[PATCH] x86/unwind: ensure stack pointer is aligned

[Josh Poimboeuf]

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
  CPU: 48 PID: 3700 Comm: mosh-server Not tainted 4.9.0-9.el7.nodebug2.x86_64 #1
  Hardware name: Dell Inc. PowerEdge R820/0RN9TC, BIOS 2.3.4 01/22/2016
  task: ffff88081f4de040 task.stack: ffffc900217c0000
  RIP: 0010:[<ffffffff810203b2>] [<ffffffff810203b2>] show_trace_log_lvl+0xe2/0x200
  RSP: 0018:ffff88081fb057b0  EFLAGS: 00010012
  RAX: 0000000000000000 RBX: ffffc900217c3ffc RCX: 0000000000000001
  RDX: 0000000000000000 RSI: ffff88081fb057b8 RDI: 0000002b00007ffc
  RBP: ffff88081fb05850 R08: 0000000000000a91 R09: ffff88081fb0ecf3
  R10: 0000000000000003 R11: 000000000000000f R12: 0000002b00007ffc
  R13: ffff88081f4de040 R14: ffffffff81950d37 R15: 0000000000000000
  FS:  00007fd0ffe61840(0000) GS:ffff88081fb00000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: ffffc900217c4000 CR3: 000000081c44a000 CR4: 00000000001406e0
  Call Trace:
   <NMI>
    ? show_stack_log_lvl+0x15e/0x1f0
    ? vmalloc_fault+0x31d/0x330
    ? vmalloc_fault+0x31d/0x330
    ? dump_stack+0x5c/0x7c
    ? __warn+0xba/0xe0
    ? vmalloc_fault+0x31d/0x330
    ? __do_page_fault+0x330/0x4b0
    ? do_page_fault+0x2b/0x70
    ? page_fault+0x28/0x30
    ? unwind_next_frame+0x35/0x70
    ? vsnprintf+0x3c5/0x590
    ? sprintf+0x56/0x80
    ? kallsyms_lookup+0x7b/0xb0
    ? __sprint_symbol+0xd0/0x100
    ? symbol_string+0x56/0xa0
    ? kallsyms_lookup+0x7b/0xb0
    ? __sprint_symbol+0xd0/0x100
    ? symbol_string+0x56/0xa0
    ? pointer+0x23d/0x410
    ? vsnprintf+0x25c/0x590
    ? irq_work_queue+0x9/0x70
    ? vprintk_nmi+0x99/0xa0
    ? vprintk_nmi+0x99/0xa0
    ? printk+0x58/0x6f
    ? __module_text_address+0x9/0x60
    ? is_module_text_address+0x5/0x10
    ? show_trace_log_lvl+0xed/0x200
    ? no_context+0x233/0x400
    ? no_context+0x233/0x400
    ? show_stack_log_lvl+0x15e/0x1f0
    ? show_regs+0xa7/0x1c0
    ? __die+0x9e/0xe0
    ? die+0x2e/0x60
    ? handle_stack_overflow+0x3a/0x50
    ? no_context+0x233/0x400
   <EOE>

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

Reported-by: Jirka Hladky <jhladky@xxxxxxxxxx>
Fixes: 7c7900f89770 ("x86/unwind: Add new unwind interface and implementations")
Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
---
 arch/x86/kernel/dumpstack.c    | 2 +-
 arch/x86/kernel/unwind_guess.c | 4 ++--
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
index 924f45e..dbce3cc 100644
--- a/arch/x86/kernel/dumpstack.c
+++ b/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;
 
 		/*
diff --git a/arch/x86/kernel/unwind_guess.c b/arch/x86/kernel/unwind_guess.c
index 22881dd..039f367 100644
--- a/arch/x86/kernel/unwind_guess.c
+++ b/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);
-- 
2.7.4