linux-kernel - Re: timerfd: use-after-free in timerfd_remove

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Date:   Tue, 31 Jan 2017 09:19:56 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Mateusz Guzik <mguzik@...hat.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Thomas Gleixner <tglx@...utronix.de>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: timerfd: use-after-free in timerfd_remove_cancel

On Tue, Jan 31, 2017 at 3:06 AM, Mateusz Guzik <mguzik@...hat.com> wrote:
> On Mon, Jan 30, 2017 at 07:41:59PM +0100, Dmitry Vyukov wrote:
>> Hello,
>>
>> The following program triggers use-after-free in timerfd_remove_cancel:
>> https://gist.githubusercontent.com/dvyukov/202576d437c84ffbbe52e9ccd77e1b44/raw/5562bff8626a73627157331ea2b837f59080ac84/gistfile1.txt
>>
>> BUG: KASAN: use-after-free in __list_del include/linux/list.h:104
>> [inline] at addr ffff88006bab1410
>> BUG: KASAN: use-after-free in __list_del_entry
>> include/linux/list.h:119 [inline] at addr ffff88006bab1410
>> BUG: KASAN: use-after-free in list_del_rcu include/linux/rculist.h:129
>> [inline] at addr ffff88006bab1410
>> BUG: KASAN: use-after-free in timerfd_remove_cancel fs/timerfd.c:120
>> [inline] at addr ffff88006bab1410
>> BUG: KASAN: use-after-free in timerfd_release+0x28e/0x310
>> fs/timerfd.c:209 at addr ffff88006bab1410
>> Write of size 8 by task a.out/2897
>>
> [..]
>> Seems that ctx->might_cancel is racy.
>>
>
> Indeed it is. Can you try the patch below please. If it works I'll send
> it in a nicer form.

If the reproducer does not crash kernel (assuming you tested the
patch), than there is nothing else I can do to test it.


> diff --git a/fs/timerfd.c b/fs/timerfd.c
> index c173cc1..63f91c3 100644
> --- a/fs/timerfd.c
> +++ b/fs/timerfd.c
> @@ -112,14 +112,30 @@ void timerfd_clock_was_set(void)
>         rcu_read_unlock();
>  }
>
> +static void timerfd_set_cancel(struct timerfd_ctx *ctx)
> +{
> +       if (ctx->might_cancel)
> +               return;

/\/\/\/\/\/\

But this is not OK. This is a data race. We will get back to you with
a data race report soon.
If you want to play smart, you need at least READ_ONCE/WRITE_ONCE for
variables not protected with locks.
However, it looks like this code still has atomicity violation: if I
do two calls, one that needs to setup cancel and one that does not, I
can end up with an inconsistent outcome -- e.g. timer is setup as if
it needs to be in cancel_list but it is not added to the cancel_list;
or vice versa  -- timer is setup as if it does not need cancel but it
is added to the cancel_list.


> +       spin_lock(&cancel_lock);
> +       if (!ctx->might_cancel) {
> +               ctx->might_cancel = true;
> +               list_add_rcu(&ctx->clist, &cancel_list);
> +       }
> +       spin_unlock(&cancel_lock);
> +}
> +
>  static void timerfd_remove_cancel(struct timerfd_ctx *ctx)
>  {
> +       if (!ctx->might_cancel)
> +               return;
> +
> +       spin_lock(&cancel_lock);
>         if (ctx->might_cancel) {
>                 ctx->might_cancel = false;
> -               spin_lock(&cancel_lock);
>                 list_del_rcu(&ctx->clist);
> -               spin_unlock(&cancel_lock);
>         }
> +       spin_unlock(&cancel_lock);
>  }
>
>  static bool timerfd_canceled(struct timerfd_ctx *ctx)
> @@ -134,16 +150,10 @@ static void timerfd_setup_cancel(struct timerfd_ctx *ctx, int flags)
>  {
>         if ((ctx->clockid == CLOCK_REALTIME ||
>              ctx->clockid == CLOCK_REALTIME_ALARM) &&
> -           (flags & TFD_TIMER_ABSTIME) && (flags & TFD_TIMER_CANCEL_ON_SET)) {
> -               if (!ctx->might_cancel) {
> -                       ctx->might_cancel = true;
> -                       spin_lock(&cancel_lock);
> -                       list_add_rcu(&ctx->clist, &cancel_list);
> -                       spin_unlock(&cancel_lock);
> -               }
> -       } else if (ctx->might_cancel) {
> +           (flags & TFD_TIMER_ABSTIME) && (flags & TFD_TIMER_CANCEL_ON_SET))
> +               timerfd_set_cancel(ctx);
> +       else
>                 timerfd_remove_cancel(ctx);
> -       }
>  }
>
>  static ktime_t timerfd_get_remaining(struct timerfd_ctx *ctx)