linux-kernel - Re: [PATCH 0/7] ensure bios aren't split in middle of crypto data unit

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Date:   Thu, 25 Mar 2021 21:41:06 +0000
From:   Satya Tangirala <satyat@...gle.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jens Axboe <axboe@...nel.dk>,
        Eric Biggers <ebiggers@...gle.com>
Subject: Re: [PATCH 0/7] ensure bios aren't split in middle of crypto data
 unit

On Thu, Jan 21, 2021 at 05:11:29PM +0000, Christoph Hellwig wrote:
> On Thu, Jan 14, 2021 at 03:47:16PM +0000, Satya Tangirala wrote:
> > When a bio has an encryption context, its size must be aligned to its
> > crypto data unit size. A bio must not be split in the middle of a data
> > unit. Currently, bios are split at logical block boundaries, but a crypto
> > data unit size might be larger than the logical block size - e.g. a machine
> > could be using fscrypt (which uses 4K crypto data units) with an eMMC block
> > device with inline encryption hardware that has a logical block size of
> > 512 bytes. So we need to support cases where the data unit size is larger
> > than the logical block size.
> 
> I think this model is rather broken.  Instead of creating an -EIO path
> we can't handle anywhere make sure that the size limits exposed by the
> driver that wants to split always align to the crypto data units to
> avoid this issue to start with.
Hey Christoph,
Thanks for the suggestion. I finally sent out v2 for this patch at
https://lore.kernel.org/linux-block/20210325212609.492188-1-satyat@google.com/

I tried doing something similar to what you suggested to avoid
creating an -EIO path, but instead of changing the size limits exposed
by the driver, I changed the allowed data unit sizes based on the
exposed size limits. I did it that way because the limits that
interfere with inline encryption happen to be "hard limits" a driver
can't lie about, like having support for SG gaps or not requiring
chunk sectors. Another reason for doing it this way is so that we
don't interfere with regular unencrypted I/O by changing driver
exposed limits unconditionally (and I didn't think it was
straightforward to expose two different sets of limits of encrypted
and unencrypted I/O respectively). Please take a look at the new patch
series if you're able to. Thanks!