linux-kernel - Re: Use after free in bcm2835_spi

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Date:   Thu, 22 Oct 2020 14:12:54 +0200
From:   Lukas Wunner <lukas@...ner.de>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     Mark Brown <broonie@...nel.org>,
        Vladimir Oltean <olteanv@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-spi <linux-spi@...r.kernel.org>
Subject: Re: Use after free in bcm2835_spi_remove()

On Wed, Oct 14, 2020 at 02:20:16PM -0700, Florian Fainelli wrote:
> In bcm2835_spi_remove(), spi_controller_unregister() will free the ctlr
> reference which will lead to an use after free in bcm2835_release_dma().
> 
> To avoid this use after free, allocate the bcm2835_spi structure with a
> different lifecycle than the spi_controller structure such that we
> unregister the SPI controller, free up all the resources and finally let
> device managed allocations free the bcm2835_spi structure.
[...]
> -	if (ctlr->dma_tx) {
> -		dmaengine_terminate_sync(ctlr->dma_tx);
> +	if (dma_tx) {
> +		dmaengine_terminate_sync(dma_tx);
>  
>  		if (bs->fill_tx_desc)
>  			dmaengine_desc_free(bs->fill_tx_desc);
>  
>  		if (bs->fill_tx_addr)
> -			dma_unmap_page_attrs(ctlr->dma_tx->device->dev,
> +			dma_unmap_page_attrs(dma_tx->device->dev,
>  					     bs->fill_tx_addr, sizeof(u32),
>  					     DMA_TO_DEVICE,
>  					     DMA_ATTR_SKIP_CPU_SYNC);
>  
> -		dma_release_channel(ctlr->dma_tx);
> -		ctlr->dma_tx = NULL;
> +		dma_release_channel(dma_tx);
>  	}

You must set ctlr->dma_tx and ctlr->dma_rx to NULL because the driver
checks their value in a couple of places.

E.g. bcm2835_spi_setup() checks ctlr->dma_rx.

Likewise, the error paths of bcm2835_dma_init() and bcm2835_spi_probe()
call bcm2835_dma_release() and the latter checks ctlr->dma_tx and
ctlr->dma_rx to determine whether DMA was set up, hence needs to be
torn down.


> +	bs = devm_kzalloc(&pdev->dev, sizeof(*bs), GFP_KERNEL);
> +	if (!bs)
> +		return -ENOMEM;
> +
>  	ctlr = spi_alloc_master(&pdev->dev, ALIGN(sizeof(*bs),
>  						  dma_get_cache_alignment()));

You can set the second argument to spi_alloc_master() to 0
to conserve memory.

Thanks,

Lukas