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

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Message-ID: <CAMj1kXFMe6WqKXJsK1Lqk++naM8D=MZADhgpO_FjUd0RuFQU2w@mail.gmail.com>
Date: Sun, 21 Jan 2024 17:49:44 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: Borislav Petkov <bp@...en8.de>
Cc: Kevin Loughlin <kevinloughlin@...gle.com>, 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>, 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 Sun, 21 Jan 2024 at 16:38, Borislav Petkov <bp@...en8.de> wrote:
>
> On Sun, Jan 21, 2024 at 03:12:56PM +0100, Ard Biesheuvel wrote:
> > The SEV boot code is especially tricky here as very few
> > people can even test it,
>
> No worries about that - us, the Google cloud folks, AWS and a bunch of
> others are people I could think of who could help out. :-)
>

Yeah. I have been trying to find people internally at Google that can
help me set up some CI that I can throw kernel builds at and they will
be test booted in a SEV guest, but so far progress has been slow.

> > 1)
> > WARNING: modpost: vmlinux: section mismatch in reference:
> > startup_64_pi+0x33 (section: .pi.text) -> sme_enable (section:
> > .init.text)
>
> sme_enable() is in the 1:1 mapping TU
> arch/x86/mm/mem_encrypt_identity.c, see
>
>   1cd9c22fee3a ("x86/mm/encrypt: Move page table helpers into separate translation unit")
>
> so might as well move it to .pi.text
>

Ack.

> The rest below look like they'd need more serious untangling.
>
> Btw, I just had another idea: we could remove -mcmodel=kernel from the
> build flags of the whole kernel once -fPIC is enabled so that gcc can be
> forced to do rIP-relative addressing.
>
> I'm being told the reason it doesn't allow mcmodel=kernel with -fPIC is
> only a matter of removing that check and that it *should* otherwise work
> but someone needs to try that. And then there are older gccs which we
> cannot fix.
>

-fPIE -mcmodel=small should work fine afaik. The only problem i
encountered is that it changes the default per-CPU base register to FS
but that can be overridden on the command line.

The problem with building the entire kernel -fPIE is that it increases
code size: RIP-relative LEA instructions are 1 byte longer than
absolute 32-bit MOVs.