linux-kernel - Re: [tip:efi/urgent] efi: Align 'efi_guid

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Date:   Fri, 21 Dec 2018 01:08:10 +0100
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Heinrich Schuchardt <xypron.glpk@....de>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     Qian Cai <cai@....us>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Rik van Riel <riel@...riel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-tip-commits@...r.kernel.org
Subject: Re: [tip:efi/urgent] efi: Align 'efi_guid_t' to 64 bits

On Wed, 19 Dec 2018 at 23:50, Ard Biesheuvel <ard.biesheuvel@...aro.org> wrote:
>
> On Tue, 18 Dec 2018 at 00:20, Heinrich Schuchardt <xypron.glpk@....de> wrote:
> >
> > On 12/17/18 11:42 PM, Ard Biesheuvel wrote:
> > > On Mon, 17 Dec 2018 at 23:33, Heinrich Schuchardt <xypron.glpk@....de> wrote:
> > >>
> > >> On 12/17/18 7:16 PM, tip-bot for Heinrich Schuchardt wrote:
> > >>> Commit-ID:  793423cf07e51e3185b8680167115813589c057d
> > >>> Gitweb:     https://git.kernel.org/tip/793423cf07e51e3185b8680167115813589c057d
> > >>> Author:     Heinrich Schuchardt <xypron.glpk@....de>
> > >>> AuthorDate: Mon, 17 Dec 2018 19:02:14 +0100
> > >>> Committer:  Ingo Molnar <mingo@...nel.org>
> > >>> CommitDate: Mon, 17 Dec 2018 19:12:48 +0100
> > >>>
> > >>> efi: Align 'efi_guid_t' to 64 bits
> > >>>
> > >>> The UEFI Specification Version 2.7 Errata A defines:
> > >>>
> > >>>   "EFI_GUID
> > >>>   128-bit buffer containing a unique identifier value.
> > >>>   Unless otherwise specified, aligned on a 64-bit boundary."
> > >>>
> > >>> Before this patch efi_guid_t was only 8-bit aligned.
> > >>>
> > >>> Note that this could potentially trigger alignment faults during
> > >>> EFI runtime services calls on 32-bit ARM, given that it does not
> > >>> permit load/store double or load/store multiple instructions to
> > >>> operate on memory addresses that are not 32-bit aligned.
> > >>>
> > >>> Signed-off-by: Heinrich Schuchardt <xypron.glpk@....de>
> > >>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> > >>> Cc: <stable@...r.kernel.org> # v4.9+, or earlier if possible
> > >>> Cc: Andy Lutomirski <luto@...nel.org>
> > >>> Cc: Borislav Petkov <bp@...en8.de>
> > >>> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> > >>> Cc: H. Peter Anvin <hpa@...or.com>
> > >>> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> > >>> Cc: Peter Zijlstra <peterz@...radead.org>
> > >>> Cc: Qian Cai <cai@....us>
> > >>> Cc: Rik van Riel <riel@...riel.com>
> > >>> Cc: Thomas Gleixner <tglx@...utronix.de>
> > >>> Cc: linux-efi@...r.kernel.org
> > >>> Link: http://lkml.kernel.org/r/20181217180214.9436-3-ard.biesheuvel@linaro.org
> > >>> Signed-off-by: Ingo Molnar <mingo@...nel.org>
> > >>> ---
> > >>>  include/linux/efi.h | 2 +-
> > >>>  1 file changed, 1 insertion(+), 1 deletion(-)
> > >>>
> > >>> diff --git a/include/linux/efi.h b/include/linux/efi.h
> > >>> index 100ce4a4aff6..e6480c805932 100644
> > >>> --- a/include/linux/efi.h
> > >>> +++ b/include/linux/efi.h
> > >>> @@ -48,7 +48,7 @@ typedef u16 efi_char16_t;           /* UNICODE character */
> > >>>  typedef u64 efi_physical_addr_t;
> > >>>  typedef void *efi_handle_t;
> > >>>
> > >>> -typedef guid_t efi_guid_t;
> > >>> +
> > >>>
> > >>>  #define EFI_GUID(a,b,c,d0,d1,d2,d3,d4,d5,d6,d7) \
> > >>>       GUID_INIT(a, b, c, d0, d1, d2, d3, d4, d5, d6, d7)
> > >>>
> > >>
> > >> Before rushing this patch in, we should carefully review its side
> > >> effects, e.g. on 32bit system this changes the size of
> > >> efi_config_table_32_t from 20 to 24, which is part of the interface to
> > >> the UEFI firmware.
> > >>
> > >
> > > grmbl.
> > >
> > > Thanks for spotting that.
> > >
> > > The UEFI spec defines a GUID struct as { UINT32; UINT16; UINT16;
> > > UINT8[8]; } so its natural alignment is 32 bits not 64 bits. The
> > > alignment issue on ARM would be solved by using __aligned(4) rather
> > > than __aligned(8), while not affecting the size of the config table
> > > struct (and potentially others) on 32-bit architectures.
> > >
> > > Ingo, apologies for the breakage. Do you prefer a replacement patch or
> > > a followup patch?
> > >
> >
> > The UEFI spec explicitly requires EFI_GUID to be 64-bit aligned. On the
> > other hand neither EDK2 nor GRUB not U-Boot cared about this requirement
> > up to now. So the array of efi_config_table_32_t had 20 byte long
> > members at least on Linux, EDK2, GRUB, U-Boot, and possibly on other
> > UEFI implementations though the UEFI spec does not mention packing here.
> >
> > As the 4.20 kernel release is imminent, please, keep this patch out
> > before causing breakage.
> >
>
> Ingo,
>
> Let's just drop Heinrich's patch for now, and I will come back to it
> for the next release. There is very little EFI on 32-bit ARM in the
> field, and this patch in its current form will break IA-32 as well, so
> a quick followup fix is probably not the best approach here.

Ingo,

Could you please let me know what your plans are with this patch? It
has been pulled into -next now.

Please either drop it or revert it.