module: harden ELF info handling

5fdc7db6 ("module: setup load info before module_sig_check()")
moved the ELF setup, so that it was done before the signature
check. This made the module name available to signature error
messages.

However, the checks for ELF correctness in setup_load_info
are not sufficient to prevent bad memory references due to
corrupted offset fields, indices, etc.

So, there's a regression in behavior here: a corrupt and unsigned
(or badly signed) module, which might previously have been rejected
immediately, can now cause an oops/crash.

Harden ELF handling for module loading by doing the following:

- Move the signature check back up so that it comes before ELF
  initialization. It's best to do the signature check to see
  if we can trust the module, before using the ELF structures
  inside it. This also makes checks against info->len
  more accurate again, as this field will be reduced by the
  length of the signature in mod_check_sig().

  The module name is now once again not available for error
  messages during the signature check, but that seems like
  a fair tradeoff.

- Check if sections have offset / size fields that at least don't
  exceed the length of the module.

- Check if sections have section name offsets that don't fall
  outside the section name table.

- Add a few other sanity checks against invalid section indices,
  etc.

This is not an exhaustive consistency check, but the idea is to
at least get through the signature and blacklist checks without
crashing because of corrupted ELF info, and to error out gracefully
for most issues that would have caused problems later on.

Fixes: 5fdc7db6 ("module: setup load info before module_sig_check()")
Signed-off-by: Frank van der Linden <fllinden@amazon.com>
Signed-off-by: Jessica Yu <jeyu@kernel.org>