linux-kernel - Re: [PATCH 3/9] efi/x86: Move efi stub globals from .bss to .data

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Message-ID: <CAMzpN2gJWwVun1Kp6vGuza9LM5KpB=0EwsP8x8eOJQuDGh38Hg@mail.gmail.com>
Date:   Thu, 9 Apr 2020 16:05:33 -0400
From:   Brian Gerst <brgerst@...il.com>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     linux-efi@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Borislav Petkov <bp@...e.de>,
        Colin Ian King <colin.king@...onical.com>,
        Gary Lin <glin@...e.com>, Jiri Slaby <jslaby@...e.cz>,
        Sergey Shatunov <me@...k.pw>, Takashi Iwai <tiwai@...e.de>
Subject: Re: [PATCH 3/9] efi/x86: Move efi stub globals from .bss to .data

On Thu, Apr 9, 2020 at 9:07 AM Ard Biesheuvel <ardb@...nel.org> wrote:
>
> From: Arvind Sankar <nivedita@...m.mit.edu>
>
> Commit
>
>   3ee372ccce4d ("x86/boot/compressed/64: Remove .bss/.pgtable from bzImage")
>
> removed the .bss section from the bzImage.
>
> However, while a PE loader is required to zero-initialize the .bss
> section before calling the PE entry point, the EFI handover protocol
> does not currently document any requirement that .bss be initialized by
> the bootloader prior to calling the handover entry.
>
> When systemd-boot is used to boot a unified kernel image [1], the image
> is constructed by embedding the bzImage as a .linux section in a PE
> executable that contains a small stub loader from systemd together with
> additional sections and potentially an initrd. As the .bss section
> within the bzImage is no longer explicitly present as part of the file,
> it is not initialized before calling the EFI handover entry.
> Furthermore, as the size of the embedded .linux section is only the size
> of the bzImage file itself, the .bss section's memory may not even have
> been allocated.
>
> In particular, this can result in efi_disable_pci_dma being true even
> when it was not specified via the command line or configuration option,
> which in turn causes crashes while booting on some systems.
>
> To avoid issues, place all EFI stub global variables into the .data
> section instead of .bss. As of this writing, only boolean flags for a
> few command line arguments and the sys_table pointer were in .bss and
> will now move into the .data section.
>
> [1] https://systemd.io/BOOT_LOADER_SPECIFICATION/#type-2-efi-unified-kernel-images
>
> Signed-off-by: Arvind Sankar <nivedita@...m.mit.edu>
> Reported-by: Sergey Shatunov <me@...k.pw>
> Fixes: 3ee372ccce4d ("x86/boot/compressed/64: Remove .bss/.pgtable from bzImage")
> Link: https://lore.kernel.org/r/20200406180614.429454-1-nivedita@alum.mit.edu
> Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
> ---
>  drivers/firmware/efi/libstub/efistub.h  | 2 +-
>  drivers/firmware/efi/libstub/x86-stub.c | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/firmware/efi/libstub/efistub.h b/drivers/firmware/efi/libstub/efistub.h
> index cc90a748bcf0..67d26949fd26 100644
> --- a/drivers/firmware/efi/libstub/efistub.h
> +++ b/drivers/firmware/efi/libstub/efistub.h
> @@ -25,7 +25,7 @@
>  #define EFI_ALLOC_ALIGN                EFI_PAGE_SIZE
>  #endif
>
> -#ifdef CONFIG_ARM
> +#if defined(CONFIG_ARM) || defined(CONFIG_X86)
>  #define __efistub_global       __section(.data)
>  #else
>  #define __efistub_global
> diff --git a/drivers/firmware/efi/libstub/x86-stub.c b/drivers/firmware/efi/libstub/x86-stub.c
> index e02ea51273ff..867a57e28980 100644
> --- a/drivers/firmware/efi/libstub/x86-stub.c
> +++ b/drivers/firmware/efi/libstub/x86-stub.c
> @@ -20,7 +20,7 @@
>  /* Maximum physical address for 64-bit kernel with 4-level paging */
>  #define MAXMEM_X86_64_4LEVEL (1ull << 46)
>
> -static efi_system_table_t *sys_table;
> +static efi_system_table_t *sys_table __efistub_global;
>  extern const bool efi_is64;
>  extern u32 image_offset;
>
> --
> 2.17.1
>

Can we use the -fno-zero-initialized-in-bss compiler flag instead of
explicitly marking global variables?

--
Brian Gerst