linux-kernel - Re: [PATCH] binder: Address corner cases in deferred copy and fixup

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Date:   Wed, 13 Apr 2022 12:00:03 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Alessandro Astone <ales.astone@...il.com>
Cc:     tkjos@...roid.com, brauner@...nel.org, arve@...roid.com,
        linux-kernel@...r.kernel.org, maco@...roid.com,
        Joel Fernandes <joel@...lfernandes.org>,
        Hridya Valsaraju <hridya@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [PATCH] binder: Address corner cases in deferred copy and fixup

On Wed, Apr 13, 2022 at 10:54:27AM +0200, Alessandro Astone wrote:
> When handling BINDER_TYPE_FDA object we are pushing a parent fixup
> with a certain skip_size but no scatter-gather copy object, since
> the copy is handled standalone.
> If BINDER_TYPE_FDA is the last children the scatter-gather copy
> loop will never stop to skip it, thus we are left with an item in
> the parent fixup list. This will trigger the BUG_ON().
> 
> Furthermore, it is possible to receive BINDER_TYPE_FDA object
> with num_fds=0 which will confuse the scatter-gather code.
> 
> In the android userspace I could only find these usecases in the
> libstagefright OMX implementation, so it might be that they're
> doing something very weird, but nonetheless the kernel should not
> panic about it.
> 
> Fixes: 09184ae9b575 ("binder: defer copies of pre-patched txn data")
> Signed-off-by: Alessandro Astone <ales.astone@...il.com>
> ---
>  drivers/android/binder.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/android/binder.c b/drivers/android/binder.c
> index 8351c5638880..18ad6825ba30 100644
> --- a/drivers/android/binder.c
> +++ b/drivers/android/binder.c
> @@ -2295,7 +2295,7 @@ static int binder_do_deferred_txn_copies(struct binder_alloc *alloc,
>  {
>  	int ret = 0;
>  	struct binder_sg_copy *sgc, *tmpsgc;
> -	struct binder_ptr_fixup *pf =
> +	struct binder_ptr_fixup *tmppf, *pf =

Just make this a new line:
	struct binder_ptr_fixup *tmppf;
above the existing line.

>  		list_first_entry_or_null(pf_head, struct binder_ptr_fixup,
>  					 node);
>  
> @@ -2349,7 +2349,11 @@ static int binder_do_deferred_txn_copies(struct binder_alloc *alloc,
>  		list_del(&sgc->node);
>  		kfree(sgc);
>  	}
> -	BUG_ON(!list_empty(pf_head));

So you are hitting this BUG_ON() today?

> +	list_for_each_entry_safe(pf, tmppf, pf_head, node) {
> +		BUG_ON(pf->skip_size == 0);
> +		list_del(&pf->node);
> +		kfree(pf);
> +	}
>  	BUG_ON(!list_empty(sgc_head));
>  
>  	return ret > 0 ? -EINVAL : ret;
> @@ -2486,6 +2490,9 @@ static int binder_translate_fd_array(struct list_head *pf_head,
>  	struct binder_proc *proc = thread->proc;
>  	int ret;
>  
> +	if (fda->num_fds == 0)
> +		return 0;

Why return 0?

This feels like a separate issue from above, should this be 2 different
commits?

thanks,

greg k-h