Re: [PATCH v2] efi/x86: Setup stack correctly for efi_pe_entry

[Ard Biesheuvel <ardb@kernel.org>]

On Wed, 17 Jun 2020 at 15:20, Arvind Sankar <nivedita@alum.mit.edu> wrote:
>
> Commit
>   17054f492dfd ("efi/x86: Implement mixed mode boot without the handover protocol")
> introduced a new entry point for the EFI stub to be booted in mixed mode
> on 32-bit firmware.
>
> When entered via efi32_pe_entry, control is first transferred to
> startup_32 to setup for the switch to long mode, and then the EFI stub
> proper is entered via efi_pe_entry. efi_pe_entry is an MS ABI function,
> and the ABI requires 32 bytes of shadow stack space to be allocated by
> the caller, as well as the stack being aligned to 8 mod 16 on entry.
>
> Allocate 40 bytes on the stack before switching to 64-bit mode when
> calling efi_pe_entry to account for this.
>
> For robustness, explicitly align boot_stack_end to 16 bytes. It is
> currently implicitly aligned since .bss is cacheline-size aligned,
> head_64.o is the first object file with a .bss section, and the heap and
> boot sizes are aligned.
>
> Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>

Cheers

> ---
>  arch/x86/boot/compressed/head_64.S | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
> index e821a7d7d5c4..97d37f0a34f5 100644
> --- a/arch/x86/boot/compressed/head_64.S
> +++ b/arch/x86/boot/compressed/head_64.S
> @@ -213,7 +213,6 @@ SYM_FUNC_START(startup_32)
>          * We place all of the values on our mini stack so lret can
>          * used to perform that far jump.
>          */
> -       pushl   $__KERNEL_CS
>         leal    startup_64(%ebp), %eax
>  #ifdef CONFIG_EFI_MIXED
>         movl    efi32_boot_args(%ebp), %edi
> @@ -224,11 +223,20 @@ SYM_FUNC_START(startup_32)
>         movl    efi32_boot_args+8(%ebp), %edx   // saved bootparams pointer
>         cmpl    $0, %edx
>         jnz     1f
> +       /*
> +        * efi_pe_entry uses MS calling convention, which requires 32 bytes of
> +        * shadow space on the stack even if all arguments are passed in
> +        * registers. We also need an additional 8 bytes for the space that
> +        * would be occupied by the return address, and this also results in
> +        * the correct stack alignment for entry.
> +        */
> +       subl    $40, %esp
>         leal    efi_pe_entry(%ebp), %eax
>         movl    %edi, %ecx                      // MS calling convention
>         movl    %esi, %edx
>  1:
>  #endif
> +       pushl   $__KERNEL_CS
>         pushl   %eax
>
>         /* Enter paged protected Mode, activating Long Mode */
> @@ -784,6 +792,7 @@ SYM_DATA_LOCAL(boot_heap,   .fill BOOT_HEAP_SIZE, 1, 0)
>
>  SYM_DATA_START_LOCAL(boot_stack)
>         .fill BOOT_STACK_SIZE, 1, 0
> +       .balign 16
>  SYM_DATA_END_LABEL(boot_stack, SYM_L_LOCAL, boot_stack_end)
>
>  /*
> --
> 2.26.2
>