x86/unwind: Warn on bad frame pointer

Detect situations in the unwinder where the frame pointer refers to a
bad address, and print an appropriate warning.

Use printk_deferred_once() because the unwinder can be called with the
console lock by lockdep via save_stack_trace().

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/03c888f6f7414d54fa56b393ea25482be6899b5f.1477496147.git.jpoimboe@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/unwind_frame.c

@@ -123,8 +123,17 @@ bool unwind_next_frame(struct unwind_state *state)
 	}
 
 	/* make sure the next frame's data is accessible */
-	if (!update_stack_state(state, next_frame, next_len))
-		return false;
+	if (!update_stack_state(state, next_frame, next_len)) {
+		/*
+		 * Don't warn on bad regs->bp.  An interrupt in entry code
+		 * might cause a false positive warning.
+		 */
+		if (state->regs)
+			goto the_end;
+
+		goto bad_address;
+	}
+
 	/* move to the next frame */
 	if (regs) {
 		state->regs = regs;
@@ -136,6 +145,11 @@ bool unwind_next_frame(struct unwind_state *state)
 
 	return true;
 
+bad_address:
+	printk_deferred_once(KERN_WARNING
+		"WARNING: kernel stack frame pointer at %p in %s:%d has bad value %p\n",
+		state->bp, state->task->comm,
+		state->task->pid, next_bp);
 the_end:
 	state->stack_info.type = STACK_TYPE_UNKNOWN;
 	return false;