[dm-devel] Deadlock when using crypto API for block devices

[Mikulas Patocka]

On Fri, 24 Aug 2018, Herbert Xu wrote:

> On Thu, Aug 23, 2018 at 04:39:23PM -0400, Mikulas Patocka wrote:
> >
> > 1. don't set CRYPTO_TFM_REQ_MAY_SLEEP in dm-crypt
> > =================================================
> > If we don't set it, dm-crypt will use GFP_ATOMIC and GFP_ATOMIC may fail. 
> > The function init_crypt in xts.c handles the failure gracefully - but the 
> > question is - does the whole crypto code handle allocation failures 
> > gracefully? If not and if it returns -ENOMEM somewhere, it would result in 
> > I/O errors and data corruption.
> 
> It is safe to use GFP_ATOMIC.  First of the allocation is really
> small (less than a page) so it will only fail when the system is
> almost completely out of memory.

GFP_ATOMIC is used by networking code. If the system is in a situation 
when packets arrive faster than the swapper is able to swap, it will 
happen. It does happen during netwoking surge and corrupting the 
filesystem in tris situation is not acceptable.

> Even when it does fail the crypto
> operation will still succeed, albeit it will process things at a
> smaller granularity so the performance will degrade.

A quick search through the crypto code shows that ahash_save_req and 
seqiv_aead_encrypt return -ENOMEM.

Will you fix them?

> The sleeping part of that flag is also not an issue because it
> only kicks in once per page.  As this is going to be less than
> or equal to a page it shouldn't matter.
>  
> > 3. introduce new flag CRYPTO_TFM_REQ_MAY_SLEEP_NOIO
> > ===================================================
> > Would you like to introduce it?
> 
> For now I don't think this is necessary given that this allocation
> and similar ones in lrw and other places in the crypto API are just
> performance enhancements and unlikely to fail except in very dire
> situations.
> 
> But if new problems arise I'm certainly not opposed to this.
> 
> Thanks,
> -- 
> Email: Herbert Xu <herbert at gondor.apana.org.au>
> Home Page: http://gondor.apana.org.au/~herbert/
> PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

Mikulas