linux-kernel - [PATCH v3] devtmpfs: mount with noexec and nosuid

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Message-ID: <20121120215059.GA1859@www.outflux.net>
Date:	Tue, 20 Nov 2012 13:50:59 -0800
From:	Kees Cook <keescook@...omium.org>
To:	linux-kernel@...r.kernel.org
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	ellyjones@...omium.org, Kay Sievers <kay@...y.org>,
	Roland Eggner <edvx1@...temanalysen.net>
Subject: [PATCH v3] 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@...omium.org
Cc: Kay Sievers <kay@...y.org>
Cc: Roland Eggner <edvx1@...temanalysen.net>
Signed-off-by: Kees Cook <keescook@...omium.org>

---
v3:
- use a single define for the mount flags, suggested by Greg K.H.
v2:
- use CONFIG_DEVTMPFS_SAFE to wrap the logic.
---
 drivers/base/Kconfig    |   12 ++++++++++++
 drivers/base/devtmpfs.c |   11 +++++++++--
 2 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/drivers/base/Kconfig b/drivers/base/Kconfig
index b34b5cd..a37fcf2 100644
--- a/drivers/base/Kconfig
+++ b/drivers/base/Kconfig
@@ -56,6 +56,18 @@ config DEVTMPFS_MOUNT
 	  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" if EXPERIMENTAL
 	default y
diff --git a/drivers/base/devtmpfs.c b/drivers/base/devtmpfs.c
index 147d1a4..e44ca1d 100644
--- a/drivers/base/devtmpfs.c
+++ b/drivers/base/devtmpfs.c
@@ -25,6 +25,12 @@
 #include <linux/slab.h>
 #include <linux/kthread.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
@@ -347,7 +353,8 @@ int devtmpfs_mount(const char *mntdir)
 	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
@@ -372,7 +379,7 @@ static int devtmpfsd(void *p)
 	*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 */
-- 
1.7.9.5


-- 
Kees Cook
Chrome OS Security
--
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