[PATCH v3 7/9] crypto alg: Introduce generic max blocksize and alignmask

Kees Cook
2018-06-29T00:28:41Z

In the quest to remove all stack VLA usage from the kernel[1], this
exposes a new general upper bound on crypto blocksize and alignmask
(higher than for the existing cipher limits) for VLA removal,
and introduces new checks.

At present, the highest cra_alignmask in the kernel is 63. The highest
cra_blocksize is 144 (SHA3_224_BLOCK_SIZE, 18 8-byte words). For the
new blocksize limit, I went with 160 (20 8-byte words).

[1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com

Signed-off-by: Kees Cook <keescook@chromium.org>
---
crypto/algapi.c | 7 ++++++-
include/crypto/algapi.h | 4 +++-
2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/crypto/algapi.c b/crypto/algapi.c
index c0755cf4f53f..496fc51bf215 100644
--- a/crypto/algapi.c
+++ b/crypto/algapi.c
@@ -57,9 +57,14 @@ static int crypto_check_alg(struct crypto_alg *alg)
if (alg->cra_alignmask & (alg->cra_alignmask + 1))
return -EINVAL;

- if (alg->cra_blocksize > PAGE_SIZE / 8)
+ /* General maximums for all algs. */
+ if (alg->cra_alignmask > MAX_ALGAPI_ALIGNMASK)
return -EINVAL;

+ if (alg->cra_blocksize > MAX_ALGAPI_BLOCKSIZE)
+ return -EINVAL;
+
+ /* Lower maximums for specific alg types. */
if (!alg->cra_type && (alg->cra_flags & CRYPTO_ALG_TYPE_MASK) ==
CRYPTO_ALG_TYPE_CIPHER) {
if (alg->cra_alignmask > MAX_CIPHER_ALIGNMASK)
diff --git a/include/crypto/algapi.h b/include/crypto/algapi.h
index bd5e8ccf1687..21371ac8f355 100644
--- a/include/crypto/algapi.h
+++ b/include/crypto/algapi.h
@@ -20,8 +20,10 @@
/*
* Maximum values for blocksize and alignmask, used to allocate
* static buffers that are big enough for any combination of
- * ciphers and architectures.
+ * algs and architectures. Ciphers have a lower maximum size.
*/
+#define MAX_ALGAPI_BLOCKSIZE 160
+#define MAX_ALGAPI_ALIGNMASK 63
#define MAX_CIPHER_BLOCKSIZE 16
#define MAX_CIPHER_ALIGNMASK 15

--
2.17.1