efi/libstub: Take noinitrd cmdline argument into account for devpath initrd

One of the advantages of using what basically amounts to a callback
interface into the bootloader for loading the initrd is that it provides
a natural place for the bootloader or firmware to measure the initrd
contents while they are being passed to the kernel.

Unfortunately, this is not a guarantee that the initrd will in fact be
loaded and its /init invoked by the kernel, since the command line may
contain the 'noinitrd' option, in which case the initrd is ignored, but
this will not be reflected in the PCR that covers the initrd measurement.

This could be addressed by measuring the command line as well, and
including that PCR in the attestation policy, but this locks down the
command line completely, which may be too restrictive.

So let's take the noinitrd argument into account in the stub, too. This
forces any PCR that covers the initrd to assume a different value when
noinitrd is passed, allowing an attestation policy to disregard the
command line if there is no need to take its measurement into account
for other reasons.

As Peter points out, this would still require the agent that takes the
measurements to measure a separator event into the PCR in question at
ExitBootServices() time, to prevent replay attacks using the known
measurement from the TPM log.

Cc: Peter Jones <pjones@redhat.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

1 files changed, 7 insertions, 0 deletions

diff --git a/drivers/firmware/efi/libstub/efi-stub-helper.c b/drivers/firmware/efi/libstub/efi-stub-helper.c
index 6017b968cef7..b1da58141a4d 100644
--- a/drivers/firmware/efi/libstub/efi-stub-helper.c
+++ b/drivers/firmware/efi/libstub/efi-stub-helper.c
@@ -14,6 +14,7 @@
 
 static bool __efistub_global efi_nochunk;
 static bool __efistub_global efi_nokaslr;
+static bool __efistub_global efi_noinitrd;
 static bool __efistub_global efi_quiet;
 static bool __efistub_global efi_novamap;
 static bool __efistub_global efi_nosoftreserve;
@@ -28,6 +29,10 @@ bool __pure nokaslr(void)
 {
 	return efi_nokaslr;
 }
+bool __pure noinitrd(void)
+{
+	return efi_noinitrd;
+}
 bool __pure is_quiet(void)
 {
 	return efi_quiet;
@@ -87,6 +92,8 @@ efi_status_t efi_parse_options(char const *cmdline)
 			efi_nokaslr = true;
 		} else if (!strcmp(param, "quiet")) {
 			efi_quiet = true;
+		} else if (!strcmp(param, "noinitrd")) {
+			efi_noinitrd = true;
 		} else if (!strcmp(param, "efi") && val) {
 			efi_nochunk = parse_option_str(val, "nochunk");
 			efi_novamap = parse_option_str(val, "novamap");