Re: [PATCH] crypto: user - prevent operating on larval algorithms

[Eric Biggers <ebiggers@kernel.org>]

Hi Michal,

On Wed, Jul 03, 2019 at 10:21:08PM +0200, Michal Suchánek wrote:
> On Wed, 3 Jul 2019 22:30:57 +0800
> Herbert Xu <herbert@gondor.apana.org.au> wrote:
> 
> > On Tue, Jul 02, 2019 at 02:17:00PM -0700, Eric Biggers wrote:
> > > From: Eric Biggers <ebiggers@google.com>
> > > 
> > > Michal Suchanek reported [1] that running the pcrypt_aead01 test from
> > > LTP [2] in a loop and holding Ctrl-C causes a NULL dereference of
> > > alg->cra_users.next in crypto_remove_spawns(), via crypto_del_alg().
> > > The test repeatedly uses CRYPTO_MSG_NEWALG and CRYPTO_MSG_DELALG.
> > > 
> > > The crash occurs when the instance that CRYPTO_MSG_DELALG is trying to
> > > unregister isn't a real registered algorithm, but rather is a "test
> > > larval", which is a special "algorithm" added to the algorithms list
> > > while the real algorithm is still being tested.  Larvals don't have
> > > initialized cra_users, so that causes the crash.  Normally pcrypt_aead01
> > > doesn't trigger this because CRYPTO_MSG_NEWALG waits for the algorithm
> > > to be tested; however, CRYPTO_MSG_NEWALG returns early when interrupted.
> > > 
> 
> Do you have some way to reproduce this reliably?
> 
> I suppose you would have to send a signal to the process for the call
> to get interrupted, right?
> 

It reproduced pretty reliably for me with what you suggested.  Just typing in
terminal:

	while true; do pcrypt_aead01; done

and then holding Ctrl-C.

If I have time I'll try writing an LTP test that specifically reproduces it.
Yes, it would involve sending a signal to a thread or process that's executing
CRYPTO_MSG_NEWALG (unless I find a better way).

- Eric