Commit-ID:  74327a3e884a0ff895ba7b51d3488e6a177407b2
Gitweb:     http://git.kernel.org/tip/74327a3e884a0ff895ba7b51d3488e6a177407b2
Author:     Andy Lutomirski <l...@kernel.org>
AuthorDate: Thu, 15 Sep 2016 22:45:46 -0700
Committer:  Ingo Molnar <mi...@kernel.org>
CommitDate: Fri, 16 Sep 2016 09:18:53 +0200

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 <l...@kernel.org>
Cc: Borislav Petkov <b...@alien8.de>
Cc: Brian Gerst <brge...@gmail.com>
Cc: Denys Vlasenko <dvlas...@redhat.com>
Cc: H. Peter Anvin <h...@zytor.com>
Cc: Jann Horn <j...@thejh.net>
Cc: Josh Poimboeuf <jpoim...@redhat.com>
Cc: Linus Torvalds <torva...@linux-foundation.org>
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: Thomas Gleixner <t...@linutronix.de>
Link: 
http://lkml.kernel.org/r/337aeca8614024aa4d8d9c81053bbf8fcffbe4ad.1474003868.git.l...@kernel.org
Signed-off-by: Ingo Molnar <mi...@kernel.org>
---
 arch/x86/kernel/process.c | 22 +++++++++++++++-------
 1 file changed, 15 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 0b9ed8e..4002b47 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;
 }

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