ima: change how MODULE_SIG_FORCE is checked on modules checking policy

This patchset ensure that IMA's modules checking policy:

measure func=MODULE_CHECK uid=0

rely on the correct value of CONFIG_MODULE_SIG_FORCE, since the way it is today the code completely ignores the module.sig_enforce cmdline param, which behaves in a OR logic with the CONFIG value (CONFIG_MODULE_SIG_FORCE || module.sig_enforce). That said, everytime a module would load, in the current checking code, when the kernel was not compiled with the CONFIG set the call to init_module syscall fails with -EACCES:

# strace -f -v modprobe <any-module> | grep init_module
init_module(0x55b9bcc9bba0, 17763, "") = -1 EACCES (Permission denied)

With this patchset the result would rely on the module.sig_enforce cmdline as well. Once the CONFIG is not set, but the param is, the result would be 'success', as it should be:

# strace -f -v modprobe <any-module> | grep init_module
init_module(0x7f9602d6e010, 386646, "") = 0

The patchset was tested in two different kernels: 4.13.6 (Fedora 27) and 4.14.0-rc4 (integrity-next tree).

Changes:
v3:
1/2 - added dummy is_module_sig_enforced() function to the case where CONFIG_MODULES is not set and IMA=y.
v2:
1/2 - code changes to correct checkpatch.pl warnings.

Bruno E. O. Meneguele (2):
module: export module signature enforcement status
ima: check signature enforcement against cmdline param instead of
CONFIG

 include/linux/module.h            |  7 +++++++
 kernel/module.c                   | 10 ++++++++++
 security/integrity/ima/ima_main.c |  6 +++---
 3 files changed, 20 insertions(+), 3 deletions(-)

2.13.6

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