[PATCH V34 05/29] Restrict /dev/{mem,kmem,port} when the kernel is locked down

Matthew Garrett
2019-06-22T00:04:18Z

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>
Cc: x86@kernel.org
---
drivers/char/mem.c | 6 +++++-
include/linux/security.h | 1 +
security/lockdown/lockdown.c | 1 +
3 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index b08dc50f9f26..93c02493f0fa 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>
@@ -786,6 +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)
{
+ int ret = security_locked_down(LOCKDOWN_DEV_MEM);
+
+ if (ret)
+ return ret;
return capable(CAP_SYS_RAWIO) ? 0 : -EPERM;
}

diff --git a/include/linux/security.h b/include/linux/security.h
index 46d85cd63b06..200175c8605a 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -83,6 +83,7 @@ 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 25a3a5b0aa9c..565c87451f0f 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",
};
--
2.22.0.410.gd8fdbe21b5-goog