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

[Arvind Sankar]

On Wed, Oct 21, 2020 at 05:28:33PM +0200, Daniel Kiper wrote:
> On Mon, Oct 19, 2020 at 01:18:22PM -0400, Arvind Sankar wrote:
> > On Mon, Oct 19, 2020 at 04:51:53PM +0200, Daniel Kiper wrote:
> > > On Fri, Oct 16, 2020 at 04:51:51PM -0400, 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?
> > >
> > > What do you think about my MLE_HEADER_OFFSET and related stuff proposal?
> > >
> >
> > I'm wondering if it would be easier to just allow relocations in these
> > special "header" sections. I need to check how easy/hard it is to do
> > that without triggering linker warnings.
> 
> Ross and I still bouncing some ideas. We came to the conclusion that
> putting mle_header into kernel .rodata.kernel_info section or even
> arch/x86/boot/compressed/kernel_info.S file would be the easiest thing
> to do at this point. Of course I would suggest some renaming too. E.g.
> .rodata.kernel_info to .rodata.kernel_headers, etc. Does it make sense
> for you?
> 
> Daniel

I haven't been able to come up with any different options that don't
require post-processing of the kernel image. Allowing relocations in
specific sections seems to not be possible with lld, and anyway would
require the fields to be 64-bit sized so it doesn't really help.

Putting mle_header into kernel_info seems like a reasonable thing to me,
and if you do that, putting it into kernel_info.S would seem to be
necessary?  Would you also have a fixed field with the offset of the
mle_header from kernel_info?  That seems nicer than having the
bootloader scan the variable data for magic strings.