lkml.org 
[lkml]   [2017]   [Nov]   [28]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    From
    Date
    SubjectRe: [PATCH v5 next 1/5] modules:capabilities: add request_module_cap()
    On Tue, Nov 28, 2017 at 10:16 PM, Luis R. Rodriguez <mcgrof@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 <mcgrof@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

    \
     
     \ /
      Last update: 2017-11-28 22:42    [W:6.528 / U:0.360 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site