Re: [dm-devel] [PATCH 1/3] dm integrity: do not filter algos with CRYPTO_ALG_ALLOCATES_MEMORY

[Mikulas Patocka <mpatocka@redhat.com>]

Hi

If you allocate memory in crypto processing in dm-integrity, you risk the 
low-memory deadlock when swapping to dm-integrity.

I.e. the machine runs out of memory, it needs to swap out pages to free 
some memory, the swap-out bio goes to dm-integrity and dm-integrity calls 
the crypto API and tries to allocate more memory => deadlock.



On Wed, 5 Jul 2023, Eric Biggers wrote:

> On Wed, Jul 05, 2023 at 05:40:07PM +0100, Giovanni Cabiddu wrote:
> > The flag CRYPTO_ALG_ALLOCATES_MEMORY indicates that an algorithm might
> > allocate memory in the datapath and therefore sleep.
> > Dm-integrity is filtering out implementations of skcipher algorithms
> > that have this flag set. However, in the same function it does
> > allocations with GFP_KERNEL.

It's OK to use GFP_KERNEL in the device mapper target constructor (because 
at this point there is no I/O going to the device). But it's not OK to use 
it for individual bio processing.

> Which function is the above referring to?  The actual encryption/decryption
> happens in crypt_journal(), and I don't see any memory allocations there.
> 
> > As dm-integrity is re-entrant and capable of handling sleeps that could
> > occur during allocations with GFP_KERNEL, then it is also capable of
> > using skcipher algorithm implementations that have
> > CRYPTO_ALG_ALLOCATES_MEMORY set.
> > 
> > Remove the filtering of skcipher implementations with the flag
> > CRYPTO_ALG_ALLOCATES_MEMORY set.
> 
> What about the use of CRYPTO_ALG_ALLOCATES_MEMORY in get_mac()?
> 
> > 
> > Suggested-by: Herbert Xu <herbert@gondor.apana.org.au>
> > Link: https://lore.kernel.org/linux-crypto/ZILvtASXQKLG43y9@gondor.apana.org.au/
> > Signed-off-by: Giovanni Cabiddu <giovanni.cabiddu@intel.com>
> > Reviewed-by: Fiona Trahe <fiona.trahe@intel.com>
> 
> This needs:
> 
>     Fixes: a7a10bce8a04 ("dm integrity: don't use drivers that have CRYPTO_ALG_ALLOCATES_MEMORY")
>     Cc: stable@vger.kernel.org
> 
> But, are you 100% sure the explanation in commit a7a10bce8a04 was incorrect?
> 
> - Eric

Mikulas