Re: [PATCH v3] devtmpfs: mount with noexec and nosuid

From: Roland Eggner
Date: Wed Nov 21 2012 - 01:44:42 EST


On 2012-11-20 Tuesday at 13:50 -0800 Kees Cook wrote:
> 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@xxxxxxxxxxxx
> Cc: Kay Sievers <kay@xxxxxxxx>
> Cc: Roland Eggner <edvx1@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
>
> ---
> 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

Can we afford 2 additional characters and name it âDEVTMPFS_NOEXECâ?

> + 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.
Proposal:
help
This instructs the kernel to include the MS_NOEXEC and MS_NOSUID mount
flags when mounting devtmpfs.
In-kernel separation of executable and non-executable code combined
with a proper executability policy is a basic technique to protect
against exploits by buggy or malicious code or hardware errors. In
terms of overhead it is a low-cost-high-effect technique especially on
platforms with dedicated hardware support, e.g. x86_64 (look for "NX"
feature in BIOS settings). Mounting devtmpfs with MS_NOEXEC flag is
an essential building-block for this security technique.

Notice: If enabled, software which depends on execution of
runtime-generated code can only be used with restricted feature set or
not at all, e.g. proprietary video drivers, JIT-compilers, most modern
web browsers. The grsecurity-patchset provides exception mechanisms to
solve this problem for e.g. desktop systems.

For server and embedded systems with HA-requirements consider Y.
For desktop systems say N unless you know what you do.

Apart from that â
Acked-by: Roland Eggner

> +
> 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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