x86/cet: Fix shskt manipulation error with BUGFRAME_{warn,run_fn}

x86/cet: Fix shskt manipulation error with BUGFRAME_{warn,run_fn}

This was a clear oversight in the original CET work.  The BUGFRAME_run_fn and
BUGFRAME_warn paths update regs->rip without an equivalent adjustment to the
shadow stack, causing IRET to suffer #CP because of the mismatch.

One subtle, and therefore fragile, aspect of extable_shstk_fixup() was that it
required regs->rip to have its old value as a cross-check that the right word
in the shadow stack was being edited.

Rework extable_shstk_fixup() into fixup_exception_return() which takes
ownership of the update to both the regular and shadow stacks, ensuring that
the regs->rip update is ordered correctly.

Use the new fixup_exception_return() for BUGFRAME_run_fn and BUGFRAME_warn to
ensure that the shadow stack is updated too.

Fixes: 209fb9919b50 ("x86/extable: Adjust extable handling to be shadow stack compatible")
Reported-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Tested-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
x86/cet: Fix build on newer versions of GCC

Some versions of GCC complain with:

  traps.c:405:22: error: 'get_shstk_bottom' defined but not used [-Werror=unused-function]
   static unsigned long get_shstk_bottom(unsigned long sp)
                        ^~~~~~~~~~~~~~~~
  cc1: all warnings being treated as errors

Change #ifdef to if ( IS_ENABLED(...) ) to make the sole user of
get_shstk_bottom() visible to the compiler.

Fixes: 35727551c070 ("x86/cet: Fix shskt manipulation error with BUGFRAME_{warn,run_fn}")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Compile-tested-by: Jan Beulich <jbeulich@suse.com>
Tested-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
master commit: 35727551c0703493a2240e967cffc3063b13d49c
master date: 2021-08-16 16:03:20 +0100
master commit: 54c9736382e0d558a6acd820e44185e020131c48
master date: 2021-08-17 12:55:48 +0100