devtmpfs: mount with noexec and nosuid
Since devtmpfs is writable, make the default noexec,nosuid as well. This
protects from the case of a privileged process having an arbitrary file
write flaw and an argumentless arbitrary execution (i.e. it would lack
the ability to run "mount -o remount,exec,suid /dev").
Rather than relying on userspace "mount -o remount,noexec,nosuid /dev",
accomplish this from the kernel. This means no additional exec during
(potentially time-sensitive) boot is needed. The kernel is responsible
for this mount, so the mount flags should be configurable.
Cc: ellyjones@chromium.org
Cc: Kay Sievers <kay@vrfy.org>
Cc: Roland Eggner <edvx1@systemanalysen.net>
Signed-off-by: Kees Cook <keescook@chromium.org>
---
v3:
- use a single define for the mount flags, suggested by Greg K.H.
v2:
- use CONFIG_DEVTMPFS_SAFE to wrap the logic.
diff --git a/drivers/base/Kconfig b/drivers/base/Kconfig
index 98504ec..2f3bc44 100644
--- a/drivers/base/Kconfig
+++ b/drivers/base/Kconfig
@@ -59,6 +59,18 @@
rescue mode with init=/bin/sh, even when the /dev directory
on the rootfs is completely empty.
+config DEVTMPFS_SAFE
+ bool "Use nosuid,noexec mount options on devtmpfs"
+ depends on DEVTMPFS
+ help
+ This instructs the kernel to include the MS_NOEXEC and
+ MS_NOSUID mount flags when mounting devtmpfs. This prevents
+ certain kinds of code-execution attacks on embedded platforms.
+
+ Notice: If enabled, things like /dev/mem cannot be mmapped
+ with the PROT_EXEC flag. This can break, for example, non-KMS
+ video drivers.
+
config STANDALONE
bool "Select only drivers that don't need compile-time external firmware"
default y
diff --git a/drivers/base/devtmpfs.c b/drivers/base/devtmpfs.c
index 44a74cf..5648490 100644
--- a/drivers/base/devtmpfs.c
+++ b/drivers/base/devtmpfs.c
@@ -26,6 +26,12 @@
#include <linux/kthread.h>
#include "base.h"
+#ifdef CONFIG_DEVTMPFS_SAFE
+# define DEVTMPFS_MFLAGS (MS_SILENT | MS_NOEXEC | MS_NOSUID)
+#else
+# define DEVTMPFS_MFLAGS MS_SILENT
+#endif
+
static struct task_struct *thread;
#if defined CONFIG_DEVTMPFS_MOUNT
@@ -354,7 +360,8 @@
if (!thread)
return 0;
- err = sys_mount("devtmpfs", (char *)mntdir, "devtmpfs", MS_SILENT, NULL);
+ err = sys_mount("devtmpfs", (char *)mntdir, "devtmpfs",
+ DEVTMPFS_MFLAGS, NULL);
if (err)
printk(KERN_INFO "devtmpfs: error mounting %i\n", err);
else
@@ -380,7 +387,7 @@
*err = sys_unshare(CLONE_NEWNS);
if (*err)
goto out;
- *err = sys_mount("devtmpfs", "/", "devtmpfs", MS_SILENT, options);
+ *err = sys_mount("devtmpfs", "/", "devtmpfs", DEVTMPFS_MFLAGS, options);
if (*err)
goto out;
sys_chdir("/.."); /* will traverse into overmounted root */
