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

[Juergen Gross <jgross@xxxxxxxx>]

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

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