security/keys: rewrite all of big_key crypto
commit 428490e38b2e352812e0b765d8bceafab0ec441d upstream.
This started out as just replacing the use of crypto/rng with
get_random_bytes_wait, so that we wouldn't use bad randomness at boot
time. But, upon looking further, it appears that there were even deeper
underlying cryptographic problems, and that this seems to have been
committed with very little crypto review. So, I rewrote the whole thing,
trying to keep to the conventions introduced by the previous author, to
fix these cryptographic flaws.
It makes no sense to seed crypto/rng at boot time and then keep
using it like this, when in fact there's already get_random_bytes_wait,
which can ensure there's enough entropy and be a much more standard way
of generating keys. Since this sensitive material is being stored
untrusted, using ECB and no authentication is simply not okay at all. I
find it surprising and a bit horrifying that this code even made it past
basic crypto review, which perhaps points to some larger issues. This
patch moves from using AES-ECB to using AES-GCM. Since keys are uniquely
generated each time, we can set the nonce to zero. There was also a race
condition in which the same key would be reused at the same time in
different threads. A mutex fixes this issue now.
So, to summarize, this commit fixes the following vulnerabilities:
* Low entropy key generation, allowing an attacker to potentially
guess or predict keys.
* Unauthenticated encryption, allowing an attacker to modify the
cipher text in particular ways in order to manipulate the plaintext,
which is is even more frightening considering the next point.
* Use of ECB mode, allowing an attacker to trivially swap blocks or
compare identical plaintext blocks.
* Key re-use.
* Faulty memory zeroing.
[Note that in backporting this commit to 4.9, get_random_bytes_wait was
replaced with get_random_bytes, since 4.9 does not have the former
function. This might result in slightly worse entropy in key generation,
but common use cases of big_keys makes that likely not a huge deal. And,
this is the best we can do with this old kernel. Alas.]
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Eric Biggers <ebiggers3@gmail.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Kirill Marinushkin <k.marinushkin@gmail.com>
Cc: security@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/security/keys/Kconfig b/security/keys/Kconfig
index d942c7c..e0a3978 100644
--- a/security/keys/Kconfig
+++ b/security/keys/Kconfig
@@ -41,10 +41,8 @@
bool "Large payload keys"
depends on KEYS
depends on TMPFS
- depends on (CRYPTO_ANSI_CPRNG = y || CRYPTO_DRBG = y)
select CRYPTO_AES
- select CRYPTO_ECB
- select CRYPTO_RNG
+ select CRYPTO_GCM
help
This option provides support for holding large keys within the kernel
(for example Kerberos ticket caches). The data may be stored out to