[RFC PATCH] efi/libstub/arm64: Retain 2MB kernel Image alignment if !KASLR

[Ard Biesheuvel]

On Fri, 10 Jul 2020 at 00:12, Will Deacon <will at kernel.org> wrote:
>
> Since commit 82046702e288 ("efi/libstub/arm64: Replace 'preferred' offset
> with alignment check"), loading a relocatable arm64 kernel at a physical
> address which is not 2MB aligned and subsequently booting with EFI will
> leave the Image in-place, relying on the kernel to relocate itself early
> during boot. In conjunction with commit dd4bc6076587 ("arm64: warn on
> incorrect placement of the kernel by the bootloader"), which enables
> CONFIG_RELOCATABLE by default, this effectively means that entering an
> arm64 kernel loaded at an alignment smaller than 2MB with EFI (e.g. using
> QEMU) will result in silent relocation at runtime.
>
> Unfortunately, this has a subtle but confusing affect for developers
> trying to inspect the PC value during a crash and comparing it to the
> symbol addresses in vmlinux using tools such as 'nm' or 'addr2line';
> all text addresses will be displaced by a sub-2MB offset, resulting in
> the wrong symbol being identified in many cases. Passing "nokaslr" on
> the command line or disabling "CONFIG_RANDOMIZE_BASE" does not help,
> since the EFI stub only copies the kernel Image to a 2MB boundary if it
> is not relocatable.
>
> Adjust the EFI stub for arm64 so that the minimum Image alignment is 2MB
> unless KASLR is in use.
>
> Cc: Ard Biesheuvel <ardb at kernel.org>
> Cc: Mark Rutland <mark.rutland at arm.com>
> Cc: Catalin Marinas <catalin.marinas at arm.com>
> Cc: Marc Zyngier <maz at kernel.org>
> Cc: David Brazdil <dbrazdil at google.com>
> Signed-off-by: Will Deacon <will at kernel.org>
> ---
>
> Not massively happy with this patch, but also not massively happy that
> addr2line is so misleading without it. Thoughts?
>

I think the approach is fine, as disabling kaslr at compile time as
well as at boot time will produce the desired results. The only reason
we enabled relocatable kernels without kaslr in the first place was to
work around broken non-efi loaders.

Acked-by: Ard Biesheuvel <ardb at kernel.org>

>  drivers/firmware/efi/libstub/arm64-stub.c     | 25 +++++++++++--------
>  .../firmware/efi/libstub/efi-stub-helper.c    |  2 +-
>  2 files changed, 15 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/firmware/efi/libstub/arm64-stub.c b/drivers/firmware/efi/libstub/arm64-stub.c
> index 7f6a57dec513..e5bfac79e5ac 100644
> --- a/drivers/firmware/efi/libstub/arm64-stub.c
> +++ b/drivers/firmware/efi/libstub/arm64-stub.c
> @@ -35,13 +35,16 @@ efi_status_t check_platform_features(void)
>  }
>
>  /*
> - * Relocatable kernels can fix up the misalignment with respect to
> - * MIN_KIMG_ALIGN, so they only require a minimum alignment of EFI_KIMG_ALIGN
> - * (which accounts for the alignment of statically allocated objects such as
> - * the swapper stack.)
> + * Although relocatable kernels can fix up the misalignment with respect to
> + * MIN_KIMG_ALIGN, the resulting virtual text addresses are subtly out of
> + * sync with those recorded in the vmlinux when kaslr is disabled but the
> + * image required relocation anyway. Therefore retain 2M alignment unless
> + * KASLR is in use.
>   */
> -static const u64 min_kimg_align = IS_ENABLED(CONFIG_RELOCATABLE) ? EFI_KIMG_ALIGN
> -                                                                : MIN_KIMG_ALIGN;
> +static u64 min_kimg_align(void)
> +{
> +       return efi_nokaslr ? MIN_KIMG_ALIGN : EFI_KIMG_ALIGN;
> +}
>
>  efi_status_t handle_kernel_image(unsigned long *image_addr,
>                                  unsigned long *image_size,
> @@ -74,21 +77,21 @@ efi_status_t handle_kernel_image(unsigned long *image_addr,
>
>         kernel_size = _edata - _text;
>         kernel_memsize = kernel_size + (_end - _edata);
> -       *reserve_size = kernel_memsize + TEXT_OFFSET % min_kimg_align;
> +       *reserve_size = kernel_memsize + TEXT_OFFSET % min_kimg_align();
>
>         if (IS_ENABLED(CONFIG_RANDOMIZE_BASE) && phys_seed != 0) {
>                 /*
>                  * If KASLR is enabled, and we have some randomness available,
>                  * locate the kernel at a randomized offset in physical memory.
>                  */
> -               status = efi_random_alloc(*reserve_size, min_kimg_align,
> +               status = efi_random_alloc(*reserve_size, min_kimg_align(),
>                                           reserve_addr, phys_seed);
>         } else {
>                 status = EFI_OUT_OF_RESOURCES;
>         }
>
>         if (status != EFI_SUCCESS) {
> -               if (IS_ALIGNED((u64)_text - TEXT_OFFSET, min_kimg_align)) {
> +               if (IS_ALIGNED((u64)_text - TEXT_OFFSET, min_kimg_align())) {
>                         /*
>                          * Just execute from wherever we were loaded by the
>                          * UEFI PE/COFF loader if the alignment is suitable.
> @@ -99,7 +102,7 @@ efi_status_t handle_kernel_image(unsigned long *image_addr,
>                 }
>
>                 status = efi_allocate_pages_aligned(*reserve_size, reserve_addr,
> -                                                   ULONG_MAX, min_kimg_align);
> +                                                   ULONG_MAX, min_kimg_align());
>
>                 if (status != EFI_SUCCESS) {
>                         efi_err("Failed to relocate kernel\n");
> @@ -108,7 +111,7 @@ efi_status_t handle_kernel_image(unsigned long *image_addr,
>                 }
>         }
>
> -       *image_addr = *reserve_addr + TEXT_OFFSET % min_kimg_align;
> +       *image_addr = *reserve_addr + TEXT_OFFSET % min_kimg_align();
>         memcpy((void *)*image_addr, _text, kernel_size);
>
>         return EFI_SUCCESS;
> diff --git a/drivers/firmware/efi/libstub/efi-stub-helper.c b/drivers/firmware/efi/libstub/efi-stub-helper.c
> index d40fd68c6bb2..6bca70bbb43d 100644
> --- a/drivers/firmware/efi/libstub/efi-stub-helper.c
> +++ b/drivers/firmware/efi/libstub/efi-stub-helper.c
> @@ -19,7 +19,7 @@
>  #include "efistub.h"
>
>  bool efi_nochunk;
> -bool efi_nokaslr;
> +bool efi_nokaslr = !IS_ENABLED(CONFIG_RANDOMIZE_BASE);
>  bool efi_noinitrd;
>  int efi_loglevel = CONSOLE_LOGLEVEL_DEFAULT;
>  bool efi_novamap;
> --
> 2.27.0.383.g050319c2ae-goog
>