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Date:   Tue, 17 May 2022 19:19:09 +0000
From:   Carlos Llamas <cmllamas@google.com>
To:     Schspa Shi <schspa@gmail.com>
Cc:     gregkh@linuxfoundation.org, arve@android.com, tkjos@android.com,
        maco@android.com, joel@joelfernandes.org, brauner@kernel.org,
        hridya@google.com, surenb@google.com, linux-kernel@vger.kernel.org,
        syzbot+46fff6434a7f968ecb39@syzkaller.appspotmail.com
Subject: Re: [PATCH] binder: fix atomic sleep when get extended error
Message-ID: <YoP1LSSzgg9sHJDW@google.com>
References: <20220517175225.65283-1-schspa@gmail.com>
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In-Reply-To: <20220517175225.65283-1-schspa@gmail.com>
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On Wed, May 18, 2022 at 01:52:25AM +0800, Schspa Shi wrote:
> binder_inner_proc_lock(thread->proc) is a spin lock, copy_to_user can't
> be called with in this lock.
> 
> Copy it as a local variable, and check the id to make sure the user space
> gets the latest error message
> 
> Reported-by: syzbot+46fff6434a7f968ecb39@syzkaller.appspotmail.com
> Fixes: bd32889e841c ("binder: add BINDER_GET_EXTENDED_ERROR ioctl")
> Signed-off-by: Schspa Shi <schspa@gmail.com>
> ---
>  drivers/android/binder.c | 13 ++++++++++---
>  1 file changed, 10 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/android/binder.c b/drivers/android/binder.c
> index d9253b2a7bd9..5f2e1fa3da74 100644
> --- a/drivers/android/binder.c
> +++ b/drivers/android/binder.c
> @@ -5163,13 +5163,20 @@ static int binder_ioctl_get_extended_error(struct binder_thread *thread,
>  					   void __user *ubuf)
>  {
>  	struct binder_extended_error *ee = &thread->ee;
> +	struct binder_extended_error eeb;
>  
>  	binder_inner_proc_lock(thread->proc);
> -	if (copy_to_user(ubuf, ee, sizeof(*ee))) {
> -		binder_inner_proc_unlock(thread->proc);
> +retry:
> +	eeb = *ee;
> +	binder_inner_proc_unlock(thread->proc);
> +	if (copy_to_user(ubuf, &eeb, sizeof(eeb)))
>  		return -EFAULT;
> -	}
>  
> +	binder_inner_proc_lock(thread->proc);
> +	if (eeb.id != ee->id) {
> +		/* retry to get newest error info */
> +		goto retry;
> +	}
>  	ee->id = 0;
>  	ee->command = BR_OK;
>  	ee->param = 0;
> -- 
> 2.24.3 (Apple Git-128)
>

Oops! Thank you for your patch. In this case the local copy would be
enough, no need for the retry logic as this is already taken care of.
You could also leverage the binder_set_extended_error() to reset the
thread->ee. Would you mind sending these updates? I was thinking
something like this:

 drivers/android/binder.c | 15 ++++++---------
 1 file changed, 6 insertions(+), 9 deletions(-)

diff --git a/drivers/android/binder.c b/drivers/android/binder.c
index 83facfa1a5c3..f92021cd384b 100644
--- a/drivers/android/binder.c
+++ b/drivers/android/binder.c
@@ -5163,19 +5163,16 @@ static int binder_ioctl_get_freezer_info(
 static int binder_ioctl_get_extended_error(struct binder_thread *thread,
 					   void __user *ubuf)
 {
-	struct binder_extended_error *ee = &thread->ee;
+	struct binder_extended_error ee;

 	binder_inner_proc_lock(thread->proc);
-	if (copy_to_user(ubuf, ee, sizeof(*ee))) {
-		binder_inner_proc_unlock(thread->proc);
-		return -EFAULT;
-	}
-
-	ee->id = 0;
-	ee->command = BR_OK;
-	ee->param = 0;
+	ee = thread->ee;
+	binder_set_extended_error(&thread->ee, 0, BR_OK, 0);
 	binder_inner_proc_unlock(thread->proc);

+	if (copy_to_user(ubuf, &ee, sizeof(ee)))
+		return -EFAULT;
+
 	return 0;
 }

--

Carlos Llamas