Re: [PATCH v4 2/3] x86/efistub: Obtain SEV CC blob address from the stub

Tom Lendacky
2025-09-11T21:27:34Z

On 9/9/25 03:06, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@kernel.org>
>
> The x86 EFI stub no longer boots the core kernel via the traditional
> decompressor but jumps straight to it, avoiding all the page fault
> handling and other complexity that is entirely unnecessary when booting
> via EFI, which guarantees that all system memory is mapped 1:1.
>
> The SEV startup code in the core kernel expects the address of the CC
> blob configuration table in boot_params, so store it there when booting
> from EFI with SEV-SNP enabled. This removes the need to call
> sev_enable() from the EFI stub.
>
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
> drivers/firmware/efi/libstub/x86-stub.c | 21 +++++++++++++++-----
> 1 file changed, 16 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/firmware/efi/libstub/x86-stub.c b/drivers/firmware/efi/libstub/x86-stub.c
> index 0d05eac7c72b..c4ef645762ec 100644
> --- a/drivers/firmware/efi/libstub/x86-stub.c
> +++ b/drivers/firmware/efi/libstub/x86-stub.c
> @@ -681,17 +681,28 @@ static efi_status_t exit_boot(struct boot_params *boot_params, void *handle)
> return EFI_SUCCESS;
> }
>
> -static bool have_unsupported_snp_features(void)
> +static bool check_snp_features(struct boot_params *bp)
> {
> + u64 status = sev_get_status();
> u64 unsupported;
>
> - unsupported = snp_get_unsupported_features(sev_get_status());
> + unsupported = snp_get_unsupported_features(status);
> if (unsupported) {
> efi_err("Unsupported SEV-SNP features detected: 0x%llx\n",
> unsupported);
> - return true;
> + return false;
> }
> - return false;
> +
> + if (status & MSR_AMD64_SEV_SNP_ENABLED) {
> + void *tbl = get_efi_config_table(EFI_CC_BLOB_GUID);
> +
> + if (!tbl) {
> + efi_err("SEV-SNP is enabled but CC blob not found\n");
> + return false;
> + }
> + bp->cc_blob_address = (u32)(unsigned long)tbl;

I think we ran into bugs where the cc_blob_address was random data from a
boot loader when SNP wasn't active and that's why we always initialize it
to 0:

4b1c74240757 ("x86/boot: Don't propagate uninitialized boot_params->cc_blob_address")

So we probably need the same statement that is at the beginning of the
decompressor sev_enable() at the very beginning of this function to ensure
cc_blob_address is set to zero:

/*
* bp->cc_blob_address should only be set by boot/compressed kernel.
* Initialize it to 0 to ensure that uninitialized values from
* buggy bootloaders aren't propagated.
*/
if (bp)
bp->cc_blob_address = 0;

Thanks,
Tom

> + }
> + return true;
> }
>
> static void efi_get_seed(void *seed, int size)
> @@ -831,7 +842,7 @@ void __noreturn efi_stub_entry(efi_handle_t handle,
>
> hdr = &boot_params->hdr;
>
> - if (have_unsupported_snp_features())
> + if (!check_snp_features(boot_params))
> efi_exit(handle, EFI_UNSUPPORTED);
>
> if (IS_ENABLED(CONFIG_EFI_DXE_MEM_ATTRIBUTES)) {