x86/ibt: Keep IBT disabled during alternative patching

commit ebebe307 upstream.

cfi_rewrite_callers() updates the fineIBT hash matching at the caller side,
but except for paranoid-mode it relies on apply_retpoline() and friends for
any ENDBR relocation. This could temporarily cause an indirect branch to
land on a poisoned ENDBR.

For instance, with para-virtualization enabled, a simple wrmsrl() could
have an indirect branch pointing to native_write_msr() who's ENDBR has been
relocated due to fineIBT:

<wrmsrl>:
       push   %rbp
       mov    %rsp,%rbp
       mov    %esi,%eax
       mov    %rsi,%rdx
       shr    $0x20,%rdx
       mov    %edi,%edi
       mov    %rax,%rsi
       call   *0x21e65d0(%rip)        # <pv_ops+0xb8>
       ^^^^^^^^^^^^^^^^^^^^^^^

Such an indirect call during the alternative patching could #CP if the
caller is not *yet* adjusted for the new target ENDBR. To prevent a false
 #CP, keep CET-IBT disabled until all callers are patched.

Patching during the module load does not need to be guarded by IBT-disable
because the module code is not executed until the patching is complete.

  [ pawan: Since apply_paravirt() happens before __apply_fineibt()
	   relocates the ENDBR, pv_ops in the example above is not relevant.
	   It is still safer to keep this commit because missing an ENDBR
	   means an oops. ]

Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>