linux-kernel - Re: [PATCH] dma-direct: Fix potential NULL pointer dereference

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Message-ID: <f5cba632-421a-f375-3697-51a182a53a32@oracle.com>
Date:   Tue, 15 Sep 2020 10:11:51 -0400
From:   Thomas Tai <thomas.tai@...cle.com>
To:     Christoph Hellwig <hch@....de>
Cc:     konrad.wilk@...cle.com, m.szyprowski@...sung.com,
        robin.murphy@....com, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dma-direct: Fix potential NULL pointer dereference



On 2020-09-15 10:07 a.m., Christoph Hellwig wrote:
> On Tue, Sep 15, 2020 at 08:03:14AM -0600, Thomas Tai wrote:
>> When booting the kernel v5.9-rc4 on a VM, the kernel would panic when
>> printing a warning message in swiotlb_map(). It is because dev->dma_mask
>> can potentially be a null pointer. Using the dma_get_mask() macro can
>> avoid the NULL pointer dereference.
> 
> dma_mask must not be zero.  This means drm is calling DMA API functions
> on something weird.  This needs to be fixed in the caller.
> 

Thanks, Christoph for your comment. The caller already fixed the null 
pointer in the latest v5.9-rc5. I am thinking that if we had used the 
dma_get_mask(), the kernel couldn't panic and could properly print out 
the warning message.

Thomas