Re: PLEASE REVERT URGENTLY: Re: [PATCH v5 2/3] x86/boot: add acpi rsdp address to setup_header

From: Juergen Gross
Date: Sat Nov 10 2018 - 01:26:19 EST


On 09/11/2018 23:23, H. Peter Anvin wrote:
> I just noticed this patch -- I missed it because the cover message
> seemed far more harmless so I didn't notice this change.
>
> THIS PATCH IS FATALLY WRONG AND NEEDS TO BE IMMEDIATELY REVERTED BEFORE
> ANYONE STARTS RELYING ON IT; IT HAS THE POTENTIAL OF BREAKING THE
> BOOTLOADER PROTOCOL FOR ALL FUTURE.

It is already broken and this patch tries to repair it.

> It seems to be based on fundamental misconceptions about the various
> data structures in the protocol, and does so in a way that completely
> breaks the way the protocol is designed to work.
>
> The protocol is specifically designed such that fields are not version
> dependencies. The version number is strictly to inform the boot loader
> about which capabilities the kernel has, so that the boot loader can
> know if a certain data field is meaningful and/or honored.

Right. That was where I started in early 2018.

Unfortunately there are many major distros shipping boot loaders which
write crap data past the end of setup_header.

>
>> +Protocol 2.14: (Kernel 4.20) Added acpi_rsdp_addr holding the physical
>> + address of the ACPI RSDP table.
>> + The bootloader updates version with:
>> + 0x8000 | min(kernel-version, bootloader-version)
>> + kernel-version being the protocol version supported by
>> + the kernel and bootloader-version the protocol version
>> + supported by the bootloader.
>
> [...]
>
>> **** MEMORY LAYOUT
>>
>> The traditional memory map for the kernel loader, used for Image or
>> @@ -197,6 +209,7 @@ Offset Proto Name Meaning
>> 0258/8 2.10+ pref_address Preferred loading address
>> 0260/4 2.10+ init_size Linear memory required during initialization
>> 0264/4 2.11+ handover_offset Offset of handover entry point
>> +0268/8 2.14+ acpi_rsdp_addr Physical address of RSDP table
>
> NO.
>
> That is not how struct setup_header works, nor does this belong here.
>
> struct setup_header contains *initialized data*, and has a length byte
> at offset 0x201. The bootloader is responsible for copying the full
> structure into the appropriate offset (0x1f1) in struct boot_params.

Yes, but some boot loaders copy more than that clobbering initialized
kernel data (like in my case acpi_rsdp_addr).

> The length byte isn't actually a requirement, since the maximum possible
> size of this structure is 144 bytes, and the kernel will (obviously) not
> look at the older fields anyway, but it is good practice. The kernel or
> any other entity is free to zero out the bytes past this length pointer.
>
> There are only 24 bytes left in this structure, and this would occupy 8
> of them for no valid reason. The *only* valid reason to put a
> zero-initialized field in struct setup_header is if it used by the
> 16-bit legacy BIOS boot, which is obviously not the case here.
>
> This field thus belongs in struct boot_params, not struct setup_header.

Okay, I can change that. Hoping that all boot loaders really write
zeroes to that field in case they don't know it.

>> @@ -317,6 +330,12 @@ Protocol: 2.00+
>> e.g. 0x0204 for version 2.04, and 0x0a11 for a hypothetical version
>> 10.17.
>>
>> + Up to protocol version 2.13 this information is only read by the
>> + bootloader. From protocol version 2.14 onwards the bootloader will
>> + write the used protocol version ored with 0x8000 to the field. The
>> + used protocol version will be the minimum of the supported protocol
>> + versions of the bootloader and the kernel.
>> +
>
> Again, this is completely wrong. The version number is communication to
> the bootloader, which may end up going through multiple stages.
> Modifying this field breaks this invariant in a not-very-subtle way.
>
> Fields in struct setup_header are to be initialized from the image
> provided in the kernel header.
>
> Fields in struct boot_params are to be initialized to zero.

See above. grub2 in Debian, RHEL, ... doesn't do that reliably.

> There is a field called "sentinel" which attempts to detect broken
> bootloaders which do not do this correctly; however, when enabling new
> bootloaders the Right Thing to do is to make sure they adhere to the
> protocol as defined, rather than pushing a new hack onto the kernel.
>
> Thus:
>
> 1. Please revert this patch immediately, and destroy any boot loaders
> which tries to implement this.> 2. Add the acpi_rsdp_addr to struct boot_params.
> 3. DO NOT modify the boot protocol version header field. Instead
> make sure that the bootloader follows the protocol and zeroes
> all unknown fields in struct boot_params.

How can I do this for boot loaders shipped since several years?

> 4. Possibly make the kernel panic if it notices that the boot version
> header has been mucked with, in case some of these boot loaders
> have already escaped into the field.

So don't let a new kernel boot from a disk with above grub2?

I don't think so.


Juergen