crypto: api - Check spawn->alg under lock in crypto_drop_spawn

BugLink: https://bugs.launchpad.net/bugs/1864775

commit 7db3b61b upstream.

We need to check whether spawn->alg is NULL under lock as otherwise
the algorithm could be removed from under us after we have checked
it and found it to be non-NULL.  This could cause us to remove the
spawn from a non-existent list.

Fixes: 7ede5a5b ("crypto: api - Fix crypto_drop_spawn crash...")
Cc: <stable@vger.kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

crypto/algapi.c

@@ -653,10 +653,8 @@ EXPORT_SYMBOL_GPL(crypto_grab_spawn);
 
 void crypto_drop_spawn(struct crypto_spawn *spawn)
 {
-	if (!spawn->alg)
-		return;
-
 	down_write(&crypto_alg_sem);
+	if (spawn->alg)
 		list_del(&spawn->list);
 	up_write(&crypto_alg_sem);
 }