[PATCH V40 05/29] lockdown: Restrict /dev/{mem,kmem,port} when the kernel is locked down

From: Matthew Garrett <matthewgarrett@google.com>
To: jmorris@namei.org
Cc: linux-security-module@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-api@vger.kernel.org,
	Matthew Garrett <mjg59@srcf.ucam.org>,
	David Howells <dhowells@redhat.com>,
	Matthew Garrett <mjg59@google.com>,
	Kees Cook <keescook@chromium.org>,
	x86@kernel.org
Subject: [PATCH V40 05/29] lockdown: Restrict /dev/{mem,kmem,port} when the kernel is locked down
Date: Mon, 19 Aug 2019 17:17:41 -0700	[thread overview]
Message-ID: <20190820001805.241928-6-matthewgarrett@google.com> (raw)
In-Reply-To: <20190820001805.241928-1-matthewgarrett@google.com>

From: Matthew Garrett <mjg59@srcf.ucam.org>

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           | 7 +++++--
 include/linux/security.h     | 1 +
 security/lockdown/lockdown.c | 1 +
 3 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index b08dc50f9f26..d0148aee1aab 100644
--- a/drivers/char/mem.c
+++ b/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
diff --git a/include/linux/security.h b/include/linux/security.h
index 9e8abb60a99f..e5dd446ef35b 100644
--- a/include/linux/security.h
+++ b/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,
 };
diff --git a/security/lockdown/lockdown.c b/security/lockdown/lockdown.c
index d8e42125a5dd..240ecaa10a1d 100644
--- a/security/lockdown/lockdown.c
+++ b/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",
 };
-- 
2.23.0.rc1.153.gdeed80330f-goog

