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

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Date: Tue, 16 Jan 2024 15:44:09 -0800
From: Kevin Loughlin <kevinloughlin@...gle.com>
To: Tom Lendacky <thomas.lendacky@....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>, 
	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 Mon, Jan 15, 2024 at 7:53 AM Tom Lendacky <thomas.lendacky@....com> wrote:
>
> On 1/11/24 16:36, Kevin Loughlin wrote:
> >
> > @@ -61,33 +66,34 @@ static __maybe_unused __always_inline bool amd_cc_platform_vtom(enum cc_attr att
> >   static bool noinstr amd_cc_platform_has(enum cc_attr attr)
> >   {
> >   #ifdef CONFIG_AMD_MEM_ENCRYPT
> > +     const u64 sev_status_fixed_up = sev_get_status_fixup();
>
> Why not also have a variable for sme_me_mask?

`sme_get_me_mask_fixup()` is only used on certain conditional paths in
the calculation of the return value (therefore, a max of 1 times per
invocation of `amd_cc_platform_has()`). As such, calling
`sme_get_me_mask()` unconditionally at the beginning of the function
would be unnecessary for some invocations of `amd_cc_platform_has()`.
In contrast, the sev_status is needed on every invocation of
`amd_cc_platform_has()`. Additionally, the `sev_get_status_fixup()`
result is potentially used multiple times in the same invocation of
`amd_cc_platform_has()`, motivating the use of a local variable.

> > @@ -130,6 +130,7 @@ static unsigned long __head sme_postprocess_startup(struct boot_params *bp, pmdv
> >   {
> >       unsigned long vaddr, vaddr_end;
> >       int i;
> > +     const u64 sme_me_mask_fixed_up = sme_get_me_mask_fixup();
>
> Should be the first variable listed given the length of the line.

I will incorporate this and all other stylistic changes that you
mentioned in v3.

> > @@ -110,8 +115,9 @@ static void __noreturn sev_es_terminate(unsigned int set, unsigned int reason)
> >   static u64 get_hv_features(void)
> >   {
> >       u64 val;
> > +     const u16 *ghcb_version_ptr = (const u16 *) GET_RIP_RELATIVE_PTR(ghcb_version);
>
> Is this one really needed? The ghcb_version variable isn't referenced
> before fixup, right? It's referenced in both decompression and early boot,
> but I didn't think a fixup is needed.

You're right; it looks like we do *not* need the fixup for
ghcb_version in both instances that you identified. I will remove
these particular fixups in v3.

Thanks!