On Tue, Nov 28, 2017 at 10:16 PM, Luis R. Rodriguez <mcg...@kernel.org> wrote: > On Tue, Nov 28, 2017 at 12:11:34PM -0800, Kees Cook wrote: >> On Tue, Nov 28, 2017 at 11:14 AM, Luis R. Rodriguez <mcg...@kernel.org> >> wrote: >> > kmod is just a helper to poke userpsace to load a module, that's it. >> > >> > The old init_module() and newer finit_module() do the real handy work or >> > module loading, and both currently only use may_init_module(): >> > >> > static int may_init_module(void) >> > { >> > if (!capable(CAP_SYS_MODULE) || modules_disabled) >> > return -EPERM; >> > >> > return 0; >> > } >> > >> > This begs the question: >> > >> > o If userspace just tries to just use raw finit_module() do we want >> > similar >> > checks? >> > >> > Otherwise, correct me if I'm wrong this all seems pointless. >> >> Hm? That's direct-loading, not auto-loading. This series is only about >> auto-loading. > > And *all* auto-loading uses aliases? What's the difference between > auto-loading > and direct-loading?
Not all auto-loading uses aliases, auto-loading is when kernel code calls request_module() to loads the feature that was not present, and direct-loading in this thread is the direct syscalls like finit_module(). >> We already have a global sysctl for blocking direct-loading >> (modules_disabled). > > My point was that even if you have a CAP_NET_ADMIN check on request_module(), > finit_module() will not check for it, so a crafty userspace could still try > to just finit_module() directly, and completely then bypass the CAP_NET_ADMIN > check. The finit_module() uses CAP_SYS_MODULE which should allow all modules and in this context it should be more privileged than CAP_NET_ADMIN which is only for "netdev-%s" (to not load arbitrary modules with it). finit_module() coming from request_module() always has the CAP_NET_ADMIN, hence the check is done before. > So unless I'm missing something, I see no point in adding extra checks for > request_module() but nothing for the respective load_module(). I see, request_module() is called from kernel context which runs in init namespace will full capabilities, the spawned userspace modprobe will get CAP_SYS_MODULE and all other caps, then after comes modprobe and load_module(). Btw as suggested by Linus I will update with request_module_cap() and I can offer my help maintaining these bits too. > > Luis -- tixxdz