linux-kernel - Re: [PATCH] iommu/iova: wait 'fq_timer' handler to finish before destroying 'fq'

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <ebfebc58-10b5-c12e-edbe-a22181721c2d@arm.com>
Date:   Thu, 9 Dec 2021 17:48:05 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Xiongfeng Wang <wangxiongfeng2@...wei.com>, joro@...tes.org,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Cc:     yaohongbo@...wei.com, huawei.libin@...wei.com
Subject: Re: [PATCH] iommu/iova: wait 'fq_timer' handler to finish before
 destroying 'fq'

On 2021-12-09 13:17, Robin Murphy wrote:
> Sorry I missed this before...
> 
> On 2019-07-27 10:21, Xiongfeng Wang wrote:
>> Fix following crash that occurs when 'fq_flush_timeout()' access
>> 'fq->lock' while 'iovad->fq' has been cleared. This happens when the
>> 'fq_timer' handler is being executed and we call
>> 'free_iova_flush_queue()'. When the timer handler is being executed,
>> its pending state is cleared and it is detached. This patch use
>> 'del_timer_sync()' to wait for the timer handler 'fq_flush_timeout()' to
>> finish before destroying the flush queue.
> 
> So if I understand correctly, you shut down the device - which naturally 
> frees some DMA mappings into the FQ - then hotplug it out, such that 
> tearing down its group and default domain can end up racing with the 
> timeout firing on a different CPU? It would help if the commit message 
> actually explained that - I've just reverse-engineered it from the given 
> symptom - rather than focusing on details that aren't really important. 
> fq->lock is hardly significant, since *any* access to the FQ while it's 
> being destroyed is fundamentally unsound. I also spent way too long 
> trying to understand the significance of the full stack trace below 
> before realising that it is in fact just irrelevant - there's only one 
> way fq_flush_timeout() ever gets called, and it's the obvious one.
> 
> The fix itself seems reasonable - the kerneldoc for del_timer_sync() is 
> slightly scary, but since free_iova_flush_queue() doesn't touch any of 
> the locks and definitely shouldn't run in IRQ context I believe we're OK.
> 
> This will affect my IOVA refactoring series a little, so I'm happy to 
> help improve the writeup if you like - provided that my understanding is 
> actually correct - and include it in a v2 of that.

FWIW, this is what I came up with:

https://gitlab.arm.com/linux-arm/linux-rm/-/commit/ecea6835baca75b945bd8ecfaa636ff01dabcc1d

Let me know what you think.

Thanks,
Robin.