Re: [PATCH] crypto: algboss - don't wait during notifier callback

[Eric Biggers <ebiggers@kernel.org>]

On Thu, Jun 04, 2020 at 11:52:53AM -0700, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.com>
> 
> When a crypto template needs to be instantiated, CRYPTO_MSG_ALG_REQUEST
> is sent to crypto_chain.  cryptomgr_schedule_probe() handles this by
> starting a thread to instantiate the template, then waiting for this
> thread to complete via crypto_larval::completion.
> 
> This can deadlock because instantiating the template may require loading
> modules, and this (apparently depending on userspace) may need to wait
> for the crc-t10dif module (lib/crc-t10dif.c) to be loaded.  But
> crc-t10dif's module_init function uses crypto_register_notifier() and
> therefore takes crypto_chain.rwsem for write.  That can't proceed until
> the notifier callback has finished, as it holds this semaphore for read.
> 
> Fix this by removing the wait on crypto_larval::completion from within
> cryptomgr_schedule_probe().  It's actually unnecessary because
> crypto_alg_mod_lookup() calls crypto_larval_wait() itself after sending
> CRYPTO_MSG_ALG_REQUEST.
> 
> This only actually became a problem in v4.20 due to commit b76377543b73
> ("crc-t10dif: Pick better transform if one becomes available"), but the
> unnecessary wait was much older.
> 
> BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=207159
> Reported-by: Mike Gerow <gerow@google.com>

I forgot about the person who originally filed the bugzilla bug.
Kai, if you're okay with it, let's also add:

Reported-by: Kai Lüke <kai@kinvolk.io>

> Fixes: 398710379f51 ("crypto: algapi - Move larval completion into algboss")
> Cc: <stable@vger.kernel.org> # v3.6+
> Cc: Martin K. Petersen <martin.petersen@oracle.com>
> Signed-off-by: Eric Biggers <ebiggers@google.com>
> ---
>  crypto/algboss.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/crypto/algboss.c b/crypto/algboss.c
> index 535f1f87e6c1..5ebccbd6b74e 100644
> --- a/crypto/algboss.c
> +++ b/crypto/algboss.c
> @@ -178,8 +178,6 @@ static int cryptomgr_schedule_probe(struct crypto_larval *larval)
>  	if (IS_ERR(thread))
>  		goto err_put_larval;
>  
> -	wait_for_completion_interruptible(&larval->completion);
> -
>  	return NOTIFY_STOP;
>  
>  err_put_larval:
> -- 
> 2.27.0.rc2.251.g90737beb825-goog
>