Re: [PATCH] x86/process: Silence KASAN warnings in get_wchan()

[Dmitry Vyukov]

LGTM

Reviewed-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>


On Mon, Oct 5, 2015 at 12:28 PM, Andrey Ryabinin
<aryabinin@xxxxxxxxxxxxx> wrote:
> get_wchan() is racy by design, it may access volatile stack
> of running task, thus it may access redzone in a stack frame
> and cause KASAN to warn about this.
>
> Use kasan_disable_current()/kasan_enable_current() to silence
> these warnings.
>
> Reported-by: Sasha Levin <sasha.levin@xxxxxxxxxx>
> Signed-off-by: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
> ---
>
>  Perhaps it would be better to add something like this:
>         READ_ONCE_NOCHECK()
>         {
>                 kasan_disable_current();
>                 READ_ONCE();
>                 kasan_enable_current();
>         }
>   ?
>
>  arch/x86/kernel/process.c | 18 +++++++++++++-----
>  1 file changed, 13 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> index 39e585a..0488eb9 100644
> --- a/arch/x86/kernel/process.c
> +++ b/arch/x86/kernel/process.c
> @@ -13,6 +13,7 @@
>  #include <linux/random.h>
>  #include <linux/user-return-notifier.h>
>  #include <linux/dmi.h>
> +#include <linux/kasan.h>
>  #include <linux/utsname.h>
>  #include <linux/stackprotector.h>
>  #include <linux/tick.h>
> @@ -514,7 +515,7 @@ unsigned long arch_randomize_brk(struct mm_struct *mm)
>   */
>  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)
> @@ -550,14 +551,21 @@ unsigned long get_wchan(struct task_struct *p)
>         if (sp < bottom || sp > top)
>                 return 0;
>
> +       kasan_disable_current();
>         fp = READ_ONCE(*(unsigned long *)sp);
>         do {
>                 if (fp < bottom || fp > top)
> -                       return 0;
> +                       goto out;
> +
>                 ip = READ_ONCE(*(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(*(unsigned long *)fp);
>         } while (count++ < 16 && p->state != TASK_RUNNING);
> -       return 0;
> +
> +out:
> +       kasan_enable_current();
> +       return ret;
>  }
> --
> 2.4.9
>
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