linux-kernel - Re: [PATCH] x86/efistub: fix the missing KASLR_FLAG bit in boot

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Date: Tue, 2 Jan 2024 16:29:14 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: Yuntao Wang <ytcoode@...il.com>
Cc: linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org, 
	"Borislav Petkov (AMD)" <bp@...en8.de>, "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, 
	Tom Lendacky <thomas.lendacky@....com>, Ingo Molnar <mingo@...nel.org>, 
	Dionna Glaze <dionnaglaze@...gle.com>, Nikolay Borisov <nik.borisov@...e.com>
Subject: Re: [PATCH] x86/efistub: fix the missing KASLR_FLAG bit in boot_params->hdr.loadflags

On Tue, 26 Dec 2023 at 15:03, Yuntao Wang <ytcoode@...il.com> wrote:
>
> When KASLR is enabled, the KASLR_FLAG bit in boot_params->hdr.loadflags
> should be set to 1 to propagate KASLR status from compressed kernel to
> kernel, just as the choose_random_location() function does.
>
> Currently, when kernel is booted via efi stub, the KASLR_FLAG bit in
> boot_params->hdr.loadflags is not set, even though it should be. This
> causes some functions, such as kernel_randomize_memory(), not to execute
> as expected. Fix it.
>
> Signed-off-by: Yuntao Wang <ytcoode@...il.com>
> ---
>  drivers/firmware/efi/libstub/x86-stub.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/firmware/efi/libstub/x86-stub.c b/drivers/firmware/efi/libstub/x86-stub.c
> index da9b7b8d0716..b0c68593ad40 100644
> --- a/drivers/firmware/efi/libstub/x86-stub.c
> +++ b/drivers/firmware/efi/libstub/x86-stub.c
> @@ -787,7 +787,10 @@ static efi_status_t efi_decompress_kernel(unsigned long *kernel_entry)
>                         efi_debug("AMI firmware v2.0 or older detected - disabling physical KASLR\n");
>                         seed[0] = 0;
>                 }
> -       }
> +
> +               boot_params_ptr->hdr.loadflags |= KASLR_FLAG;
> +       } else
> +               boot_params_ptr->hdr.loadflags &= ~KASLR_FLAG;
>

Thanks for the fix.

I'll queue this up right away, but I am going to drop the 'else' part,
given that the KASLR flag is never set by the EFI stub so clearing it
should never be needed.


>         status = efi_random_alloc(alloc_size, CONFIG_PHYSICAL_ALIGN, &addr,
>                                   seed[0], EFI_LOADER_CODE,
> --
> 2.43.0
>