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: [email protected] > Cc: Kay Sievers <[email protected]> > Cc: Roland Eggner <[email protected]> > Signed-off-by: Kees Cook <[email protected]> > > --- > 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 “DEVTMPFS_NOEXEC”?
> + 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 …
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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