modules: allow modprobe load regular elf binaries
As the first step in development of bpfilter project [1] the request_module()
code is extended to allow user mode helpers to be invoked. Idea is that
user mode helpers are built as part of the kernel build and installed as
traditional kernel modules with .ko file extension into distro specified
location, such that from a distribution point of view, they are no different
than regular kernel modules. Thus, allow request_module() logic to load such
user mode helper (umh) modules via:
request_module("foo") ->
call_umh("modprobe foo") ->
sys_finit_module(FD of /lib/modules/.../foo.ko) ->
call_umh(struct file)
Such approach enables kernel to delegate functionality traditionally done
by kernel modules into user space processes (either root or !root) and
reduces security attack surface of such new code, meaning in case of
potential bugs only the umh would crash but not the kernel. Another
advantage coming with that would be that bpfilter.ko can be debugged and
tested out of user space as well (e.g. opening the possibility to run
all clang sanitizers, fuzzers or test suites for checking translation).
Also, such architecture makes the kernel/user boundary very precise:
control plane is done by the user space while data plane stays in the kernel.
It's easy to distinguish "umh module" vs traditional kernel module:
$ readelf -h bld/net/bpfilter/bpfilter.ko|grep Type
Type: EXEC (Executable file)
$ readelf -h bld/net/ipv4/netfilter/iptable_filter.ko |grep Type
Type: REL (Relocatable file)
Since umh can crash, can be oom-ed by the kernel, 'kill -9' by admin,
the subsystem that uses them (like bpfilter) need to manage life
time of umh on its own, so module infra doesn't do any accounting
of them. They don't appear in "lsmod" and cannot be "rmmod".
Multiple request_module("umh") will load multiple umh.ko processes.
Similar to kernel modules the kernel will be tainted if "umh module"
has invalid signature.
[1] https://lwn.net/Articles/747551/
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
5 files changed
