Commit 9a006742 authored Dec 29, 2017 by Eric Biggers Committed by Herbert Xu Jan 05, 2018

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>

parent 2973633e

crypto/algapi.c

+12 −0

Original line number	Diff line number	Diff line
		@@ -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)));

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