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

From: Borislav Petkov <bp@alien8.de>
To: Ard Biesheuvel <ardb@kernel.org>
Cc: Kevin Loughlin <kevinloughlin@google.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	Andy Lutomirski <luto@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Nathan Chancellor <nathan@kernel.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Bill Wendling <morbo@google.com>,
	Justin Stitt <justinstitt@google.com>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	Michael Kelley <mikelley@microsoft.com>,
	Pankaj Gupta <pankaj.gupta@amd.com>,
	Stephen Rothwell <sfr@canb.auug.org.au>,
	Arnd Bergmann <arnd@arndb.de>,
	Steve Rutherford <srutherford@google.com>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Hou Wenlong <houwenlong.hwl@antgroup.com>,
	Vegard Nossum <vegard.nossum@oracle.com>,
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	Yuntao Wang <ytcoode@gmail.com>,
	Wang Jinchao <wangjinchao@xfusion.com>,
	David Woodhouse <dwmw@amazon.co.uk>,
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	Randy Dunlap <rdunlap@infradead.org>,
	Bjorn Helgaas <bhelgaas@google.com>,
	Dionna Glaze <dionnaglaze@google.com>,
	Brijesh Singh <brijesh.singh@amd.com>,
	Michael Roth <michael.roth@amd.com>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	linux-kernel@vger.kernel.org, llvm@lists.linux.dev,
	linux-coco@lists.linux.dev, Ashish Kalra <ashish.kalra@amd.com>,
	Andi Kleen <ak@linux.intel.com>,
	Adam Dunlap <acdunlap@google.com>,
	Peter Gonda <pgonda@google.com>, Jacob Xu <jacobhxu@google.com>,
	Sidharth Telang <sidtelang@google.com>
Subject: Re: [RFC PATCH v2] x86/sev: enforce RIP-relative accesses in early SEV/SME code
Date: Wed, 17 Jan 2024 14:05:57 +0100	[thread overview]
Message-ID: <20240117130557.GDZafQtfRyeVFbBUXA@fat_crate.local> (raw)
In-Reply-To: <CAMj1kXH=k26nNyB+LQJ7WUJgbD2f3PREyjCzSngMCGc+72XJ6w@mail.gmail.com>

On Wed, Jan 17, 2024 at 11:59:14AM +0100, Ard Biesheuvel wrote:
> Fully agree. All this fiddling with RIP relative references from C
> code is going to be a maintenance burden going forward.

Yah.

> The proper way to do this is use PIC codegen for the objects that
> matter.

And we have arch/x86/mm/mem_encrypt_identity.c which is supposed to deal
with stuff running from the ident mappings and PA == VA.

We could put the rest of those special SEV things there or do a separate
TU to be built using something like PIE_FLAGS, as in your patch.

> I had a stab [0] at this a while ago (for the purpose of increasing
> the KASLR range, which requires PIE linking) but I didn't pursue it in
> the end.

FWIW, that looks a lot more like a natural kernel code with
__va_symbol() etc. Definitely better and we talked about it at some
point already as it does ring a bell.

> On arm64, we use a separate pseudo-namespace for code that can run
> safely at any offset, using the __pi_ prefix (for Position
> Independent). Using symbol prefixing at the linker level, we ensure
> that __pi_ code can only call other __pi_ code, or code that has been
> made available to it via an explicit __pi_ prefixed alias. (Happy to
> elaborate more but we should find a smaller audience - your cc list is
> a tad long). Perhaps this is something we should explore on x86 as
> well (note that the EFI stub does something similar for architectures
> that link the EFI stub into the core kernel rather than into the
> decompressor)

Grepping through the tree, is __pi_memcpy one example for that?

It sure looks like it with the alias and all. From a quick scan, that is
not that bad either. It gives you the clear distinction what that
symbol is and who can call it.

Thx.

-- 
Regards/Gruss,
    Boris.

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