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>

drivers/firmware/efi/libstub/arm-stub.c

@@ -153,7 +153,7 @@ efi_status_t efi_entry(efi_handle_t handle, efi_system_table_t *sys_table_arg)
 	unsigned long image_size = 0;
 	unsigned long dram_base;
 	/* addr/point and size pairs for memory management*/
-	unsigned long initrd_addr;
+	unsigned long initrd_addr = 0;
 	unsigned long initrd_size = 0;
 	unsigned long fdt_addr = 0;  /* Original DTB */
 	unsigned long fdt_size = 0;
@@ -268,8 +268,10 @@ efi_status_t efi_entry(efi_handle_t handle, efi_system_table_t *sys_table_arg)
 	if (!fdt_addr)
 		pr_efi("Generating empty DTB\n");
 
+	if (!noinitrd()) {
 		max_addr = efi_get_max_initrd_addr(dram_base, image_addr);
-	status = efi_load_initrd_dev_path(&initrd_addr, &initrd_size, max_addr);
+		status = efi_load_initrd_dev_path(&initrd_addr, &initrd_size,
+						  max_addr);
 		if (status == EFI_SUCCESS) {
 			pr_efi("Loaded initrd from LINUX_EFI_INITRD_MEDIA_GUID device path\n");
 		} else if (status == EFI_NOT_FOUND) {
@@ -280,6 +282,7 @@ efi_status_t efi_entry(efi_handle_t handle, efi_system_table_t *sys_table_arg)
 		}
 		if (status != EFI_SUCCESS)
 			pr_efi_err("Failed to load initrd!\n");
+	}
 
 	efi_random_get_seed();
 

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");

drivers/firmware/efi/libstub/efistub.h

@@ -33,6 +33,7 @@
 
 extern bool __pure nochunk(void);
 extern bool __pure nokaslr(void);
+extern bool __pure noinitrd(void);
 extern bool __pure is_quiet(void);
 extern bool __pure novamap(void);
 

drivers/firmware/efi/libstub/x86-stub.c

@@ -421,15 +421,18 @@ efi_status_t __efiapi efi_pe_entry(efi_handle_t handle,
 	if (status != EFI_SUCCESS)
 		goto fail2;
 
+	if (!noinitrd()) {
 		status = efi_load_initrd(image, &ramdisk_addr, &ramdisk_size,
 					 hdr->initrd_addr_max,
-				 above4g ? ULONG_MAX : hdr->initrd_addr_max);
+					 above4g ? ULONG_MAX
+						 : hdr->initrd_addr_max);
 		if (status != EFI_SUCCESS)
 			goto fail2;
 		hdr->ramdisk_image = ramdisk_addr & 0xffffffff;
 		hdr->ramdisk_size  = ramdisk_size & 0xffffffff;
 		boot_params->ext_ramdisk_image = (u64)ramdisk_addr >> 32;
 		boot_params->ext_ramdisk_size  = (u64)ramdisk_size >> 32;
+	}
 
 	efi_stub_entry(handle, sys_table, boot_params);
 	/* not reached */
@@ -699,14 +702,9 @@ struct boot_params *efi_main(efi_handle_t handle,
 {
 	unsigned long bzimage_addr = (unsigned long)startup_32;
 	struct setup_header *hdr = &boot_params->hdr;
-	unsigned long max_addr = hdr->initrd_addr_max;
-	unsigned long initrd_addr, initrd_size;
 	efi_status_t status;
 	unsigned long cmdline_paddr;
 
-	if (hdr->xloadflags & XLF_CAN_BE_LOADED_ABOVE_4G)
-		max_addr = ULONG_MAX;
-
 	sys_table = sys_table_arg;
 
 	/* Check if we were booted by the EFI firmware */
@@ -746,16 +744,24 @@ struct boot_params *efi_main(efi_handle_t handle,
 	 * permit an initrd loaded from the LINUX_EFI_INITRD_MEDIA_GUID device
 	 * path to supersede it.
 	 */
-	status = efi_load_initrd_dev_path(&initrd_addr, &initrd_size, max_addr);
+	if (!noinitrd()) {
+		unsigned long addr, size;
+		unsigned long max_addr = hdr->initrd_addr_max;
+
+		if (hdr->xloadflags & XLF_CAN_BE_LOADED_ABOVE_4G)
+			max_addr = ULONG_MAX;
+
+		status = efi_load_initrd_dev_path(&addr, &size, max_addr);
 		if (status == EFI_SUCCESS) {
-		hdr->ramdisk_image		= (u32)initrd_addr;
-		hdr->ramdisk_size 		= (u32)initrd_size;
-		boot_params->ext_ramdisk_image	= (u64)initrd_addr >> 32;
-		boot_params->ext_ramdisk_size 	= (u64)initrd_size >> 32;
+			hdr->ramdisk_image		= (u32)addr;
+			hdr->ramdisk_size 		= (u32)size;
+			boot_params->ext_ramdisk_image	= (u64)addr >> 32;
+			boot_params->ext_ramdisk_size 	= (u64)size >> 32;
 		} else if (status != EFI_NOT_FOUND) {
 			efi_printk("efi_load_initrd_dev_path() failed!\n");
 			goto fail;
 		}
+	}
 
 	/*
 	 * If the boot loader gave us a value for secure_boot then we use that,