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

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Date: Mon, 15 Jan 2024 21:46:34 +0100
From: Borislav Petkov <bp@...en8.de>
To: Kevin Loughlin <kevinloughlin@...gle.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
	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>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.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 Thu, Jan 11, 2024 at 10:36:50PM +0000, Kevin Loughlin wrote:
> SEV/SME code can execute prior to page table fixups for kernel
> relocation. However, as with global variables accessed in
> __startup_64(), the compiler is not required to generate RIP-relative
> accesses for SEV/SME global variables, causing certain flavors of SEV
> hosts and guests built with clang to crash during boot.

So, about that. If I understand my gcc toolchain folks correctly:

mcmodel=kernel - everything fits into the high 31 bit of the address
space

-fPIE/PIC - position independent

And supplied both don't make a whole lotta of sense: if you're building
position-independent, then mcmodel=kernel would be overridden by the
first.

I have no clue why clang enabled it...

So, *actually* the proper fix here should be not to add this "fixed_up"
gunk everywhere but remove mcmodel=kernel from the build and simply do
-fPIE/PIC.

I'd say...

I could also be missing something obvious ofc.

> Fixes: 95d33bfaa3e1 ("x86/sev: Register GHCB memory when SEV-SNP is active")
> Fixes: ee0bfa08a345 ("x86/compressed/64: Add support for SEV-SNP CPUID table in #VC handlers")
> Fixes: 1cd9c22fee3a ("x86/mm/encrypt: Move page table helpers into separate translation unit")
> Fixes: c9f09539e16e ("x86/head/64: Check SEV encryption before switching to kernel page-table")
> Fixes: b577f542f93c ("x86/coco: Add API to handle encryption mask")
> Tested-by: Kevin Loughlin <kevinloughlin@...gle.com>

You don't need to add your Tested-by tag - it is kinda assumed that
people submit patches *after* testing them. Although I have a gazillion
examples where that is not the case...

:-\

> Signed-off-by: Kevin Loughlin <kevinloughlin@...gle.com>


-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette