Re: [PATCH v1 3/3] crypto: ccp - Protect against poorly marked end of sg list

[Tom Lendacky <thomas.lendacky@amd.com>]

On 05/27/2015 04:45 AM, Herbert Xu wrote:
> On Wed, May 27, 2015 at 05:43:05PM +0800, Herbert Xu wrote:
>> Tom Lendacky <thomas.lendacky@amd.com> wrote:
>>> Scatter gather lists can be created with more available entries than are
>>> actually used (e.g. using sg_init_table() to reserve a specific number
>>> of sg entries, but in actuality using something less than that based on
>>> the data length).  The caller sometimes fails to mark the last entry
>>> with sg_mark_end().  In these cases, sg_nents() will return the original
>>> size of the sg list as opposed to the actual number of sg entries that
>>> contain valid data.
>>>
>>> On arm64, if the sg_nents() value is used in a call to dma_map_sg() in
>>> this situation, then it causes a BUG_ON in lib/swiotlb.c because an
>>> "empty" sg list entry results in dma_capable() returning false and
>>> swiotlb trying to create a bounce buffer of size 0. This occurred in
>>> the userspace crypto interface before being fixed by
>>>
>>> 0f477b655a52 ("crypto: algif - Mark sgl end at the end of data")
>>>
>>> Protect against this in the future by counting the number of sg entries
>>> needed to meet the length requirement and supplying that value to
>>> dma_map_sg().
>>
>> Is this needed for any reason other than this bug that's already
>> been fixed?
>
> Could this be needed if you have a properly marked SG list say of
> 100 bytes but len is only 10 bytes?

I don't think that situation matters because the DMA mapping should
succeed just fine at 100 bytes even if only needing/using 10 bytes.

Thanks,
Tom

>
> Cheers,
>