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

[Arvind Sankar <nivedita@alum.mit.edu>]

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
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
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.
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