Date: Tue, 17 Jan 2017 17:26:27 +0100 From: Greg KH <gregkh@...uxfoundation.org> To: Jeff Layton <jlayton@...chiereds.net> Cc: kernel-hardening@...ts.openwall.com, linux-kernel@...r.kernel.org, Benjamin Herrenschmidt <benh@...nel.crashing.org>, Thomas Sailer <t.sailer@...mni.ethz.ch>, "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>, Johan Hovold <johan@...nel.org>, Alex Elder <elder@...nel.org>, "J. Bruce Fields" <bfields@...ldses.org>, David Howells <dhowells@...hat.com>, NeilBrown <neilb@...e.com> Subject: Re: [PATCH 3/3] Introduce STATIC_USERMODEHELPER to mediate call_usermodehelper() On Tue, Jan 17, 2017 at 11:20:23AM -0500, Jeff Layton wrote: > On Mon, 2017-01-16 at 17:50 +0100, Greg KH wrote: > > From: Greg Kroah-Hartman <gregkh@...uxfoundation.org> > > > > Some usermode helper applications are defined at kernel build time, while > > others can be changed at runtime. To provide a sane way to filter these, add a > > new kernel option "STATIC_USERMODEHELPER". This option routes all > > call_usermodehelper() calls through this binary, no matter what the caller > > wishes to have called. > > > > The new binary (by default set to /sbin/usermode-helper, but can be changed > > through the STATIC_USERMODEHELPER_PATH option) can properly filter the > > requested programs to be run by the kernel by looking at the first argument > > that is passed to it. All other options should then be passed onto the proper > > program if so desired. > > > > To disable all call_usermodehelper() calls by the kernel, set > > STATIC_USERMODEHELPER_PATH to an empty string. > > > > Thanks to Neil Brown for the idea of this feature. > > > > Cc: NeilBrown <neilb@...e.com> > > Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org> > > --- > > kernel/kmod.c | 14 ++++++++++++++ > > security/Kconfig | 35 +++++++++++++++++++++++++++++++++++ > > 2 files changed, 49 insertions(+) > > > > diff --git a/kernel/kmod.c b/kernel/kmod.c > > index 426a614e97fe..0c407f905ca4 100644 > > --- a/kernel/kmod.c > > +++ b/kernel/kmod.c > > @@ -528,7 +528,12 @@ struct subprocess_info *call_usermodehelper_setup(const char *path, char **argv, > > goto out; > > > > INIT_WORK(&sub_info->work, call_usermodehelper_exec_work); > > + > > +#ifdef CONFIG_STATIC_USERMODEHELPER > > + sub_info->path = CONFIG_STATIC_USERMODEHELPER_PATH; > > +#else > > sub_info->path = path; > > +#endif > > sub_info->argv = argv; > > sub_info->envp = envp; > > > > @@ -566,6 +571,15 @@ int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait) > > retval = -EBUSY; > > goto out; > > } > > + > > + /* > > + * If there is no binary for us to call, then just return and get out of > > + * here. This allows us to set STATIC_USERMODEHELPER_PATH to "" and > > + * disable all call_usermodehelper() calls. > > + */ > > + if (strlen(sub_info->path) == 0) > > + goto out; > > + > > /* > > * Set the completion pointer only if there is a waiter. > > * This makes it possible to use umh_complete to free > > diff --git a/security/Kconfig b/security/Kconfig > > index 118f4549404e..d900f47eaa68 100644 > > --- a/security/Kconfig > > +++ b/security/Kconfig > > @@ -158,6 +158,41 @@ config HARDENED_USERCOPY_PAGESPAN > > been removed. This config is intended to be used only while > > trying to find such users. > > > > +config STATIC_USERMODEHELPER > > + bool "Force all usermode helper calls through a single binary" > > + help > > + By default, the kernel can call many different userspace > > + binary programs through the "usermode helper" kernel > > + interface. Some of these binaries are statically defined > > + either in the kernel code itself, or as a kernel configuration > > + option. However, some of these are dynamically created at > > + runtime, or can be modified after the kernel has started up. > > + To provide an additional layer of security, route all of these > > + calls through a single executable that can not have its name > > + changed. > > + > > + Note, it is up to this single binary to then call the relevant > > + "real" usermode helper binary, based on the first argument > > + passed to it. If desired, this program can filter and pick > > + and choose what real programs are called. > > + > > + If you wish for all usermode helper programs are to be > > + disabled, choose this option and then set > > + STATIC_USERMODEHELPER_PATH to an empty string. > > + > > +config STATIC_USERMODEHELPER_PATH > > + string "Path to the static usermode helper binary" > > + depends on STATIC_USERMODEHELPER > > + default "/sbin/usermode-helper" > > + help > > + The binary called by the kernel when any usermode helper > > + program is wish to be run. The "real" application's name will > > + be in the first argument passed to this program on the command > > + line. > > + > > + If you wish for all usermode helper programs to be disabled, > > + specify an empty string here (i.e. ""). > > + > > source security/selinux/Kconfig > > source security/smack/Kconfig > > source security/tomoyo/Kconfig > > I like the core of this idea (having a single dispatch binary) a lot. It > seems like a good way to limit the attack surface. We could even > consider signing this binary as well, etc... Or use a read-only partition that is checked with dm-verity at boot so you know it's safe. Lots of ways to protect this file. > I'm less excited though about using the binary pathnames in argv. > Would it be better to pass a non-path string of some sort that the > binary would have to turn into a path to the binary to exec? What exactly do you mean by "non-path" string? You can do whatever you want with the argv value in your new binary, but rewriting all of the users of this interface in the kernel to pass in some other type of value instead of a full path, that doesn't make much sense (hint a path is just as good as anything else.) thanks, greg k-h
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