Re: [PATCH] x86/efi: Restore Firmware IDT in before ExitBootServices()

[Ard Biesheuvel <ardb@kernel.org>]

On Fri, 20 Aug 2021 at 09:34, Joerg Roedel <joro@8bytes.org> wrote:
>
> From: Joerg Roedel <jroedel@suse.de>
>
> Commit 79419e13e808 ("x86/boot/compressed/64: Setup IDT in startup_32
> boot path") introduced an IDT into the 32 bit boot path of the
> decompressor stub.  But the IDT is set up before ExitBootServices() is
> called and some UEFI firmwares rely on their own IDT.
>
> Save the firmware IDT on boot and restore it before calling into EFI
> functions to fix boot failures introduced by above commit.
>
> Reported-by: Fabio Aiuto <fabioaiuto83@gmail.com>
> Fixes: 79419e13e808 ("x86/boot/compressed/64: Setup IDT in startup_32 boot path")
> Cc: stable@vger.kernel.org # 5.13+
> Signed-off-by: Joerg Roedel <jroedel@suse.de>

Acked by: Ard Biesheuvel <ardb@kernel.org>

One nit below

> ---
>  arch/x86/boot/compressed/efi_thunk_64.S | 23 ++++++++++++++++++-----
>  arch/x86/boot/compressed/head_64.S      |  3 +++
>  2 files changed, 21 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/boot/compressed/efi_thunk_64.S b/arch/x86/boot/compressed/efi_thunk_64.S
> index 95a223b3e56a..99cfd5dea23c 100644
> --- a/arch/x86/boot/compressed/efi_thunk_64.S
> +++ b/arch/x86/boot/compressed/efi_thunk_64.S
> @@ -39,7 +39,7 @@ SYM_FUNC_START(__efi64_thunk)
>         /*
>          * Convert x86-64 ABI params to i386 ABI
>          */
> -       subq    $32, %rsp
> +       subq    $64, %rsp

Any reason in particular for the increase by 32?

>         movl    %esi, 0x0(%rsp)
>         movl    %edx, 0x4(%rsp)
>         movl    %ecx, 0x8(%rsp)
> @@ -49,14 +49,19 @@ SYM_FUNC_START(__efi64_thunk)
>         leaq    0x14(%rsp), %rbx
>         sgdt    (%rbx)
>
> +       addq    $16, %rbx
> +       sidt    (%rbx)
> +
>         /*
> -        * Switch to gdt with 32-bit segments. This is the firmware GDT
> -        * that was installed when the kernel started executing. This
> -        * pointer was saved at the EFI stub entry point in head_64.S.
> +        * Switch to idt and gdt with 32-bit segments. This is the firmware GDT
> +        * and IDT that was installed when the kernel started executing. The
> +        * pointers were saved at the EFI stub entry point in head_64.S.
>          *
>          * Pass the saved DS selector to the 32-bit code, and use far return to
>          * restore the saved CS selector.
>          */
> +       leaq    efi32_boot_idt(%rip), %rax
> +       lidt    (%rax)
>         leaq    efi32_boot_gdt(%rip), %rax
>         lgdt    (%rax)
>
> @@ -67,7 +72,7 @@ SYM_FUNC_START(__efi64_thunk)
>         pushq   %rax
>         lretq
>
> -1:     addq    $32, %rsp
> +1:     addq    $64, %rsp
>         movq    %rdi, %rax
>
>         pop     %rbx
> @@ -132,6 +137,9 @@ SYM_FUNC_START_LOCAL(efi_enter32)
>          */
>         cli
>
> +       lidtl   (%ebx)
> +       subl    $16, %ebx
> +
>         lgdtl   (%ebx)
>
>         movl    %cr4, %eax
> @@ -166,6 +174,11 @@ SYM_DATA_START(efi32_boot_gdt)
>         .quad   0
>  SYM_DATA_END(efi32_boot_gdt)
>
> +SYM_DATA_START(efi32_boot_idt)
> +       .word   0
> +       .quad   0
> +SYM_DATA_END(efi32_boot_idt)
> +
>  SYM_DATA_START(efi32_boot_cs)
>         .word   0
>  SYM_DATA_END(efi32_boot_cs)
> diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
> index a2347ded77ea..572c535cf45b 100644
> --- a/arch/x86/boot/compressed/head_64.S
> +++ b/arch/x86/boot/compressed/head_64.S
> @@ -319,6 +319,9 @@ SYM_INNER_LABEL(efi32_pe_stub_entry, SYM_L_LOCAL)
>         movw    %cs, rva(efi32_boot_cs)(%ebp)
>         movw    %ds, rva(efi32_boot_ds)(%ebp)
>
> +       /* Store firmware IDT descriptor */
> +       sidtl   rva(efi32_boot_idt)(%ebp)
> +
>         /* Disable paging */
>         movl    %cr0, %eax
>         btrl    $X86_CR0_PG_BIT, %eax
> --
> 2.32.0
>