linux-kernel - Re: [PATCH 1/1] riscv: efi: Set NX compat flag in PE/COFF header

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Message-ID: <811ea10e-3bf1-45a5-a407-c09ec5756b48@canonical.com>
Date: Tue, 1 Oct 2024 17:24:21 +0200
From: Heinrich Schuchardt <heinrich.schuchardt@...onical.com>
To: Alexandre Ghiti <alex@...ti.fr>
Cc: Ard Biesheuvel <ardb@...nel.org>,
 Emil Renner Berthing <emil.renner.berthing@...onical.com>,
 linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
 stable@...r.kernel.org, Paul Walmsley <paul.walmsley@...ive.com>,
 Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>
Subject: Re: [PATCH 1/1] riscv: efi: Set NX compat flag in PE/COFF header

On 01.10.24 15:51, Alexandre Ghiti wrote:
> Hi Heinrich,
> 
> On 29/09/2024 16:02, Heinrich Schuchardt wrote:
>> The IMAGE_DLLCHARACTERISTICS_NX_COMPAT informs the firmware that the
>> EFI binary does not rely on pages that are both executable and
>> writable.
>>
>> The flag is used by some distro versions of GRUB to decide if the EFI
>> binary may be executed.
>>
>> As the Linux kernel neither has RWX sections nor needs RWX pages for
>> relocation we should set the flag.
>>
>> Cc: Ard Biesheuvel <ardb@...nel.org>
>> Cc: <stable@...r.kernel.org>
>> Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@...onical.com>
>> ---
>>   arch/riscv/kernel/efi-header.S | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/riscv/kernel/efi-header.S b/arch/riscv/kernel/efi- 
>> header.S
>> index 515b2dfbca75..c5f17c2710b5 100644
>> --- a/arch/riscv/kernel/efi-header.S
>> +++ b/arch/riscv/kernel/efi-header.S
>> @@ -64,7 +64,7 @@ extra_header_fields:
>>       .long    efi_header_end - _start            // SizeOfHeaders
>>       .long    0                    // CheckSum
>>       .short    IMAGE_SUBSYSTEM_EFI_APPLICATION        // Subsystem
>> -    .short    0                    // DllCharacteristics
>> +    .short    IMAGE_DLL_CHARACTERISTICS_NX_COMPAT    // 
>> DllCharacteristics
>>       .quad    0                    // SizeOfStackReserve
>>       .quad    0                    // SizeOfStackCommit
>>       .quad    0                    // SizeOfHeapReserve
> 
> 
> I don't understand if this fixes something or not: what could go wrong 
> if we don't do this?
> 
> Thanks,
> 
> Alex
> 


Hello Alexandre,

https://learn.microsoft.com/en-us/windows-hardware/drivers/bringup/uefi-ca-memory-mitigation-requirements
describes Microsoft's effort to improve security by avoiding memory 
pages that are both executable and writable.

IMAGE_DLL_CHARACTERISTICS_NX_COMPAT is an assertion by the EFI binary 
that it does not use RWX pages. It may use the 
EFI_MEMORY_ATTRIBUTE_PROTOCOL to set whether a page is writable or 
executable (but not both).

When using secure boot, compliant firmware will not allow loading a 
binary if the flag is not set.

Best regards

Heinrich