lockdown: Restrict /dev/{mem,kmem,port} when the kernel is locked down

Allowing users to read and write to core kernel memory makes it possible
for the kernel to be subverted, avoiding module loading restrictions, and
also to steal cryptographic information.

Disallow /dev/mem and /dev/kmem from being opened this when the kernel has
been locked down to prevent this.

Also disallow /dev/port from being opened to prevent raw ioport access and
thus DMA from being used to accomplish the same thing.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Matthew Garrett <mjg59@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Cc: x86@kernel.org
Signed-off-by: James Morris <jmorris@namei.org>

drivers/char/mem.c

@@ -29,8 +29,8 @@
 #include <linux/export.h>
 #include <linux/io.h>
 #include <linux/uio.h>
-
 #include <linux/uaccess.h>
+#include <linux/security.h>
 
 #ifdef CONFIG_IA64
 # include <linux/efi.h>
@@ -786,7 +786,10 @@ static loff_t memory_lseek(struct file *file, loff_t offset, int orig)
 
 static int open_port(struct inode *inode, struct file *filp)
 {
-	return capable(CAP_SYS_RAWIO) ? 0 : -EPERM;
+	if (!capable(CAP_SYS_RAWIO))
+		return -EPERM;
+
+	return security_locked_down(LOCKDOWN_DEV_MEM);
 }
 
 #define zero_lseek	null_lseek

include/linux/security.h

@@ -104,6 +104,7 @@ enum lsm_event {
 enum lockdown_reason {
 	LOCKDOWN_NONE,
 	LOCKDOWN_MODULE_SIGNATURE,
+	LOCKDOWN_DEV_MEM,
 	LOCKDOWN_INTEGRITY_MAX,
 	LOCKDOWN_CONFIDENTIALITY_MAX,
 };

security/lockdown/lockdown.c

@@ -19,6 +19,7 @@ static enum lockdown_reason kernel_locked_down;
 static char *lockdown_reasons[LOCKDOWN_CONFIDENTIALITY_MAX+1] = {
 	[LOCKDOWN_NONE] = "none",
 	[LOCKDOWN_MODULE_SIGNATURE] = "unsigned module loading",
+	[LOCKDOWN_DEV_MEM] = "/dev/mem,kmem,port",
 	[LOCKDOWN_INTEGRITY_MAX] = "integrity",
 	[LOCKDOWN_CONFIDENTIALITY_MAX] = "confidentiality",
 };