From: Ard Biesheuvel <ardb@kernel.org>
To: Borislav Petkov <bp@alien8.de>
Cc: Ard Biesheuvel <ardb+git@google.com>,
linux-kernel@vger.kernel.org,
Kevin Loughlin <kevinloughlin@google.com>,
Tom Lendacky <thomas.lendacky@amd.com>,
Dionna Glaze <dionnaglaze@google.com>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>,
Dave Hansen <dave.hansen@linux.intel.com>,
Andy Lutomirski <luto@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
Nathan Chancellor <nathan@kernel.org>,
Nick Desaulniers <ndesaulniers@google.com>,
Justin Stitt <justinstitt@google.com>,
Kees Cook <keescook@chromium.org>,
Brian Gerst <brgerst@gmail.com>,
linux-arch@vger.kernel.org, llvm@lists.linux.dev
Subject: Re: [PATCH v3 08/19] x86/head64: Replace pointer fixups with PIE codegen
Date: Mon, 12 Feb 2024 12:52:01 +0100 [thread overview]
Message-ID: <CAMj1kXH2uwLT-EgqYFRcC0OH524W=sYtDoFC-x+isifzsia17w@mail.gmail.com> (raw)
In-Reply-To: <20240212102901.GVZcny7WeK_ZWt0HEP@fat_crate.local>
On Mon, 12 Feb 2024 at 11:29, Borislav Petkov <bp@alien8.de> wrote:
>
> On Mon, Jan 29, 2024 at 07:05:11PM +0100, Ard Biesheuvel wrote:
> > From: Ard Biesheuvel <ardb@kernel.org>
> >
> > Some of the C code in head64.c may be called from a different virtual
> > address than it was linked at. Currently, we deal with this by using
>
> Yeah, make passive pls: "Currently, this is done by using... "
>
> > ordinary, position dependent codegen, and fixing up all symbol
> > references on the fly. This is fragile and tricky to maintain. It is
> > also unnecessary: we can use position independent codegen (with hidden
> ^^^
> Ditto: "use ..."
>
> In the comments below too, pls, where it says "we".
>
Ack.
> > visibility) to ensure that all compiler generated symbol references are
> > RIP-relative, removing the need for fixups entirely.
> >
> > It does mean we need explicit references to kernel virtual addresses to
> > be generated by hand, so generate those using a movabs instruction in
> > inline asm in the handful places where we actually need this.
> >
> > Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> > ---
> > arch/x86/Makefile | 8 ++
> > arch/x86/boot/compressed/Makefile | 2 +-
> > arch/x86/include/asm/desc.h | 3 +-
> > arch/x86/include/asm/setup.h | 4 +-
> > arch/x86/kernel/Makefile | 5 ++
> > arch/x86/kernel/head64.c | 88 +++++++-------------
> > arch/x86/kernel/head_64.S | 5 +-
> > 7 files changed, 51 insertions(+), 64 deletions(-)
> >
> > diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> > index 1a068de12a56..2b5954e75318 100644
> > --- a/arch/x86/Makefile
> > +++ b/arch/x86/Makefile
> > @@ -168,6 +168,14 @@ else
> > KBUILD_CFLAGS += -mcmodel=kernel
> > KBUILD_RUSTFLAGS += -Cno-redzone=y
> > KBUILD_RUSTFLAGS += -Ccode-model=kernel
> > +
> > + PIE_CFLAGS-$(CONFIG_STACKPROTECTOR) += -fno-stack-protector
>
> Main Makefile has
>
> KBUILD_CFLAGS += -fno-PIE
>
> and this ends up being:
>
> gcc -Wp,-MMD,arch/x86/kernel/.head64.s.d -nostdinc ... -fno-PIE ... -fpie ... -fverbose-asm -S -o arch/x86/kernel/head64.s arch/x86/kernel/head64.c
>
> Can you pls remove -fno-PIE from those TUs which use PIE_CFLAGS so that
> there's no confusion when staring at V=1 output?
>
Yeah. That would means adding PIE_CFLAGS_REMOVE alongside PIE_CFLAGS
and applying both in every place it is used, but we are only dealing
with a handful of object files here.
> > + PIE_CFLAGS-$(CONFIG_LTO) += -fno-lto
> > +
> > + PIE_CFLAGS := -fpie -mcmodel=small $(PIE_CFLAGS-y) \
> > + -include $(srctree)/include/linux/hidden.h
> > +
> > + export PIE_CFLAGS
> > endif
> >
> > #
>
> Other than that, that code becomes much more readable, cool!
>
Thanks. But now that we have RIP_REL_REF(), I might split the cleanup
from the actual switch to -fpie, which I am still a bit on the fence
about, given different compiler versions, LTO, etc.
RIP_REL_REF(foo) just turns into 'foo' when compiling with -fpie and
we could drop those piecemeal once we are confident that -fpie does
not cause any regressions.
Note that I have some reservations now about .pi.text as well: it is a
bit intrusive, and on x86, we might just as well move everything that
executes from the 1:1 mapping into .head.text, and teach objtool that
those sections should not contain any ELF relocations involving
absolute addresses. But this is another thing that I want to spend a
bit more time on before I respin it, so I will just do the cleanup in
the next revision, and add the rigid correctness checks the next
cycle.
next prev parent reply other threads:[~2024-02-12 11:52 UTC|newest]
Thread overview: 52+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-01-29 18:05 [PATCH v3 00/19] x86: Confine early 1:1 mapped startup code Ard Biesheuvel
2024-01-29 18:05 ` [PATCH v3 01/19] efi/libstub: Add generic support for parsing mem_encrypt= Ard Biesheuvel
2024-01-31 7:31 ` Borislav Petkov
2024-02-01 16:23 ` Kevin Loughlin
2024-02-01 16:28 ` Ard Biesheuvel
2024-01-29 18:05 ` [PATCH v3 02/19] x86/boot: Move mem_encrypt= parsing to the decompressor Ard Biesheuvel
2024-01-31 8:35 ` Borislav Petkov
2024-01-31 9:12 ` Ard Biesheuvel
2024-01-31 9:29 ` Borislav Petkov
2024-01-31 9:59 ` Ard Biesheuvel
2024-02-01 14:17 ` Tom Lendacky
2024-02-01 16:15 ` Ard Biesheuvel
2024-02-02 16:35 ` [PATCH] x86/Kconfig: Remove CONFIG_AMD_MEM_ENCRYPT_ACTIVE_BY_DEFAULT Borislav Petkov
2024-02-02 16:47 ` Ard Biesheuvel
2024-02-03 10:50 ` [tip: x86/sev] " tip-bot2 for Borislav Petkov (AMD)
2024-01-29 18:05 ` [PATCH v3 03/19] x86/startup_64: Drop long return to initial_code pointer Ard Biesheuvel
2024-01-31 13:44 ` Borislav Petkov
2024-01-31 13:57 ` Ard Biesheuvel
2024-01-31 14:07 ` Ard Biesheuvel
2024-01-31 16:29 ` Borislav Petkov
2024-01-31 18:14 ` [tip: x86/boot] " tip-bot2 for Ard Biesheuvel
2024-01-29 18:05 ` [PATCH v3 04/19] x86/startup_64: Simplify calculation of initial page table address Ard Biesheuvel
2024-02-05 10:40 ` Borislav Petkov
2024-01-29 18:05 ` [PATCH v3 05/19] x86/startup_64: Simplify CR4 handling in startup code Ard Biesheuvel
2024-02-06 18:21 ` Borislav Petkov
2024-02-07 10:38 ` Ard Biesheuvel
2024-01-29 18:05 ` [PATCH v3 06/19] x86/startup_64: Drop global variables keeping track of LA57 state Ard Biesheuvel
2024-02-07 13:29 ` Borislav Petkov
2024-02-09 13:55 ` Ard Biesheuvel
2024-02-10 10:40 ` Borislav Petkov
2024-02-11 22:36 ` Ard Biesheuvel
2024-01-29 18:05 ` [PATCH v3 07/19] x86/startup_64: Simplify virtual switch on primary boot Ard Biesheuvel
2024-02-07 14:50 ` Borislav Petkov
2024-01-29 18:05 ` [PATCH v3 08/19] x86/head64: Replace pointer fixups with PIE codegen Ard Biesheuvel
2024-02-12 10:29 ` Borislav Petkov
2024-02-12 11:52 ` Ard Biesheuvel [this message]
2024-02-12 14:18 ` Borislav Petkov
2024-01-29 18:05 ` [PATCH v3 09/19] x86/head64: Simplify GDT/IDT initialization code Ard Biesheuvel
2024-02-12 14:37 ` Borislav Petkov
2024-02-12 15:23 ` Ard Biesheuvel
2024-01-29 18:05 ` [PATCH v3 10/19] asm-generic: Add special .pi.text section for position independent code Ard Biesheuvel
2024-01-29 18:05 ` [PATCH v3 11/19] x86: Move return_thunk to __pitext section Ard Biesheuvel
2024-01-29 18:05 ` [PATCH v3 12/19] x86/head64: Move early startup code into __pitext Ard Biesheuvel
2024-01-29 18:05 ` [PATCH v3 13/19] modpost: Warn about calls from __pitext into other text sections Ard Biesheuvel
2024-01-29 18:05 ` [PATCH v3 14/19] x86/coco: Make cc_set_mask() static inline Ard Biesheuvel
2024-01-30 23:16 ` Kevin Loughlin
2024-01-30 23:36 ` Ard Biesheuvel
2024-01-29 18:05 ` [PATCH v3 15/19] x86/sev: Make all code reachable from 1:1 mapping __pitext Ard Biesheuvel
2024-01-29 18:05 ` [PATCH v3 16/19] x86/sev: Avoid WARN() in early code Ard Biesheuvel
2024-01-29 18:05 ` [PATCH v3 17/19] x86/sev: Use PIC codegen for early SEV startup code Ard Biesheuvel
2024-01-29 18:05 ` [PATCH v3 18/19] x86/sev: Drop inline asm LEA instructions for RIP-relative references Ard Biesheuvel
2024-01-29 18:05 ` [PATCH v3 19/19] x86/startup_64: Don't bother setting up GS before the kernel is mapped Ard Biesheuvel
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