Re: [PATCH] crypto/ecc: Remove stack VLA usage

[Tudor Ambarus]

Hi, Kees,

On 03/07/2018 11:56 PM, Kees Cook wrote:

On the quest to remove all VLAs from the kernel[1], this switches to
a pair of kmalloc regions instead of using the stack. This also moves
the get_random_bytes() after all allocations (and drops the needless
"nbytes" variable).

[1] https://lkml.org/lkml/2018/3/7/621

Signed-off-by: Kees Cook <keesc...@chromium.org>
---
  crypto/ecc.c | 23 +++++++++++++++++------
  1 file changed, 17 insertions(+), 6 deletions(-)

diff --git a/crypto/ecc.c b/crypto/ecc.c
index 18f32f2a5e1c..5bfa63603da0 100644
--- a/crypto/ecc.c
+++ b/crypto/ecc.c
@@ -1025,9 +1025,7 @@ int crypto_ecdh_shared_secret(unsigned int curve_id, 
unsigned int ndigits,
  {
        int ret = 0;
        struct ecc_point *product, *pk;
-       u64 priv[ndigits];
-       u64 rand_z[ndigits];
-       unsigned int nbytes;
+       u64 *priv, *rand_z;
        const struct ecc_curve *curve = ecc_get_curve(curve_id);

if (!private_key || !public_key || !curve) {

@@ -1035,14 +1033,22 @@ int crypto_ecdh_shared_secret(unsigned int curve_id, 
unsigned int ndigits,
                goto out;
        }

- nbytes = ndigits << ECC_DIGITS_TO_BYTES_SHIFT;

+       priv = kmalloc_array(ndigits, sizeof(*priv), GFP_KERNEL);
+       if (!priv) {
+               ret = -ENOMEM;
+               goto out;
+       }

- get_random_bytes(rand_z, nbytes);

+       rand_z = kmalloc_array(ndigits, sizeof(*rand_z), GFP_KERNEL);
+       if (!rand_z) {
+               ret = -ENOMEM;
+               goto kfree_out;
+       }

pk = ecc_alloc_point(ndigits);

        if (!pk) {
                ret = -ENOMEM;
-               goto out;
+               goto kfree_out;
        }

product = ecc_alloc_point(ndigits);

@@ -1051,6 +1057,8 @@ int crypto_ecdh_shared_secret(unsigned int curve_id, 
unsigned int ndigits,
                goto err_alloc_product;
        }

+ get_random_bytes(rand_z, ndigits << ECC_DIGITS_TO_BYTES_SHIFT);

+
        ecc_swap_digits(public_key, pk->x, ndigits);
        ecc_swap_digits(&public_key[ndigits], pk->y, ndigits);
        ecc_swap_digits(private_key, priv, ndigits);
@@ -1065,6 +1073,9 @@ int crypto_ecdh_shared_secret(unsigned int curve_id, 
unsigned int ndigits,
        ecc_free_point(product);
  err_alloc_product:
        ecc_free_point(pk);
+kfree_out:
+       kfree(priv);

I think we should use kzfree here.

+       kfree(rand_z);

Probably here too. Looks like there are few intermediate buffers in ecc
that should be zeroized as well.

Best,
ta

  out:
        return ret;
  }