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Subject[PATCH 4.9 43/96] crypto: algapi - fix NULL dereference in crypto_remove_spawns()
4.9-stable review patch.  If anyone has any objections, please let me know.


From: Eric Biggers <>

commit 9a00674213a3f00394f4e3221b88f2d21fc05789 upstream.

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 <>
Signed-off-by: Eric Biggers <>
Signed-off-by: Herbert Xu <>
Signed-off-by: Greg Kroah-Hartman <>

crypto/algapi.c | 12 ++++++++++++
1 file changed, 12 insertions(+)

--- a/crypto/algapi.c
+++ b/crypto/algapi.c
@@ -167,6 +167,18 @@ void crypto_remove_spawns(struct crypto_

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 ->, 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,

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