Re: [PATCH 07/13] x86: Secure Launch kernel early boot stub

[Ross Philipson]

On 10/16/20 4:51 PM, Arvind Sankar wrote:
> On Thu, Oct 15, 2020 at 08:26:54PM +0200, Daniel Kiper wrote:
>>
>> I am discussing with Ross the other option. We can create
>> .rodata.mle_header section and put it at fixed offset as
>> kernel_info is. So, we would have, e.g.:
>>
>> arch/x86/boot/compressed/vmlinux.lds.S:
>>         .rodata.kernel_info KERNEL_INFO_OFFSET : {
>>                 *(.rodata.kernel_info)
>>         }
>>         ASSERT(ABSOLUTE(kernel_info) == KERNEL_INFO_OFFSET, "kernel_info at bad address!")
>>
>>         .rodata.mle_header MLE_HEADER_OFFSET : {
>>                 *(.rodata.mle_header)
>>         }
>>         ASSERT(ABSOLUTE(mle_header) == MLE_HEADER_OFFSET, "mle_header at bad address!")
>>
>> arch/x86/boot/compressed/sl_stub.S:
>> #define mleh_rva(X) (((X) - mle_header) + MLE_HEADER_OFFSET)
>>
>>         .section ".rodata.mle_header", "a"
>>
>> SYM_DATA_START(mle_header)
>>         .long   0x9082ac5a    /* UUID0 */
>>         .long   0x74a7476f    /* UUID1 */
>>         .long   0xa2555c0f    /* UUID2 */
>>         .long   0x42b651cb    /* UUID3 */
>>         .long   0x00000034    /* MLE header size */
>>         .long   0x00020002    /* MLE version 2.2 */
>>         .long   mleh_rva(sl_stub_entry)    /* Linear entry point of MLE (virt. address) */
>>         .long   0x00000000    /* First valid page of MLE */
>>         .long   0x00000000    /* Offset within binary of first byte of MLE */
>>         .long   0x00000000    /* Offset within binary of last byte + 1 of MLE */
>>         .long   0x00000223    /* Bit vector of MLE-supported capabilities */
>>         .long   0x00000000    /* Starting linear address of command line (unused) */
>>         .long   0x00000000    /* Ending linear address of command line (unused) */
>> SYM_DATA_END(mle_header)
>>
>> Of course MLE_HEADER_OFFSET has to be defined as a constant somewhere.
>> Anyway, is it acceptable?
>>
>> There is also another problem. We have to put into mle_header size of
>> the Linux kernel image. Currently it is done by the bootloader but
>> I think it is not a role of the bootloader. The kernel image should
>> provide all data describing its properties and do not rely on the
>> bootloader to do that. Ross and I investigated various options but we
>> did not find a good/simple way to do that. Could you suggest how we
>> should do that or at least where we should take a look to get some
>> ideas?
>>
>> Daniel
> 
> What exactly is the size you need here? Is it just the size of the
> protected mode image, that's startup_32 to _edata. Or is it the size of

It is the size of the protected mode image. Though how to reference
those symbols to get the size might all more relocation issues.

> the whole bzImage file, or something else? I guess the same question
> applies to "first valid page of MLE" and "first byte of MLE", and the

Because of the way the launch environment is setup, both "first valid
page of MLE" and "first byte of MLE" are always zero so nothing to have
to sort out there. The only fields that need to be updated are "Linear
entry point of MLE" and "Offset within binary of last byte + 1 of MLE".

Thanks
Ross

> linear entry point -- are those all relative to startup_32 or do they
> need to be relative to the start of the bzImage, i.e. you have to add
> the size of the real-mode boot stub?
> 
> If you need to include the size of the bzImage file, that's not known
> when the files in boot/compressed are built. It's only known after the
> real-mode stub is linked. arch/x86/boot/tools/build.c fills in various
> details in the setup header and creates the bzImage file, but it does
> not currently modify anything in the protected-mode portion of the
> compressed kernel (i.e. arch/x86/boot/compressed/vmlinux, which then
> gets converted to binary format as arch/x86/boot/vmlinux.bin), so it
> would need to be extended if you need to modify the MLE header to
> include the bzImage size or anything depending on the size of the
> real-mode stub.
>