[PATCH 3.16 171/254] crypto: algapi - fix NULL dereference in crypto_remove_spawns()

[Ben Hutchings]

3.16.55-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Eric Biggers <ebiggers@xxxxxxxxxx>

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 <syzkaller@xxxxxxxxxxxxxxxx>
Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
Signed-off-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 crypto/algapi.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

--- a/crypto/algapi.c
+++ b/crypto/algapi.c
@@ -147,6 +147,18 @@ 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 ->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)));