crypto: algapi - fix NULL dereference in crypto_remove_spawns()

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>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

diff --git a/crypto/algapi.c b/crypto/algapi.c
index 60d7366ed343e9ad6a76b9dd36be22fbd7543c89..9a636f961572b99af844b1d8a28bebfd9463c9e2 100644 (file)
--- a/crypto/algapi.c
+++ b/crypto/algapi.c
@@ -167,6 +167,18 @@ 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)));