Re: [Intel-gfx] [PATCH v2] debugobjects: stop accessing objects after releasing spinlock

From: Thomas Gleixner <tglx@linutronix.de>
To: Andrzej Hajda <andrzej.hajda@intel.com>,
	linux-kernel@vger.kernel.org, intel-gfx@lists.freedesktop.org,
	linux-mm@kvack.org
Cc: Nirmoy Das <nirmoy.das@intel.com>
Subject: Re: [Intel-gfx] [PATCH v2] debugobjects: stop accessing objects after releasing spinlock
Date: Mon, 23 Oct 2023 18:11:35 +0200	[thread overview]
Message-ID: <87r0llco94.ffs@tglx> (raw)
In-Reply-To: <62e16250-63f4-4fbb-b00e-db808b600664@intel.com>

On Thu, Oct 19 2023 at 12:31, Andrzej Hajda wrote:
> On 13.10.2023 15:15, Thomas Gleixner wrote:
>> It cannot be freed. If that happens then the calling code will have an
>> UAF problem on the tracked item too.
>
> Yes, and I have assumed that debugobjects are created also for detecting 
> UAFs.

Kinda.

> They should be able to handle/detect 'issues due to incorrectly 
> serialized concurrent accesses' scenarios as well, at least some of 
> them. And even if it cannot recover it should at least provide reliable 
> reporting.

Fair enough.

> Now we can have scenario:
> 1. Thread tries to deactivate destroyed object, debugobjects detects it, 
> spin lock is released, thread is preempted.
> 2. Other thread frees debugobject, then allocates new one on the same 
> memory location, ie 'obj' variable from 1st thread point to it - it is 
> possible because there is no locking.
> 3. Then preemption occurs, and 1st thread reports error for wrong object.
>
> This seems the most drastic for me, but also with lowest chances to 
> happen due to delayed freeing, but there are also other more probable 
> scenarios when we print the same object but in state different from the 
> one when debugobject detected issue, due to modification by concurrent 
> thread.

Now I understand what you mean. This information should be in the
changelog, no?

Let me stare at the patch once more.