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

[Uros Bizjak <ubizjak@xxxxxxxxx>]

On Wed, Sep 25, 2024 at 5:02 PM Ard Biesheuvel <ardb+git@xxxxxxxxxx> wrote:
>
> From: Ard Biesheuvel <ardb@xxxxxxxxxx>
>
> 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@xxxxxxxxxxxx/

Uros.

>
> Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
> ---
>  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
>