linux-kernel - Re: [PATCH 5.10 16/98] dmaengine: xilinx_dma: Fix read-after-free bug when terminating transfers

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Date:   Wed, 25 Aug 2021 10:04:50 +0200
From:   Pavel Machek <pavel@...x.de>
To:     Sasha Levin <sashal@...nel.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Adrian Larumbe <adrian.martinezlarumbe@...tec.com>,
        Vinod Koul <vkoul@...nel.org>
Subject: Re: [PATCH 5.10 16/98] dmaengine: xilinx_dma: Fix read-after-free
 bug when terminating transfers

Hi!

> [ Upstream commit 7dd2dd4ff9f3abda601f22b9d01441a0869d20d7 ]
> 
> When user calls dmaengine_terminate_sync, the driver will clean up any
> remaining descriptors for all the pending or active transfers that had
> previously been submitted. However, this might happen whilst the tasklet is
> invoking the DMA callback for the last finished transfer, so by the time it
> returns and takes over the channel's spinlock, the list of completed
> descriptors it was traversing is no longer valid. This leads to a
> read-after-free situation.
> 
> Fix it by signalling whether a user-triggered termination has happened by
> means of a boolean variable.

I see the variable is cleared and tested under spinlock, but it is set
without any locking. Do we need to take the spinlock, too?

Best regards,
								Pavel

> @@ -1049,6 +1051,13 @@ static void xilinx_dma_chan_desc_cleanup(struct xilinx_dma_chan *chan)
>  		/* Run any dependencies, then free the descriptor */
>  		dma_run_dependencies(&desc->async_tx);
>  		xilinx_dma_free_tx_descriptor(chan, desc);
> +
> +		/*
> +		 * While we ran a callback the user called a terminate function,
> +		 * which takes care of cleaning up any remaining descriptors
> +		 */
> +		if (chan->terminating)
> +			break;
>  	}
>  
>  	spin_unlock_irqrestore(&chan->lock, flags);
> @@ -1965,6 +1974,8 @@ static dma_cookie_t xilinx_dma_tx_submit(struct dma_async_tx_descriptor *tx)
>  	if (desc->cyclic)
>  		chan->cyclic = true;
>  
> +	chan->terminating = false;
> +
>  	spin_unlock_irqrestore(&chan->lock, flags);
>  
>  	return cookie;
> @@ -2436,6 +2447,7 @@ static int xilinx_dma_terminate_all(struct dma_chan *dchan)
>  
>  	xilinx_dma_chan_reset(chan);
>  	/* Remove and free all of the descriptors in the lists */
> +	chan->terminating = true;
>  	xilinx_dma_free_descriptors(chan);
>  	chan->idle = true;
>  

-- 
DENX Software Engineering GmbH,      Managing Director: Wolfgang Denk
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany

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