linux-kernel - Re: [RFC PATCH v2] x86/sev: enforce RIP-relative accesses in early SEV/SME code

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Message-ID: <20240115101239.nab725vuazvutgw6@box.shutemov.name>
Date: Mon, 15 Jan 2024 13:12:39 +0300
From: "Kirill A. Shutemov" <kirill.shutemov@...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>,
	Tom Lendacky <thomas.lendacky@....com>,
	Michael Kelley <mikelley@...rosoft.com>,
	Pankaj Gupta <pankaj.gupta@....com>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	Arnd Bergmann <arnd@...db.de>,
	Steve Rutherford <srutherford@...gle.com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Hou Wenlong <houwenlong.hwl@...group.com>,
	Vegard Nossum <vegard.nossum@...cle.com>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Yuntao Wang <ytcoode@...il.com>,
	Wang Jinchao <wangjinchao@...sion.com>,
	David Woodhouse <dwmw@...zon.co.uk>,
	Brian Gerst <brgerst@...il.com>, Hugh Dickins <hughd@...gle.com>,
	Ard Biesheuvel <ardb@...nel.org>, Joerg Roedel <jroedel@...e.de>,
	Randy Dunlap <rdunlap@...radead.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Dionna Glaze <dionnaglaze@...gle.com>,
	Brijesh Singh <brijesh.singh@....com>,
	Michael Roth <michael.roth@....com>, linux-kernel@...r.kernel.org,
	llvm@...ts.linux.dev, linux-coco@...ts.linux.dev,
	Ashish Kalra <ashish.kalra@....com>,
	Andi Kleen <ak@...ux.intel.com>, Adam Dunlap <acdunlap@...gle.com>,
	Peter Gonda <pgonda@...gle.com>, Jacob Xu <jacobhxu@...gle.com>,
	Sidharth Telang <sidtelang@...gle.com>
Subject: Re: [RFC PATCH v2] x86/sev: enforce RIP-relative accesses in early
 SEV/SME code

On Fri, Jan 12, 2024 at 10:29:36AM -0800, Kevin Loughlin wrote:
> On Fri, Jan 12, 2024 at 4:17 AM Kirill A. Shutemov
> <kirill.shutemov@...ux.intel.com> wrote:
> >
> > Can we replace existing fixup_pointer() (and other fixup_*()) with the new
> > thing? I don't think we need two confusing things for the same function.
> 
> Per my tests, yes we can; I replaced the fixup_*() functions with
> GET_RIP_RELATIVE_PTR()/PTR_TO_RIP_RELATIVE_PTR(), and guests with and
> without SEV, SEV-ES, and SEV-SNP all successfully booted under both
> clang and gcc builds.

Okay good.

BTW, do we need both macros? Caller can do &var, right?

If we are okay with single macro, maybe rename it to RIP_RELATIVE_PTR().

One other thing: I see you sprinkle casts to for every use of the macros.
But why? void* can cast to any other pointer without explicit casting.

> I have a slight preference for sending that as a
> separate follow-up commit, but please let me know if you feel
> differently. Thanks.

I'm okay with a separate patch in the same patchset.

> 
> > Also, is there any reason why GET_RIP_RELATIVE_PTR() and
> > PTR_TO_RIP_RELATIVE_PTR() have to be macros? Inline functions would be
> > cleaner.
> 
> I used macros because we need to use both the global variable itself
> and the global variable's string name (obtained via #var in the macro)
> in the inline assembly. As a secondary reason, the macro also avoids
> the need to provide separate functions for each type of variable for
> which we'd like to get RIP-relative pointers (ex: u64, unsigned int,
> unsigned long, etc.).

If we do it only on pointers, wouldn't void * -> void * be enough?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov