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    Subject[PATCH 3.16 171/254] crypto: algapi - fix NULL dereference in crypto_remove_spawns()
    3.16.55-rc1 review patch.  If anyone has any objections, please let me know.

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

    From: Eric Biggers <ebiggers@google.com>

    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@googlegroups.com>
    Signed-off-by: Eric Biggers <ebiggers@google.com>
    Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
    Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
    ---
    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)));
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