Messages in this thread | | | Date | Wed, 21 Nov 2012 07:44:50 +0100 | From | Roland Eggner <> | Subject | Re: [PATCH v3] devtmpfs: mount with noexec and nosuid |
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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@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. > --- > 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 [unhandled content-type:application/pgp-signature] | |