Merge branch 'x86-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 fixes from Thomas Gleixner:
 "A set of fixes for x86:

   - Make the unwinder more robust when it encounters a NULL pointer
     call, so the backtrace becomes more useful

   - Fix the bogus ORC unwind table alignment

   - Prevent kernel panic during kexec on HyperV caused by a cleared but
     not disabled hypercall page.

   - Remove the now pointless stacksize increase for KASAN_EXTRA, as
     KASAN_EXTRA is gone.

   - Remove unused variables from the x86 memory management code"

* 'x86-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  x86/hyperv: Fix kernel panic when kexec on HyperV
  x86/mm: Remove unused variable 'old_pte'
  x86/mm: Remove unused variable 'cpu'
  Revert "x86_64: Increase stack size for KASAN_EXTRA"
  x86/unwind: Add hardcoded ORC entry for NULL
  x86/unwind: Handle NULL pointer calls better in frame unwinder
  x86/unwind/orc: Fix ORC unwind table alignment

arch/x86/kernel/unwind_frame.c

@@ -320,10 +320,14 @@ bool unwind_next_frame(struct unwind_state *state)
 	}
 
 	/* Get the next frame pointer: */
-	if (state->regs)
+	if (state->next_bp) {
+		next_bp = state->next_bp;
+		state->next_bp = NULL;
+	} else if (state->regs) {
 		next_bp = (unsigned long *)state->regs->bp;
-	else
+	} else {
 		next_bp = (unsigned long *)READ_ONCE_TASK_STACK(state->task, *state->bp);
+	}
 
 	/* Move to the next frame if it's safe: */
 	if (!update_stack_state(state, next_bp))
@@ -398,6 +402,21 @@ void __unwind_start(struct unwind_state *state, struct task_struct *task,
 
 	bp = get_frame_pointer(task, regs);
 
+	/*
+	 * If we crash with IP==0, the last successfully executed instruction
+	 * was probably an indirect function call with a NULL function pointer.
+	 * That means that SP points into the middle of an incomplete frame:
+	 * *SP is a return pointer, and *(SP-sizeof(unsigned long)) is where we
+	 * would have written a frame pointer if we hadn't crashed.
+	 * Pretend that the frame is complete and that BP points to it, but save
+	 * the real BP so that we can use it when looking for the next frame.
+	 */
+	if (regs && regs->ip == 0 &&
+	    (unsigned long *)kernel_stack_pointer(regs) >= first_frame) {
+		state->next_bp = bp;
+		bp = ((unsigned long *)kernel_stack_pointer(regs)) - 1;
+	}
+
 	/* Initialize stack info and make sure the frame data is accessible: */
 	get_stack_info(bp, state->task, &state->stack_info,
 		       &state->stack_mask);
@@ -410,7 +429,7 @@ void __unwind_start(struct unwind_state *state, struct task_struct *task,
 	 */
 	while (!unwind_done(state) &&
 	       (!on_stack(&state->stack_info, first_frame, sizeof(long)) ||
-			state->bp < first_frame))
+			(state->next_bp == NULL && state->bp < first_frame)))
 		unwind_next_frame(state);
 }
 EXPORT_SYMBOL_GPL(__unwind_start);

arch/x86/kernel/unwind_orc.c

@@ -113,6 +113,20 @@ static struct orc_entry *orc_ftrace_find(unsigned long ip)
 }
 #endif
 
+/*
+ * If we crash with IP==0, the last successfully executed instruction
+ * was probably an indirect function call with a NULL function pointer,
+ * and we don't have unwind information for NULL.
+ * This hardcoded ORC entry for IP==0 allows us to unwind from a NULL function
+ * pointer into its parent and then continue normally from there.
+ */
+static struct orc_entry null_orc_entry = {
+	.sp_offset = sizeof(long),
+	.sp_reg = ORC_REG_SP,
+	.bp_reg = ORC_REG_UNDEFINED,
+	.type = ORC_TYPE_CALL
+};
+
 static struct orc_entry *orc_find(unsigned long ip)
 {
 	static struct orc_entry *orc;
@@ -120,6 +134,9 @@ static struct orc_entry *orc_find(unsigned long ip)
 	if (!orc_init)
 		return NULL;
 
+	if (ip == 0)
+		return &null_orc_entry;
+
 	/* For non-init vmlinux addresses, use the fast lookup table: */
 	if (ip >= LOOKUP_START_IP && ip < LOOKUP_STOP_IP) {
 		unsigned int idx, start, stop;