linux-kernel - Re: [PATCH v2 2/2] x86/efi: Apply EFI Memory Attributes after kexec

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Message-ID: <CAMj1kXGopsux6+xnsXW6vvQDJH9Y3_Ofq_QYvDa-SGt8AJ0nWQ@mail.gmail.com>
Date: Fri, 15 Nov 2024 17:39:45 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: Nicolas Saenz Julienne <nsaenz@...zon.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H . Peter Anvin" <hpa@...or.com>, Matt Fleming <matt@...eblueprint.co.uk>, linux-efi@...r.kernel.org, 
	linux-kernel@...r.kernel.org, stanspas@...zon.de, nh-open-source@...zon.com, 
	stable@...r.kernel.org
Subject: Re: [PATCH v2 2/2] x86/efi: Apply EFI Memory Attributes after kexec

On Tue, 12 Nov 2024 at 19:53, Nicolas Saenz Julienne <nsaenz@...zon.com> wrote:
>
> Kexec bypasses EFI's switch to virtual mode. In exchange, it has its own
> routine, kexec_enter_virtual_mode(), which replays the mappings made by
> the original kernel. Unfortunately, that function fails to reinstate
> EFI's memory attributes, which would've otherwise been set after
> entering virtual mode. Remediate this by calling
> efi_runtime_update_mappings() within kexec's routine.
>
> Cc: stable@...r.kernel.org
> Fixes: 18141e89a76c ("x86/efi: Add support for EFI_MEMORY_ATTRIBUTES_TABLE")
> Signed-off-by: Nicolas Saenz Julienne <nsaenz@...zon.com>
>
> ---
>
> Notes:
> - Tested with QEMU/OVMF.
>


I'll queue these up, but I am going drop the cc stable: the memory
attributes table is an overlay of the EFI memory map with restricted
permissions for EFI runtime services regions, which are only mapped
while a EFI runtime call is in progress.

So if the table is not taken into account after kexec, the runtime
code and data mappings will all be RWX but I think this is a situation
we can live with. If nothing breaks, we can always revisit this later
if there is an actual need.

Thanks,


>  arch/x86/platform/efi/efi.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/arch/x86/platform/efi/efi.c b/arch/x86/platform/efi/efi.c
> index 375ebd78296a..a7ff189421c3 100644
> --- a/arch/x86/platform/efi/efi.c
> +++ b/arch/x86/platform/efi/efi.c
> @@ -765,6 +765,7 @@ static void __init kexec_enter_virtual_mode(void)
>
>         efi_sync_low_kernel_mappings();
>         efi_native_runtime_setup();
> +       efi_runtime_update_mappings();
>  #endif
>  }
>
> --
> 2.40.1
>