x86/entry: Avoid very early RET

Commit

  ee774dac ("x86/entry: Move PUSH_AND_CLEAR_REGS out of error_entry()")

manages to introduce a CALL/RET pair that is before SWITCH_TO_KERNEL_CR3,
which means it is before RETBleed can be mitigated.

Revert to an earlier version of the commit in Fixes. Down side is that
this will bloat .text size somewhat. The alternative is fully reverting
it.

The purpose of this patch was to allow migrating error_entry() to C,
including the whole of kPTI. Much care needs to be taken moving that
forward to not re-introduce this problem of early RETs.

Fixes: ee774dac ("x86/entry: Move PUSH_AND_CLEAR_REGS out of error_entry()")
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: NBorislav Petkov <bp@suse.de>
Reviewed-by: NJosh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: NBorislav Petkov <bp@suse.de>

arch/x86/entry/entry_64.S

@@ -322,14 +322,6 @@ SYM_CODE_END(ret_from_fork)
 #endif
 .endm
 
-/* Save all registers in pt_regs */
-SYM_CODE_START_LOCAL(push_and_clear_regs)
-	UNWIND_HINT_FUNC
-	PUSH_AND_CLEAR_REGS save_ret=1
-	ENCODE_FRAME_POINTER 8
-	RET
-SYM_CODE_END(push_and_clear_regs)
-
 /**
  * idtentry_body - Macro to emit code calling the C function
  * @cfunc:		C function to be called
@@ -337,8 +329,8 @@ SYM_CODE_END(push_and_clear_regs)
  */
 .macro idtentry_body cfunc has_error_code:req
 
-	call push_and_clear_regs
-	UNWIND_HINT_REGS
+	PUSH_AND_CLEAR_REGS
+	ENCODE_FRAME_POINTER
 
 	/*
 	 * Call error_entry() and switch to the task stack if from userspace.