From: "Hou Wenlong" <houwenlong.hwl@antgroup.com>
To: Kevin Loughlin <kevinloughlin@google.com>
Cc: Borislav Petkov <bp@alien8.de>,
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>,
Vegard Nossum <vegard.nossum@oracle.com>,
Josh Poimboeuf <jpoimboe@kernel.org>,
Yuntao Wang <ytcoode@gmail.com>,
Wang Jinchao <wangjinchao@xfusion.com>,
David Woodhouse <dwmw@amazon.co.uk>,
Brian Gerst <brgerst@gmail.com>, Hugh Dickins <hughd@google.com>,
Ard Biesheuvel <ardb@kernel.org>, Joerg Roedel <jroedel@suse.de>,
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 10:47:02 +0800 [thread overview]
Message-ID: <20240117024702.GA112065@k08j02272.eu95sqa> (raw)
In-Reply-To: <CAGdbjm+fMBnkKDVFpf-fAEbYxTUWR0od2JzTTbEMoPVS3J_Faw@mail.gmail.com>
On Wed, Jan 17, 2024 at 08:07:49AM +0800, Kevin Loughlin wrote:
> On Mon, Jan 15, 2024 at 12:47 PM Borislav Petkov <bp@alien8.de> wrote:
> >
> > 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 believe that the key distinction is that using mcmodel=kernel (upper
> 2 GB of address space) or the similar mcmodel=small (lower 2 GB) means
> the compiler *can* use RIP-relative addressing for globals (because
> everything is within +/- 2GB of RIP) but is not *required* to do so.
> In contrast, fPIE/fPIC *requires* relative addressing but does not
> necessarily require a specific 2 GB placement range. Altogether, I do
> think there are use cases for both options individually. I can't think
> of a reason why gcc wouldn't be able to support mcmodel=kernel in
> conjunction with fPIE off the top of my head, but I admittedly haven't
> looked into it; I simply observed that the combination is not
> currently supported.
>
> RE: compiling the whole x86-64 kernel with fPIE/fPIC, I believe the
> required changes would be very extensive (see "[PATCH RFC 00/43]
> x86/pie: Make kernel image's virtual address flexible" at
> https://lore.kernel.org/lkml/cover.1682673542.git.houwenlong.hwl@antgroup.com/).
FYI I have sent a small patchset[0] that attempts to build the head code
(head64.c and mem_encrypt_identity.c) as PIE. However, there are still a
few functions that are called by the head code but are not in these two
files.
[0] https://lore.kernel.org/lkml/cover.1689130310.git.houwenlong.hwl@antgroup.com
next prev parent reply other threads:[~2024-01-17 2:52 UTC|newest]
Thread overview: 47+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-01-10 1:26 [RFC PATCH] x86/sev: x86/sev: enforce PC-relative addressing in clang Kevin Loughlin
2024-01-10 11:45 ` Andi Kleen
2024-01-10 17:14 ` Kevin Loughlin
2024-01-10 17:49 ` Andi Kleen
2024-01-11 22:36 ` [RFC PATCH v2] x86/sev: enforce RIP-relative accesses in early SEV/SME code Kevin Loughlin
2024-01-12 12:17 ` Kirill A. Shutemov
2024-01-12 18:29 ` Kevin Loughlin
2024-01-15 10:12 ` Kirill A. Shutemov
2024-01-16 22:13 ` Kevin Loughlin
2024-01-15 15:53 ` Tom Lendacky
2024-01-16 23:44 ` Kevin Loughlin
2024-01-15 20:46 ` Borislav Petkov
2024-01-17 0:07 ` Kevin Loughlin
2024-01-17 2:47 ` Hou Wenlong [this message]
2024-01-17 10:59 ` Ard Biesheuvel
2024-01-17 11:39 ` Andi Kleen
2024-01-17 11:55 ` Ard Biesheuvel
2024-01-17 13:05 ` Borislav Petkov
2024-01-17 13:38 ` Ard Biesheuvel
2024-01-21 14:12 ` Ard Biesheuvel
2024-01-21 15:37 ` Borislav Petkov
2024-01-21 16:49 ` Ard Biesheuvel
2024-01-21 18:20 ` Borislav Petkov
2024-01-30 22:08 ` [PATCH v3 0/2] x86: enforce and cleanup RIP-relative accesses in early boot code Kevin Loughlin
2024-01-31 14:00 ` Borislav Petkov
2024-01-31 18:16 ` Jacob Xu
2024-01-31 18:29 ` Borislav Petkov
2024-02-03 0:22 ` Kevin Loughlin
2024-02-03 10:15 ` Ard Biesheuvel
2024-02-03 10:19 ` Borislav Petkov
2024-02-03 10:27 ` Ard Biesheuvel
2024-02-03 11:25 ` Borislav Petkov
2024-02-06 15:46 ` [tip: x86/sev] x86/sev: Fix position dependent variable references in startup code tip-bot2 for Ard Biesheuvel
2024-01-30 22:08 ` [PATCH v3 1/2] x86/sev: enforce RIP-relative accesses in early SEV/SME code Kevin Loughlin
2024-01-31 8:20 ` Kirill A. Shutemov
2024-02-02 22:00 ` Kevin Loughlin
2024-02-02 22:47 ` Ard Biesheuvel
2024-02-03 0:11 ` Kevin Loughlin
2024-01-31 13:42 ` Ard Biesheuvel
2024-02-03 0:14 ` Kevin Loughlin
2024-01-30 22:08 ` [PATCH v3 2/2] x86/head64: Replace pointer fixups with RIP_RELATIVE_ADDR() Kevin Loughlin
2024-01-31 8:22 ` Kirill A. Shutemov
2024-02-01 16:38 ` Kevin Loughlin
2024-01-31 15:30 ` Tom Lendacky
2024-01-31 15:36 ` Kirill A. Shutemov
2024-01-10 13:36 ` [RFC PATCH] x86/sev: x86/sev: enforce PC-relative addressing in clang Kirill A. Shutemov
2024-01-10 17:28 ` Kevin Loughlin
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