x86/process: Pin the target stack in get_wchan()

This will prevent a crash if get_wchan() runs after the task stack
is freed.

Signed-off-by: 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: Jann Horn <jann@thejh.net>
Cc: Josh Poimboeuf <jpoimboe@redhat.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/337aeca8614024aa4d8d9c81053bbf8fcffbe4ad.1474003868.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 15 insertions, 7 deletions

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 0b9ed8ec5226..4002b475171c 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -532,15 +532,18 @@ unsigned long thread_saved_pc(struct task_struct *tsk)
  */
 unsigned long get_wchan(struct task_struct *p)
 {
-        unsigned long start, bottom, top, sp, fp, ip;
+        unsigned long start, bottom, top, sp, fp, ip, ret = 0;
         int count = 0;
 
         if (!p || p == current || p->state == TASK_RUNNING)
                 return 0;
 
+        if (!try_get_task_stack(p))
+                return 0;
+
         start = (unsigned long)task_stack_page(p);
         if (!start)
-                return 0;
+                goto out;
 
         /*
          * Layout of the stack page:
@@ -564,16 +567,21 @@ unsigned long get_wchan(struct task_struct *p)
 
         sp = READ_ONCE(p->thread.sp);
         if (sp < bottom || sp > top)
-                return 0;
+                goto out;
 
         fp = READ_ONCE_NOCHECK(((struct inactive_task_frame *)sp)->bp);
         do {
                 if (fp < bottom || fp > top)
-                        return 0;
+                        goto out;
                 ip = READ_ONCE_NOCHECK(*(unsigned long *)(fp + sizeof(unsigned long)));
-                if (!in_sched_functions(ip))
-                        return ip;
+                if (!in_sched_functions(ip)) {
+                        ret = ip;
+                        goto out;
+                }
                 fp = READ_ONCE_NOCHECK(*(unsigned long *)fp);
         } while (count++ < 16 && p->state != TASK_RUNNING);
-        return 0;
+
+out:
+        put_task_stack(p);
+        return ret;
 }