x86/unwind: Ensure stack pointer is aligned

With frame pointers disabled, on some older versions of GCC (like
4.8.3), it's possible for the stack pointer to get aligned at a
half-word boundary:

  00000000000004d0 <fib_table_lookup>:
       4d0:       41 57                   push   %r15
       4d2:       41 56                   push   %r14
       4d4:       41 55                   push   %r13
       4d6:       41 54                   push   %r12
       4d8:       55                      push   %rbp
       4d9:       53                      push   %rbx
       4da:       48 83 ec 24             sub    $0x24,%rsp

In such a case, the unwinder ends up reading the entire stack at the
wrong alignment.  Then the last read goes past the end of the stack,
hitting the stack guard page:

  BUG: stack guard page was hit at ffffc900217c4000 (stack is ffffc900217c0000..ffffc900217c3fff)
  kernel stack overflow (page fault): 0000 [#1] SMP
  ...

Fix it by ensuring the stack pointer is properly aligned before
unwinding.

Reported-by: Jirka Hladky <jhladky@redhat.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
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>
Fixes: 7c7900f ("x86/unwind: Add new unwind interface and implementations")
Link: http://lkml.kernel.org/r/cff33847cc9b02fa548625aa23268ac574460d8d.1492436590.git.jpoimboe@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/dumpstack.c

@@ -77,7 +77,7 @@ void show_trace_log_lvl(struct task_struct *task, struct pt_regs *regs,
 	 * - softirq stack
 	 * - hardirq stack
 	 */
-	for (regs = NULL; stack; stack = stack_info.next_sp) {
+	for (regs = NULL; stack; stack = PTR_ALIGN(stack_info.next_sp, sizeof(long))) {
 		const char *stack_name;
 
 		/*

arch/x86/kernel/unwind_guess.c

@@ -34,7 +34,7 @@ bool unwind_next_frame(struct unwind_state *state)
 				return true;
 		}
 
-		state->sp = info->next_sp;
+		state->sp = PTR_ALIGN(info->next_sp, sizeof(long));
 
 	} while (!get_stack_info(state->sp, state->task, info,
 				 &state->stack_mask));
@@ -49,7 +49,7 @@ void __unwind_start(struct unwind_state *state, struct task_struct *task,
 	memset(state, 0, sizeof(*state));
 
 	state->task = task;
-	state->sp   = first_frame;
+	state->sp   = PTR_ALIGN(first_frame, sizeof(long));
 
 	get_stack_info(first_frame, state->task, &state->stack_info,
 		       &state->stack_mask);