x86/its: FineIBT-paranoid vs ITS

commit e52c1dc7 upstream.

FineIBT-paranoid was using the retpoline bytes for the paranoid check,
disabling retpolines, because all parts that have IBT also have eIBRS
and thus don't need no stinking retpolines.

Except... ITS needs the retpolines for indirect calls must not be in
the first half of a cacheline :-/

So what was the paranoid call sequence:

  <fineibt_paranoid_start>:
   0:   41 ba 78 56 34 12       mov    $0x12345678, %r10d
   6:   45 3b 53 f7             cmp    -0x9(%r11), %r10d
   a:   4d 8d 5b <f0>           lea    -0x10(%r11), %r11
   e:   75 fd                   jne    d <fineibt_paranoid_start+0xd>
  10:   41 ff d3                call   *%r11
  13:   90                      nop

Now becomes:

  <fineibt_paranoid_start>:
   0:   41 ba 78 56 34 12       mov    $0x12345678, %r10d
   6:   45 3b 53 f7             cmp    -0x9(%r11), %r10d
   a:   4d 8d 5b f0             lea    -0x10(%r11), %r11
   e:   2e e8 XX XX XX XX	cs call __x86_indirect_paranoid_thunk_r11

  Where the paranoid_thunk looks like:

   1d:  <ea>                    (bad)
   __x86_indirect_paranoid_thunk_r11:
   1e:  75 fd                   jne 1d
   __x86_indirect_its_thunk_r11:
   20:  41 ff eb                jmp *%r11
   23:  cc                      int3

[ dhansen: remove initialization to false ]

[ pawan: move the its_static_thunk() definition to alternative.c. This is
	 done to avoid a build failure due to circular dependency between
	 kernel.h(asm-generic/bug.h) and asm/alternative.h which is
	 needed for WARN_ONCE(). ]

[ Just a portion of the original commit, in order to fix a build issue
  in stable kernels due to backports ]

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com>
Link: https://lore.kernel.org/r/20250514113952.GB16434@noisy.programming.kicks-ass.net


Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>