From: Eric Biggers ebiggers@google.com

syzkaller triggered a NULL pointer dereference in crypto_remove_spawns() via a program that repeatedly and concurrently requests AEADs "authenc(cmac(des3_ede-asm),pcbc-aes-aesni)" and hashes "cmac(des3_ede)" through AF_ALG, where the hashes are requested as "untested" (CRYPTO_ALG_TESTED is set in ->salg_mask but clear in ->salg_feat; this causes the template to be instantiated for every request).

Although AF_ALG users really shouldn't be able to request an "untested" algorithm, the NULL pointer dereference is actually caused by a longstanding race condition where crypto_remove_spawns() can encounter an instance which has had spawn(s) "grabbed" but hasn't yet been registered, resulting in ->cra_users still being NULL.

We probably should properly initialize ->cra_users earlier, but that would require updating many templates individually. For now just fix the bug in a simple way that can easily be backported: make crypto_remove_spawns() treat a NULL ->cra_users list as empty.

Reported-by: syzbot syzkaller@googlegroups.com Cc: stable@vger.kernel.org Signed-off-by: Eric Biggers ebiggers@google.com --- crypto/algapi.c | 12 ++++++++++++ 1 file changed, 12 insertions(+)

diff --git a/crypto/algapi.c b/crypto/algapi.c index 9895cafcce7e..395b082d03a9 100644 --- a/crypto/algapi.c +++ b/crypto/algapi.c @@ -166,6 +166,18 @@ void crypto_remove_spawns(struct crypto_alg *alg, struct list_head *list,

spawn->alg = NULL; spawns = &inst->alg.cra_users; + + /* + * We may encounter an unregistered instance here, since + * an instance's spawns are set up prior to the instance + * being registered. An unregistered instance will have + * NULL ->cra_users.next, since ->cra_users isn't + * properly initialized until registration. But an + * unregistered instance cannot have any users, so treat + * it the same as ->cra_users being empty. + */ + if (spawns->next == NULL) + break; } } while ((spawns = crypto_more_spawns(alg, &stack, &top, &secondary_spawns)));