Re: [PATCH] iommu/dma: limit iova free size to unmmaped iova

[guptap@codeaurora.org]

On 2020-05-22 14:54, Robin Murphy wrote:
> On 2020-05-22 07:25, guptap@codeaurora.org wrote:
>> On 2020-05-22 01:46, Robin Murphy wrote:
>>> On 2020-05-21 12:30, Prakash Gupta wrote:
>> I agree, we shouldn't be freeing the partial iova. Instead just making
>> sure if unmap was successful should be sufficient before freeing iova. 
>> So change
>> can instead be something like this:
>> 
>> -    iommu_dma_free_iova(cookie, dma_addr, size);
>> +    if (unmapped)
>> +        iommu_dma_free_iova(cookie, dma_addr, size);
>> 
>>> TBH my gut feeling here is that you're really just trying to treat a
>>> symptom of another bug elsewhere, namely some driver calling
>>> dma_unmap_* or dma_free_* with the wrong address or size in the first
>>> place.
>>> 
>> This condition would arise only if driver calling dma_unmap/free_* 
>> with 0
>> iova_pfn. This will be flagged with a warning during unmap but will 
>> trigger
>> panic later on while doing unrelated dma_map/unmap_*. If unmapped has 
>> already
>> failed for invalid iova, there is no reason we should consider this as 
>> valid
>> iova and free. This part should be fixed.
> 
> I disagree. In general, if drivers call the DMA API incorrectly it is
> liable to lead to data loss, memory corruption, and various other
> unpleasant misbehaviour - it is not the DMA layer's job to attempt to
> paper over driver bugs.
> 
> There *is* an argument for downgrading the BUG_ON() in
> iova_magazine_free_pfns() to a WARN_ON(), since frankly it isn't a
> sufficiently serious condition to justify killing the whole machine
> immediately, but NAK to bodging the iommu-dma mid-layer to "fix" that.
> A serious bug already happened elsewhere, so trying to hide the
> fallout really doesn't help anyone.
> 
Sorry for delayed response, it was a long weekend.
I agree that invalid DMA API call can result in unexpected issues and 
client
should fix it, but then the present behavior makes it difficult to catch 
cases
when driver is making wrong DMA API calls. When invalid iova pfn is 
passed it
doesn't fail then and there, though DMA layer is aware of iova being 
invalid. It
fails much after that in the context of an valid map/unmap, with 
BUG_ON().

Downgrading BUG_ON() to WARN_ON() in iova_magazine_free_pfns() will not 
help
much as invalid iova will cause NULL pointer dereference.

I see no reason why DMA layer wants to free an iova for which unmapped 
failed.
IMHO queuing an invalid iova (which already failed unmap) to rcache 
which
eventually going to crash the system looks like iommu-dma layer issue.

Thanks,
Prakash
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