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

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Date: Tue, 2 Jan 2024 10:26:05 -0600
From: Tom Lendacky <thomas.lendacky@....com>
To: Ard Biesheuvel <ardb@...nel.org>, 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>,
 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 1/2/24 09:29, Ard Biesheuvel wrote:
> 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.

Does it need a Fixes: tag?

Thanks,
Tom

> 
> 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
>>