Concerns about enabling retpolines by default

[Florian Weimer]

We recently discovered that our OpenSSH distribution binaries contain
retpoline thunks.  It's due to this 

            OSSH_CHECK_CFLAG_COMPILE([-mfunction-return=thunk]) # gcc
            OSSH_CHECK_CFLAG_COMPILE([-mindirect-branch=thunk]) # gcc

This was quite surprising because at least the GNU/Linux userspace has
no provisions for retpolines.  You also fail to enable -fno-plt, so you
need a special linker that produces non-standard PLT stubs.  (And this
has to be repeated for all system libraries you call.)

In our case, it caused miscompilation because it triggered two different
GCC bugs (which are being fixed upstream):

  <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87412>
  <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87414>

There have been other retpoline bugs in GCC which do not affect the
kernel (or affect only rarely used kernel features), but are potentially
visible in user space, so few distributions will backport those fixes to
their distribution compilers.  My expectation is that most of these
compilers will never get fixed because the goal was to get the kernel to
compile with retpoline support, and nothing more.

Maybe the expectation is that distributions fine-tune their build flags
according to their toolchain support level and simply configure with the
--without-hardening flag, but this still looks like an unnecessary trap
for those who build their OpenSSH binaries themselves.

Thanks,
Florian