Re: [RFC PATCH 27/28] x86/kernel: Switch to PIE linking for the core kernel

Uros Bizjak
2024-09-25T18:54:41Z

On Wed, Sep 25, 2024 at 5:02 PM Ard Biesheuvel <ardb+git@google.com> wrote:
>
> From: Ard Biesheuvel <ardb@kernel.org>
>
> Build the kernel as a Position Independent Executable (PIE). This
> results in more efficient relocation processing for the virtual
> displacement of the kernel (for KASLR). More importantly, it instructs
> the linker to generate what is actually needed (a program that can be
> moved around in memory before execution), which is better than having to
> rely on the linker to create a position dependent binary that happens to
> tolerate being moved around after poking it in exactly the right manner.
>
> Note that this means that all codegen should be compatible with PIE,
> including Rust objects, so this needs to switch to the small code model
> with the PIE relocation model as well.

I think that related to this work is the patch series [1] that
introduces the changes necessary to build the kernel as Position
Independent Executable (PIE) on x86_64 [1]. There are some more places
that need to be adapted for PIE. The patch series also introduces
objtool functionality to add validation for x86 PIE.

[1] "[PATCH RFC 00/43] x86/pie: Make kernel image's virtual address flexible"
https://lore.kernel.org/lkml/cover.1682673542.git.houwenlong.hwl@antgroup.com/

Uros.

>
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
> arch/x86/Kconfig | 2 +-
> arch/x86/Makefile | 11 +++++++----
> arch/x86/boot/compressed/misc.c | 2 ++
> arch/x86/kernel/vmlinux.lds.S | 5 +++++
> drivers/firmware/efi/libstub/x86-stub.c | 2 ++
> 5 files changed, 17 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 54cb1f14218b..dbb4d284b0e1 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -2187,7 +2187,7 @@ config RANDOMIZE_BASE
> # Relocation on x86 needs some additional build support
> config X86_NEED_RELOCS
> def_bool y
> - depends on RANDOMIZE_BASE || (X86_32 && RELOCATABLE)
> + depends on X86_32 && RELOCATABLE
>
> config PHYSICAL_ALIGN
> hex "Alignment value to which kernel should be aligned"
> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index 83d20f402535..c1dcff444bc8 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -206,9 +206,8 @@ else
> PIE_CFLAGS-$(CONFIG_SMP) += -mstack-protector-guard-reg=gs
> endif
>
> - # Don't emit relaxable GOTPCREL relocations
> - KBUILD_AFLAGS_KERNEL += -Wa,-mrelax-relocations=no
> - KBUILD_CFLAGS_KERNEL += -Wa,-mrelax-relocations=no $(PIE_CFLAGS-y)
> + KBUILD_CFLAGS_KERNEL += $(PIE_CFLAGS-y)
> + KBUILD_RUSTFLAGS_KERNEL += -Ccode-model=small -Crelocation-model=pie
> endif
>
> #
> @@ -264,12 +263,16 @@ else
> LDFLAGS_vmlinux :=
> endif
>
> +ifdef CONFIG_X86_64
> +ldflags-pie-$(CONFIG_LD_IS_LLD) := --apply-dynamic-relocs
> +ldflags-pie-$(CONFIG_LD_IS_BFD) := -z call-nop=suffix-nop
> +LDFLAGS_vmlinux += --pie -z text $(ldflags-pie-y)
> +
> #
> # The 64-bit kernel must be aligned to 2MB. Pass -z max-page-size=0x200000 to
> # the linker to force 2MB page size regardless of the default page size used
> # by the linker.
> #
> -ifdef CONFIG_X86_64
> LDFLAGS_vmlinux += -z max-page-size=0x200000
> endif
>
> diff --git a/arch/x86/boot/compressed/misc.c b/arch/x86/boot/compressed/misc.c
> index 89f01375cdb7..79e3ffe16f61 100644
> --- a/arch/x86/boot/compressed/misc.c
> +++ b/arch/x86/boot/compressed/misc.c
> @@ -495,6 +495,8 @@ asmlinkage __visible void *extract_kernel(void *rmode, unsigned char *output)
> error("Destination virtual address changed when not relocatable");
> #endif
>
> + boot_params_ptr->kaslr_va_shift = virt_addr - LOAD_PHYSICAL_ADDR;
> +
> debug_putstr("\nDecompressing Linux... ");
>
> if (init_unaccepted_memory()) {
> diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
> index f7e832c2ac61..d172e6e8eaaf 100644
> --- a/arch/x86/kernel/vmlinux.lds.S
> +++ b/arch/x86/kernel/vmlinux.lds.S
> @@ -459,6 +459,11 @@ xen_elfnote_phys32_entry_offset =
>
> DISCARDS
>
> + /DISCARD/ : {
> + *(.dynsym .gnu.hash .hash .dynamic .dynstr)
> + *(.interp .dynbss .eh_frame .sframe)
> + }
> +
> /*
> * Make sure that the .got.plt is either completely empty or it
> * contains only the lazy dispatch entries.
> diff --git a/drivers/firmware/efi/libstub/x86-stub.c b/drivers/firmware/efi/libstub/x86-stub.c
> index f8e465da344d..5c03954924fe 100644
> --- a/drivers/firmware/efi/libstub/x86-stub.c
> +++ b/drivers/firmware/efi/libstub/x86-stub.c
> @@ -912,6 +912,8 @@ static efi_status_t efi_decompress_kernel(unsigned long *kernel_entry)
> if (status != EFI_SUCCESS)
> return status;
>
> + boot_params_ptr->kaslr_va_shift = virt_addr - LOAD_PHYSICAL_ADDR;
> +
> entry = decompress_kernel((void *)addr, virt_addr, error);
> if (entry == ULONG_MAX) {
> efi_free(alloc_size, addr);
> --
> 2.46.0.792.g87dc391469-goog
>