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, 1 deletions

diff --git a/drivers/acpi/osl.c b/drivers/acpi/osl.c
index cc7507091dec..b7c3aeb175dd 100644
--- a/drivers/acpi/osl.c
+++ b/drivers/acpi/osl.c
@@ -26,6 +26,7 @@
 #include <linux/list.h>
 #include <linux/jiffies.h>
 #include <linux/semaphore.h>
+#include <linux/security.h>
 
 #include <asm/io.h>
 #include <linux/uaccess.h>
@@ -180,8 +181,19 @@ acpi_physical_address __init acpi_os_get_root_pointer(void)
 	acpi_physical_address pa;
 
 #ifdef CONFIG_KEXEC
-	if (acpi_rsdp)
+	/*
+	 * We may have been provided with an RSDP on the command line,
+	 * but if a malicious user has done so they may be pointing us
+	 * at modified ACPI tables that could alter kernel behaviour -
+	 * so, we check the lockdown status before making use of
+	 * it. If we trust it then also stash it in an architecture
+	 * specific location (if appropriate) so it can be carried
+	 * over further kexec()s.
+	 */
+	if (acpi_rsdp && !security_locked_down(LOCKDOWN_ACPI_TABLES)) {
+		acpi_arch_set_root_pointer(acpi_rsdp);
 		return acpi_rsdp;
+	}
 #endif
 	pa = acpi_arch_get_root_pointer();
 	if (pa)