Still required that EFI_MEMORY_XP and EFI_MEMORY_RO must not both be cleared?

Still required that EFI_MEMORY_XP and EFI_MEMORY_RO must not both be cleared?

[Heiner Kallweit]

I'm one of the people seeing the following warning:

efi: memattr: Entry attributes invalid: RO and XP bits both cleared
efi: memattr: ! 0x000000090000-0x000000090fff [Runtime Code|RUN|  |  |  |  |  |  |  |  |   |  |  |  |  ]

Out of curiosity I checked UEFI spec 2.9. On p.108 there's table 4.1:

                                                   EFI_MEMORY_RO EFI_MEMORY_XP EFI_MEMORY_RUNTIME
No memory access protection is possible for Entry  0             0             1
Write-protected Code                               1             0             1
Read/Write Data                                    0             1             1
Read-only Data                                     1             1             1

So it seems to be valid that both attributes are cleared.
Also in the surrounding text I didn't find a hint mandating
that at least one the attributes has to be set.

Re: Still required that EFI_MEMORY_XP and EFI_MEMORY_RO must not both be cleared?

[Ard Biesheuvel]

On Fri, 30 Apr 2021 at 10:06, Heiner Kallweit <hkallweit1@gmail.com> wrote:
>
> I'm one of the people seeing the following warning:
>
> efi: memattr: Entry attributes invalid: RO and XP bits both cleared
> efi: memattr: ! 0x000000090000-0x000000090fff [Runtime Code|RUN|  |  |  |  |  |  |  |  |   |  |  |  |  ]
>
> Out of curiosity I checked UEFI spec 2.9. On p.108 there's table 4.1:
>
>                                                    EFI_MEMORY_RO EFI_MEMORY_XP EFI_MEMORY_RUNTIME
> No memory access protection is possible for Entry  0             0             1
> Write-protected Code                               1             0             1
> Read/Write Data                                    0             1             1
> Read-only Data                                     1             1             1
>
> So it seems to be valid that both attributes are cleared.
> Also in the surrounding text I didn't find a hint mandating
> that at least one the attributes has to be set.

You are right. My assumption at the time was that not having either of
RO or XP in the memory attributes table makes little sense, as it
describes the default case. However, this interpretation may conflict
with the requirement that entries in the memory attributes table cover
an entry in the EFI memory map entirely, or not at all. IOW, if a
memory map entry consists of a code region, a data region and a region
that requires both execute and write permissions, there would be no
way to describe it unless we permit entries that have RO and XP both
cleared.

Patches welcome!

-- 
Ard.

Re: Still required that EFI_MEMORY_XP and EFI_MEMORY_RO must not both be cleared?

[Heiner Kallweit]

On 30.04.2021 11:40, Ard Biesheuvel wrote:
> On Fri, 30 Apr 2021 at 10:06, Heiner Kallweit <hkallweit1@gmail.com> wrote:
>>
>> I'm one of the people seeing the following warning:
>>
>> efi: memattr: Entry attributes invalid: RO and XP bits both cleared
>> efi: memattr: ! 0x000000090000-0x000000090fff [Runtime Code|RUN|  |  |  |  |  |  |  |  |   |  |  |  |  ]
>>
>> Out of curiosity I checked UEFI spec 2.9. On p.108 there's table 4.1:
>>
>>                                                    EFI_MEMORY_RO EFI_MEMORY_XP EFI_MEMORY_RUNTIME
>> No memory access protection is possible for Entry  0             0             1
>> Write-protected Code                               1             0             1
>> Read/Write Data                                    0             1             1
>> Read-only Data                                     1             1             1
>>
>> So it seems to be valid that both attributes are cleared.
>> Also in the surrounding text I didn't find a hint mandating
>> that at least one the attributes has to be set.
> 
> You are right. My assumption at the time was that not having either of
> RO or XP in the memory attributes table makes little sense, as it
> describes the default case. However, this interpretation may conflict
> with the requirement that entries in the memory attributes table cover
> an entry in the EFI memory map entirely, or not at all. IOW, if a
> memory map entry consists of a code region, a data region and a region
> that requires both execute and write permissions, there would be no
> way to describe it unless we permit entries that have RO and XP both
> cleared.
> 
> Patches welcome!
> 
Thanks for the comprehensive explanation. Then I'll submit a patch
for removing this check.