On 03/02/2022 8:16, Gerd Hoffmann wrote:

  Hi,

quoted

quoted

I think the module should fail noisily.  See above for autoload.  In
case the module is loaded (either manually by the admin, or because
efi.coco_secret != EFI_INVALID_TABLE_ADDR) and it can't actually load
the secrets we want know why ...

Note that the AmdSev build of OVMF always publishes
LINUX_EFI_COCO_SECRET_TABLE_GUID in the EFI table.  Even when
LAUNCH_SECRET was not executed.  In such cases the secret area will be
empty.

Hmm, ok.  Why?  I assume the secret area is filled by the host and ovmf
doesn't even look at it?

Exactly.  OVMF just reserves this area, and puts its address+size in the
EFI config table.  It doesn't care about its format and usage.

There are currently two "users" for the actual data in this memory area:

1. grub's efisecret module (which reads the disk passphrase from an
entry in the secret area)
2. linux's efi_secret module (which we're discussing here)

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If we keep only the 'efi.coco_secret != EFI_INVALID_TABLE_ADDR' check,
we'll get errors from efi_secret for every VM launch that doesn't
undergo LAUNCH_SECRET.  I don't think that's good.

Well, if that is a common case the module could either print nothing or
log KERN_INFO level instead of KERN_ERROR.

What if the user doesn't inject a secret and doesn't include the
efi_secret module at all in the initrd?  request_module("efi_secret")
will fail.

I can ignore the error code of request_module("efi_secret") but that
feels bad.

quoted

If we *do* want to check that the area starts with
EFI_SECRET_TABLE_HEADER_GUID (like I think we should), we need all the
checks before that, like checking that the area is big enough, and that
all the memremap()s succeed -- before actually comparing the
header_guid.  The checks are basically prerequisites for calling
efi_guidcmp() safely.

It is still not fully clear to me why you want do that check twice.

I want to load the module only if secrets were injected by the Guest
Owner.

Again, I'm open to ideas on how to de-duplicate these early checks, if
that's important.


-Dov