Date: Tue, 17 Jan 2017 11:52:59 -0500 From: Jeff Layton <jlayton@...chiereds.net> To: Greg KH <gregkh@...uxfoundation.org> 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, 2017-01-17 at 17:26 +0100, Greg KH wrote: > 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. > ...and the binary could be even more helpful too -- drop capabilities or change task creds before calling exec, for some binaries for instance. This may even provide a way to allow the upcalls to switch to the appropriate namespaces maybe based on info passed in env vars? In any case, there are a lot of useful possibilities here. > > 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.) > > I guess I'm thinking that this new binary is an opportunity to formalize the usermode helper upcall stuff to be more of a real ABI. It's rather haphazard now. So yeah, the pathname strings would work fine as dispatch keys, but it is a rather ugly interface. It might be nicer to pass in some set of opaque string tokens so that the upcalls being requested have a real meaning that's not defined by the legacy pathnames? Hmm...I guess in that case we could pass this token along in the env array as well. Fair enough, I think this is fine. Acked-by: Jeff Layton <jlayton@...chiereds.net>
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