il.com>,
	Arnd Bergmann <arnd@...db.de>,
	"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 v3] x86/traps: Enable UBSAN traps on x86

On Tue, Jun 25, 2024 at 11:37:19AM +0200, Peter Zijlstra wrote:
> Also, wouldn't it be saner to write this something like:
> 
> __always_inline int decode_bug(unsigned long addr, u32 *imm)
> {
> 	u8 v;
> 
> 	if (addr < TASK_SIZE)
> 		return BUG_NONE;
> 
> 	v = *(u8 *)(addr++);
> 	if (v == 0x67)
> 		v = *(u8 *)(addr++);
> 	if (v != 0x0f)
> 		return BUG_NONE;
> 	v = *(u8 *)(addr++);
> 	if (v == 0x0b)
> 		return BUG_UD2;
> 	if (v != 0xb9)
> 		return BUG_NONE;
> 
> 	if (X86_MODRM_RM(v) == 4)
> 		addr++; /* consume SiB */
> 
> 	*imm = 0;
> 	if (X86_MODRM_MOD(v) == 1)
> 		*imm = *(u8 *)addr;
> 	if (X86_MORRM_MOD(v) == 2)
> 		*imm = *(u32 *)addr;
> 
> 	// WARN on MOD(v)==3 ??
> 
> 	return BUG_UD1;
> }

Thanks for the example! (I think it should use macros instead of
open-coded "0x67", "0x0f", etc, but yeah.)

> Why does the thing emit the asop prefix at all through? afaict it
> doesn't affect the immediate you want to get at. And if it does this
> prefix, should we worry about other prefixes? Ideally we'd not accept
> any prefixes.

AFAICT it's because it's a small immediate? For an x86_64 build, this is
how Clang is generating the UD1.

-Kees

-- 
Kees Cook