crypto: api - Fix use-after-free and race in crypto_spawn_alg
There are two problems in crypto_spawn_alg. First of all it may
return spawn->alg even if spawn->dead is set. This results in a
double-free as detected by syzbot.
Secondly the setting of the DYING flag is racy because we hold
the read-lock instead of the write-lock. We should instead call
crypto_shoot_alg in a safe manner by gaining a refcount, dropping
the lock, and then releasing the refcount.
This patch fixes both problems.
Reported-by: syzbot+fc0674cde00b66844470@syzkaller.appspotmail.com
Fixes: 4f87ee118d16 ("crypto: api - Do not zap spawn->alg")
Fixes: 73669cc55646 ("crypto: api - Fix race condition in...")
Cc: <stable@vger.kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
diff --git a/crypto/algapi.c b/crypto/algapi.c
index 849254d..f1e6cca 100644
--- a/crypto/algapi.c
+++ b/crypto/algapi.c
@@ -716,17 +716,27 @@ EXPORT_SYMBOL_GPL(crypto_drop_spawn);
static struct crypto_alg *crypto_spawn_alg(struct crypto_spawn *spawn)
{
- struct crypto_alg *alg;
+ struct crypto_alg *alg = ERR_PTR(-EAGAIN);
+ struct crypto_alg *target;
+ bool shoot = false;
down_read(&crypto_alg_sem);
- alg = spawn->alg;
- if (!spawn->dead && !crypto_mod_get(alg)) {
- alg->cra_flags |= CRYPTO_ALG_DYING;
- alg = NULL;
+ if (!spawn->dead) {
+ alg = spawn->alg;
+ if (!crypto_mod_get(alg)) {
+ target = crypto_alg_get(alg);
+ shoot = true;
+ alg = ERR_PTR(-EAGAIN);
+ }
}
up_read(&crypto_alg_sem);
- return alg ?: ERR_PTR(-EAGAIN);
+ if (shoot) {
+ crypto_shoot_alg(target);
+ crypto_alg_put(target);
+ }
+
+ return alg;
}
struct crypto_tfm *crypto_spawn_tfm(struct crypto_spawn *spawn, u32 type,