acpi: Ignore acpi_rsdp kernel param when the kernel has been locked down

This option allows userspace to pass the RSDP address to the kernel, which
makes it possible for a user to modify the workings of hardware. Reject
the option when the kernel is locked down. This requires some reworking
of the existing RSDP command line logic, since the early boot code also
makes use of a command-line passed RSDP when locating the SRAT table
before the lockdown code has been initialised. This is achieved by
separating the command line RSDP path in the early boot code from the
generic RSDP path, and then copying the command line RSDP into boot
params in the kernel proper if lockdown is not enabled. If lockdown is
enabled and an RSDP is provided on the command line, this will only be
used when parsing SRAT (which shouldn't permit kernel code execution)
and will be ignored in the rest of the kernel.

(Modified by Matthew Garrett in order to handle the early boot RSDP
environment)

Signed-off-by: Josh Boyer <jwboyer@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Matthew Garrett <mjg59@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
cc: Dave Young <dyoung@redhat.com>
cc: linux-acpi@vger.kernel.org
Signed-off-by: James Morris <jmorris@namei.org>

1 files changed, 13 insertions, 6 deletions

diff --git a/arch/x86/boot/compressed/acpi.c b/arch/x86/boot/compressed/acpi.c
index ad84239e595e..e726e9b44bb1 100644
--- a/arch/x86/boot/compressed/acpi.c
+++ b/arch/x86/boot/compressed/acpi.c
@@ -26,7 +26,7 @@ struct mem_vector immovable_mem[MAX_NUMNODES*2];
  */
 #define MAX_ADDR_LEN 19
 
-static acpi_physical_address get_acpi_rsdp(void)
+static acpi_physical_address get_cmdline_acpi_rsdp(void)
 {
 	acpi_physical_address addr = 0;
 
@@ -215,10 +215,7 @@ acpi_physical_address get_rsdp_addr(void)
 {
 	acpi_physical_address pa;
 
-	pa = get_acpi_rsdp();
-
-	if (!pa)
-		pa = boot_params->acpi_rsdp_addr;
+	pa = boot_params->acpi_rsdp_addr;
 
 	if (!pa)
 		pa = efi_get_rsdp_addr();
@@ -240,7 +237,17 @@ static unsigned long get_acpi_srat_table(void)
 	char arg[10];
 	u8 *entry;
 
-	rsdp = (struct acpi_table_rsdp *)(long)boot_params->acpi_rsdp_addr;
+	/*
+	 * Check whether we were given an RSDP on the command line. We don't
+	 * stash this in boot params because the kernel itself may have
+	 * different ideas about whether to trust a command-line parameter.
+	 */
+	rsdp = (struct acpi_table_rsdp *)get_cmdline_acpi_rsdp();
+
+	if (!rsdp)
+		rsdp = (struct acpi_table_rsdp *)(long)
+			boot_params->acpi_rsdp_addr;
+
 	if (!rsdp)
 		return 0;