Re: [PATCH v1 1/1] binder: fix freeze race

[Todd Kjos]

On Thu, Sep 9, 2021 at 4:21 PM Li Li <dualli@xxxxxxxxxxxx> wrote:
>
> From: Li Li <dualli@xxxxxxxxxx>
>
> Currently cgroup freezer is used to freeze the application threads, and
> BINDER_FREEZE is used to freeze binder interface. There's already a
> mechanism for BINDER_FREEZE to wait for any existing transactions to
> drain out before actually freezing the binder interface.
>
> But freezing an app requires 2 steps, freezing the binder interface with
> BINDER_FREEZEwhich and then freezing the application main threads with
> cgroupfs. This is not an atomic operation. The following race issue
> might happen.
>
> 1) Binder interface is frozen by ioctl(BINDER_FREEZE);
> 2) Main thread initiates a new sync binder transaction;
> 3) Main thread is frozen by "echo 1 > cgroup.freeze";
> 4) The response reaches the frozen thread, which will unexpectedly fail.
>
> This patch provides a mechanism for user space freezer manager to check
> if there's any new pending transaction happening between BINDER_FREEZE
> and freezing main thread. If there's any, the main thread freezing
> operation can be rolled back.
>
> Furthermore, the response might reach the binder driver before the
> rollback actually happens. That will also cause failed transaction.
>
> As the other process doesn't wait for another response of the response,
> the failed response transaction can be fixed by treating the response
> transaction like an oneway/async one, allowing it to reach the frozen
> thread. And it will be consumed when the thread gets unfrozen later.
>
> Bug: 198493121

Please remove "Bug: " tag. Replace it with a "Fixes:" tag that cites
the patch that introduced the race.

> Signed-off-by: Li Li <dualli@xxxxxxxxxx>
> ---
>  drivers/android/binder.c          | 32 +++++++++++++++++++++++++++----
>  drivers/android/binder_internal.h |  2 ++
>  2 files changed, 30 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/android/binder.c b/drivers/android/binder.c
> index d9030cb6b1e4..f9713a97578c 100644
> --- a/drivers/android/binder.c
> +++ b/drivers/android/binder.c
> @@ -3038,9 +3038,8 @@ static void binder_transaction(struct binder_proc *proc,
>         if (reply) {
>                 binder_enqueue_thread_work(thread, tcomplete);
>                 binder_inner_proc_lock(target_proc);
> -               if (target_thread->is_dead || target_proc->is_frozen) {
> -                       return_error = target_thread->is_dead ?
> -                               BR_DEAD_REPLY : BR_FROZEN_REPLY;
> +               if (target_thread->is_dead) {
> +                       return_error = BR_DEAD_REPLY;
>                         binder_inner_proc_unlock(target_proc);
>                         goto err_dead_proc_or_thread;
>                 }
> @@ -4648,6 +4647,22 @@ static int binder_ioctl_get_node_debug_info(struct binder_proc *proc,
>         return 0;
>  }
>
> +static int binder_txns_pending(struct binder_proc *proc)
> +{
> +       struct rb_node *n;
> +       struct binder_thread *thread;
> +
> +       if (proc->outstanding_txns > 0)
> +               return 1;
> +
> +       for (n = rb_first(&proc->threads); n; n = rb_next(n)) {
> +               thread = rb_entry(n, struct binder_thread, rb_node);
> +               if (thread->transaction_stack)
> +                       return 1;
> +       }
> +       return 0;
> +}
> +
>  static int binder_ioctl_freeze(struct binder_freeze_info *info,
>                                struct binder_proc *target_proc)
>  {
> @@ -4682,6 +4697,14 @@ static int binder_ioctl_freeze(struct binder_freeze_info *info,
>         if (!ret && target_proc->outstanding_txns)
>                 ret = -EAGAIN;
>
> +       /* Also check pending transactions that wait for reply */
> +       if (ret >= 0) {
> +               binder_inner_proc_lock(target_proc);
> +               if (binder_txns_pending(target_proc))

The convention in the binder driver is to append "_ilocked" to the
function name if the function expects the inner proc lock to be held
(so "binder_txns_pending_ilocked(target_proc) in this case)".

> +                       ret = -EAGAIN;
> +               binder_inner_proc_unlock(target_proc);
> +       }
> +
>         if (ret < 0) {
>                 binder_inner_proc_lock(target_proc);
>                 target_proc->is_frozen = false;
> @@ -4705,7 +4728,8 @@ static int binder_ioctl_get_freezer_info(
>                 if (target_proc->pid == info->pid) {
>                         found = true;
>                         binder_inner_proc_lock(target_proc);
> -                       info->sync_recv |= target_proc->sync_recv;
> +                       info->sync_recv |= target_proc->sync_recv |
> +                                       (binder_txns_pending(target_proc) << 1);
>                         info->async_recv |= target_proc->async_recv;
>                         binder_inner_proc_unlock(target_proc);
>                 }
> diff --git a/drivers/android/binder_internal.h b/drivers/android/binder_internal.h
> index 810c0b84d3f8..402c4d4362a8 100644
> --- a/drivers/android/binder_internal.h
> +++ b/drivers/android/binder_internal.h
> @@ -378,6 +378,8 @@ struct binder_ref {
>   *                        binder transactions
>   *                        (protected by @inner_lock)
>   * @sync_recv:            process received sync transactions since last frozen
> + *                        bit 0: received sync transaction after being frozen
> + *                        bit 1: new pending sync transaction during freezing

Should these bit assignments be documented in binder.h since these bit
assignments end up being relevant in struct binder_frozen_status_info?


>   *                        (protected by @inner_lock)
>   * @async_recv:           process received async transactions since last frozen
>   *                        (protected by @inner_lock)
> --
> 2.33.0.309.g3052b89438-goog
>