Re: [RFC PATCH 4/5] x86/head64: Replace pointer fixups with PIE codegen

[Brian Gerst]

On Mon, Jan 22, 2024 at 4:14 AM Ard Biesheuvel <ardb+git@xxxxxxxxxx> wrote:
>
> From: Ard Biesheuvel <ardb@xxxxxxxxxx>
>
> 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
> 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
> 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.
>
> While at it, move these routines to .inittext where they belong.
>
> Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
> ---
>  arch/x86/Makefile                 |  11 ++
>  arch/x86/boot/compressed/Makefile |   2 +-
>  arch/x86/include/asm/init.h       |   2 -
>  arch/x86/include/asm/setup.h      |   2 +-
>  arch/x86/kernel/Makefile          |   4 +
>  arch/x86/kernel/head64.c          | 117 +++++++-------------
>  6 files changed, 60 insertions(+), 78 deletions(-)
>
> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index 1a068de12a56..bed0850d91b0 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -168,6 +168,17 @@ else
>          KBUILD_CFLAGS += -mcmodel=kernel
>          KBUILD_RUSTFLAGS += -Cno-redzone=y
>          KBUILD_RUSTFLAGS += -Ccode-model=kernel
> +
> +       PIE_CFLAGS := -fpie -mcmodel=small \
> +                     -include $(srctree)/include/linux/hidden.h
> +
> +       ifeq ($(CONFIG_STACKPROTECTOR),y)
> +               ifeq ($(CONFIG_SMP),y)
> +                       PIE_CFLAGS += -mstack-protector-guard-reg=gs
> +               endif

This compiler flag requires GCC 8.1 or later.  When I posted a patch
series[1] to convert the stack protector to a normal percpu variable
instead of the fixed offset, there was pushback over requiring GCC 8.1
to keep stack protector support.  I added code to objtool to convert
code from older compilers, but there hasn't been any feedback since.
Similar conversion code would be needed in objtool for this unless the
decision is made to require GCC 8.1 for stack protector support going
forward.

Brian Gerst

[1] https://lore.kernel.org/lkml/20231115173708.108316-1-brgerst@xxxxxxxxx/