Re: [dm-crypt] [dm-devel] [RFC PATCH 0/2] dm crypt: Allow unaligned buffer lengths for skcipher devices

[Sudhakar Panneerselvam <sudhakar.panneerselvam@oracle.com>]

Hello Eric,

> -----Original Message-----
> From: Eric Biggers [mailto:ebiggers@kernel.org]
> Sent: Wednesday, September 23, 2020 11:14 PM
> To: Mike Snitzer <snitzer@redhat.com>
> Cc: Sudhakar Panneerselvam <sudhakar.panneerselvam@oracle.com>;
> Damien.LeMoal@wdc.com; ssudhakarp@gmail.com; Martin Petersen
> <martin.petersen@oracle.com>; dm-crypt@saout.de; dm-devel@redhat.com;
> Shirley Ma <shirley.ma@oracle.com>; mpatocka@redhat.com; Milan Broz
> <gmazyland@gmail.com>; agk@redhat.com
> Subject: Re: [dm-devel] [RFC PATCH 0/2] dm crypt: Allow unaligned buffer
> lengths for skcipher devices
> 
> On Wed, Sep 23, 2020 at 09:27:32PM -0400, Mike Snitzer wrote:
> > You've clearly done a nice job with these changes.  Looks clean.
> >
> > BUT, I'm struggling to just accept that dm-crypt needs to go to these
> > extra lengths purely because of one bad apple usecase.
> >
> > These alignment constraints aren't new.  Are there other portions of
> > Linux's crypto subsystem that needed comparable fixes in order to work
> > with Microsfot OS initiated IO through a guest?
> >
> > You forecast that these same kinds of changes are needed for AEAD and
> > dm-integrity... that's alarming.
> >
> > Are we _certain_ there is no other way forward?
> > (Sorry I don't have suggestions.. I'm in "fact finding mode" ;)
> >
> 
> I don't understand why this is needed, since dm-crypt already sets its
> logical_block_size to its crypto sector_size.  Isn't it expected that I/O that
> isn't aligned to logical_block_size fails?  It's the I/O submitter's
> responsibility to ensure logical_block_size alignment of all I/O segments.
> Exactly how is the misaligned I/O actually being submitted here?

You are right that each I/O size should be a multiple of the block device's sector size, but I am not sure if there is any constraint that individual segment lengths should be aligned to its sector size, could you help me with how this is enforced in block layer? The closest I see is "dma_alignment" member in "struct request_queue" of the low-level block device driver and as mentioned in the patch description, iSCSI, MegaRaid, qla2xxx, nvme and others have much relaxed constraint.

To your other question, the IO stack looks like this:

Windows Guest <--> Vhost-Scsi <--> LIO(scsi/target/blockio) <-->  dm-crypt <--> iSCSI block device

One real example out of my debugging: Windows sends a I/O request with 6656 bytes to vhost-scsi interface. Vhost-scsi uses translate_desc() in drivers/vhost/vhost.c to convert windows user space memory buffers to kernel iovecs. Vhost-scsi then converts the iovecs to sg entries in vhost_scsi_mapal() which is then handed over to "target" subsystem and eventually submitted to dm-crypt. This 6656 bytes IO has got 3 segments, first segment had 1584, second 4096 and the last had 976 bytes. Dm-crypt rejects the I/O after seeing the first segment length 1584 which is not a 512 byte multiple.

Let me know if there are further questions.

Thanks
Sudhakar

> 
> - Eric