[RFC 1/1] crypto: Defer transforms destruction to a worker function

[Florent Revest]

Currently, crypto spawns can be freed in a softirq context (eg: from
sctp socket destruction's rcu callbacks). In that context, grabbing the
crypto_alg_sem is dangerous and makes CONFIG_DEBUG_ATOMIC_SLEEP scream.

Defer transform destruction to a worker function so they don't use that
semaphore in a softirq.

Reported-by: syzbot+d769eed29cc42d75e2a3@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=d769eed29cc42d75e2a3
Reported-by: syzbot+610ec0671f51e838436e@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=610ec0671f51e838436e
Signed-off-by: Florent Revest <revest@xxxxxxxxxxxx>
---
 crypto/api.c           | 26 ++++++++++++++++++--------
 include/linux/crypto.h |  3 +++
 2 files changed, 21 insertions(+), 8 deletions(-)

diff --git a/crypto/api.c b/crypto/api.c
index b9cc0c906efe..f877251954d5 100644
--- a/crypto/api.c
+++ b/crypto/api.c
@@ -640,6 +640,21 @@ void *crypto_alloc_tfm_node(const char *alg_name,
 }
 EXPORT_SYMBOL_GPL(crypto_alloc_tfm_node);
 
+void crypto_destroy_tfm_workfn(struct work_struct *w)
+{
+	struct crypto_alg *alg;
+	struct crypto_tfm *tfm;
+
+	tfm = container_of(w, struct crypto_tfm, free_work);
+	alg = tfm->__crt_alg;
+
+	if (!tfm->exit && alg->cra_exit)
+		alg->cra_exit(tfm);
+	crypto_exit_ops(tfm);
+	crypto_mod_put(alg);
+	kfree_sensitive(tfm->to_free);
+}
+
 /*
  *	crypto_destroy_tfm - Free crypto transform
  *	@mem: Start of tfm slab
@@ -650,20 +665,15 @@ EXPORT_SYMBOL_GPL(crypto_alloc_tfm_node);
  */
 void crypto_destroy_tfm(void *mem, struct crypto_tfm *tfm)
 {
-	struct crypto_alg *alg;
-
 	if (IS_ERR_OR_NULL(mem))
 		return;
 
 	if (!refcount_dec_and_test(&tfm->refcnt))
 		return;
-	alg = tfm->__crt_alg;
 
-	if (!tfm->exit && alg->cra_exit)
-		alg->cra_exit(tfm);
-	crypto_exit_ops(tfm);
-	crypto_mod_put(alg);
-	kfree_sensitive(mem);
+	tfm->to_free = mem;
+	INIT_WORK(&tfm->free_work, crypto_destroy_tfm_workfn);
+	queue_work(system_unbound_wq, &tfm->free_work);
 }
 EXPORT_SYMBOL_GPL(crypto_destroy_tfm);
 
diff --git a/include/linux/crypto.h b/include/linux/crypto.h
index 31f6fee0c36c..34ff2e1dca2b 100644
--- a/include/linux/crypto.h
+++ b/include/linux/crypto.h
@@ -430,6 +430,9 @@ struct crypto_tfm {
 	
 	struct crypto_alg *__crt_alg;
 
+	struct work_struct free_work;
+	void *to_free;
+
 	void *__crt_ctx[] CRYPTO_MINALIGN_ATTR;
 };
 
-- 
2.41.0.585.gd2178a4bd4-goog