linux-kernel - Re: [RFC PATCH] x86/sev: x86/sev: enforce PC-relative addressing in clang

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Date: Wed, 10 Jan 2024 09:49:44 -0800
From: Andi Kleen <ak@...ux.intel.com>
To: Kevin Loughlin <kevinloughlin@...gle.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>, Andy Lutomirski <luto@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>,
	Rick Edgecombe <rick.p.edgecombe@...el.com>,
	Kees Cook <keescook@...omium.org>,
	"Masami Hiramatsu (Google)" <mhiramat@...nel.org>,
	Ze Gao <zegao2021@...il.com>, Josh Poimboeuf <jpoimboe@...nel.org>,
	Pengfei Xu <pengfei.xu@...el.com>,
	Brijesh Singh <brijesh.singh@....com>,
	Michael Roth <michael.roth@....com>,
	Ashish Kalra <ashish.kalra@....com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Tom Lendacky <thomas.lendacky@....com>,
	Joerg Roedel <jroedel@...e.de>, linux-kernel@...r.kernel.org,
	llvm@...ts.linux.dev, linux-coco@...ts.linux.dev,
	Adam Dunlap <acdunlap@...gle.com>, Peter Gonda <pgonda@...gle.com>,
	Jacob Xu <jacobhxu@...gle.com>,
	Sidharth Telang <sidtelang@...gle.com>
Subject: Re: [RFC PATCH] x86/sev: x86/sev: enforce PC-relative addressing in
 clang

> On that note, I do have another version of this patch that abstracts
> snp_cpuid_get_table() into a macro along the lines of...
> 
> #define GET_RIP_RELATIVE_PTR(var) \
> ({  \
>  void *ptr; \
>  asm ("lea "#var"(%%rip), %0" \
>       : "=r" (ptr) \
>       : "p" (&var)); \
>  ptr; \
> })
> 
> ...and uses this new macro to access all SEV/SME global variables (not
> just the cpuid_table). It's similar in nature to `fixup_pointer()`
> (currently defined in arch/x86/kernel/head64.c) but doesn't require us
> to pass around `physaddr` from `__startup64()`. This wouldn't
> introduce any new execution model changes between clang vs gcc and
> would be consistent with the kernel's current approach of relying on
> developers to manually apply fixups for global variable accesses prior
> to kernel relocation. I can send an RFC v2 for the
> GET_RIP_RELATIVE_PTR() version of this patch.

That looks like a far better solution indeed.

Ideally objtool would check for this, perhaps with a new ELF
section. But actually doing that might be far more work, so perhaps not
worth it.

Thanks,

-Andi