linux-kernel - Re: tty: serial: fsl_lpuart: fix DMA mapping

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Date:   Fri, 03 Apr 2020 15:05:49 +0200
From:   Michael Walle <michael@...le.cc>
To:     Colin Ian King <colin.king@...onical.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>, linux-serial@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: tty: serial: fsl_lpuart: fix DMA mapping - static analysis bug
 report

Am 2020-04-03 14:44, schrieb Colin Ian King:
> Hi,
> 
> Static analysis with Coverity has found an issue with the following 
> commit:
> 
> From a092ab25fdaa445b821f5959e458350696fce44c Mon Sep 17 00:00:00 2001
> From: Michael Walle <michael@...le.cc>
> Date: Fri, 6 Mar 2020 22:44:31 +0100
> Subject: [PATCH] tty: serial: fsl_lpuart: fix DMA mapping
> 
> The analysis report is as follows for function lpuart_dma_rx_free in
> source drivers/tty/serial/fsl_lpuart.c :
> 
> var_compare_op: Comparing chan to null implies that chan might be null.
> 
> 1234        if (chan)
> 1235                dmaengine_terminate_all(chan);
> 1236
> 
> Dereference after null check (FORWARD_NULL)
> var_deref_op: Dereferencing null pointer chan.
> 
> 1237        dma_unmap_sg(chan->device->dev, &sport->rx_sgl, 1,
> DMA_FROM_DEVICE);
> 
> The check for chan being null implies it is may be null, however, the
> call to dma_unmap_sg dereferences chan which leads to a null pointer
> dereference issue.

Technically, this is correct. But lpuart_dma_rx_free() is guarded by
lpuart_dma_rx_use which is only true if there is a dma channel, see
lpuart_rx_dma_startup(). In any way, this looks bogus.

So actually, the "if (chan)" is superfluous. Could you double check 
that?
Then I'd make a patch which removes the if (chan) to make coverity 
happy.

-michael