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

[Heinrich Schuchardt]

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@xxxxxxxxxx>
> > Cc: <stable@xxxxxxxxxxxxxxx>
> > Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@xxxxxxxxxxxxx>
> > ---
> >   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