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>

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;
 
@@ -215,9 +215,6 @@ acpi_physical_address get_rsdp_addr(void)
 {
 	acpi_physical_address pa;
 
-	pa = get_acpi_rsdp();
-
-	if (!pa)
 	pa = boot_params->acpi_rsdp_addr;
 
 	if (!pa)
@@ -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;
 

arch/x86/include/asm/acpi.h

@@ -117,6 +117,12 @@ static inline bool acpi_has_cpu_in_madt(void)
 	return !!acpi_lapic;
 }
 
+#define ACPI_HAVE_ARCH_SET_ROOT_POINTER
+static inline void acpi_arch_set_root_pointer(u64 addr)
+{
+	x86_init.acpi.set_root_pointer(addr);
+}
+
 #define ACPI_HAVE_ARCH_GET_ROOT_POINTER
 static inline u64 acpi_arch_get_root_pointer(void)
 {
@@ -125,6 +131,7 @@ static inline u64 acpi_arch_get_root_pointer(void)
 
 void acpi_generic_reduced_hw_init(void);
 
+void x86_default_set_root_pointer(u64 addr);
 u64 x86_default_get_root_pointer(void);
 
 #else /* !CONFIG_ACPI */
@@ -138,6 +145,8 @@ static inline void disable_acpi(void) { }
 
 static inline void acpi_generic_reduced_hw_init(void) { }
 
+static inline void x86_default_set_root_pointer(u64 addr) { }
+
 static inline u64 x86_default_get_root_pointer(void)
 {
 	return 0;

arch/x86/include/asm/x86_init.h

@@ -134,10 +134,12 @@ struct x86_hyper_init {
 
 /**
  * struct x86_init_acpi - x86 ACPI init functions
+ * @set_root_poitner:		set RSDP address
  * @get_root_pointer:		get RSDP address
  * @reduced_hw_early_init:	hardware reduced platform early init
  */
 struct x86_init_acpi {
+	void (*set_root_pointer)(u64 addr);
 	u64 (*get_root_pointer)(void);
 	void (*reduced_hw_early_init)(void);
 };

arch/x86/kernel/acpi/boot.c

@@ -1760,6 +1760,11 @@ void __init arch_reserve_mem_area(acpi_physical_address addr, size_t size)
 	e820__update_table_print();
 }
 
+void x86_default_set_root_pointer(u64 addr)
+{
+	boot_params.acpi_rsdp_addr = addr;
+}
+
 u64 x86_default_get_root_pointer(void)
 {
 	return boot_params.acpi_rsdp_addr;

arch/x86/kernel/x86_init.c

@@ -95,6 +95,7 @@ struct x86_init_ops x86_init __initdata = {
 	},
 
 	.acpi = {
+		.set_root_pointer	= x86_default_set_root_pointer,
 		.get_root_pointer	= x86_default_get_root_pointer,
 		.reduced_hw_early_init	= acpi_generic_reduced_hw_init,
 	},

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)

include/linux/acpi.h

@@ -632,6 +632,12 @@ bool acpi_gtdt_c3stop(int type);
 int acpi_arch_timer_mem_init(struct arch_timer_mem *timer_mem, int *timer_count);
 #endif
 
+#ifndef ACPI_HAVE_ARCH_SET_ROOT_POINTER
+static inline void acpi_arch_set_root_pointer(u64 addr)
+{
+}
+#endif
+
 #ifndef ACPI_HAVE_ARCH_GET_ROOT_POINTER
 static inline u64 acpi_arch_get_root_pointer(void)
 {