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Subject[PATCH v3] crypto: ecc: Remove stack VLA usage
On the quest to remove all VLAs from the kernel[1], this avoids VLAs
by just using the maximum allocation size (4 bytes) for stack arrays.
All the VLAs in ecc were either 3 or 4 bytes (or a multiple), so just
make it 4 bytes all the time. Initialization routines are adjusted to
check that ndigits does not end up larger than the arrays.

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

Signed-off-by: Kees Cook <keescook@chromium.org>
---
This expects 14de52112ee70ca289fa77bf2d9cbc79fd2c811f to be reverted.
---
crypto/ecc.c | 47 ++++++++++++++++++++++++-----------------------
crypto/ecc.h | 4 +++-
crypto/ecdh.c | 4 ++--
3 files changed, 29 insertions(+), 26 deletions(-)

diff --git a/crypto/ecc.c b/crypto/ecc.c
index 18f32f2a5e1c..815541309a95 100644
--- a/crypto/ecc.c
+++ b/crypto/ecc.c
@@ -515,7 +515,7 @@ static void vli_mmod_fast_256(u64 *result, const u64 *product,
static bool vli_mmod_fast(u64 *result, u64 *product,
const u64 *curve_prime, unsigned int ndigits)
{
- u64 tmp[2 * ndigits];
+ u64 tmp[2 * ECC_MAX_DIGITS];

switch (ndigits) {
case 3:
@@ -536,7 +536,7 @@ static bool vli_mmod_fast(u64 *result, u64 *product,
static void vli_mod_mult_fast(u64 *result, const u64 *left, const u64 *right,
const u64 *curve_prime, unsigned int ndigits)
{
- u64 product[2 * ndigits];
+ u64 product[2 * ECC_MAX_DIGITS];

vli_mult(product, left, right, ndigits);
vli_mmod_fast(result, product, curve_prime, ndigits);
@@ -546,7 +546,7 @@ static void vli_mod_mult_fast(u64 *result, const u64 *left, const u64 *right,
static void vli_mod_square_fast(u64 *result, const u64 *left,
const u64 *curve_prime, unsigned int ndigits)
{
- u64 product[2 * ndigits];
+ u64 product[2 * ECC_MAX_DIGITS];

vli_square(product, left, ndigits);
vli_mmod_fast(result, product, curve_prime, ndigits);
@@ -560,8 +560,8 @@ static void vli_mod_square_fast(u64 *result, const u64 *left,
static void vli_mod_inv(u64 *result, const u64 *input, const u64 *mod,
unsigned int ndigits)
{
- u64 a[ndigits], b[ndigits];
- u64 u[ndigits], v[ndigits];
+ u64 a[ECC_MAX_DIGITS], b[ECC_MAX_DIGITS];
+ u64 u[ECC_MAX_DIGITS], v[ECC_MAX_DIGITS];
u64 carry;
int cmp_result;

@@ -649,8 +649,8 @@ static void ecc_point_double_jacobian(u64 *x1, u64 *y1, u64 *z1,
u64 *curve_prime, unsigned int ndigits)
{
/* t1 = x, t2 = y, t3 = z */
- u64 t4[ndigits];
- u64 t5[ndigits];
+ u64 t4[ECC_MAX_DIGITS];
+ u64 t5[ECC_MAX_DIGITS];

if (vli_is_zero(z1, ndigits))
return;
@@ -711,7 +711,7 @@ static void ecc_point_double_jacobian(u64 *x1, u64 *y1, u64 *z1,
static void apply_z(u64 *x1, u64 *y1, u64 *z, u64 *curve_prime,
unsigned int ndigits)
{
- u64 t1[ndigits];
+ u64 t1[ECC_MAX_DIGITS];

vli_mod_square_fast(t1, z, curve_prime, ndigits); /* z^2 */
vli_mod_mult_fast(x1, x1, t1, curve_prime, ndigits); /* x1 * z^2 */
@@ -724,7 +724,7 @@ static void xycz_initial_double(u64 *x1, u64 *y1, u64 *x2, u64 *y2,
u64 *p_initial_z, u64 *curve_prime,
unsigned int ndigits)
{
- u64 z[ndigits];
+ u64 z[ECC_MAX_DIGITS];

vli_set(x2, x1, ndigits);
vli_set(y2, y1, ndigits);
@@ -750,7 +750,7 @@ static void xycz_add(u64 *x1, u64 *y1, u64 *x2, u64 *y2, u64 *curve_prime,
unsigned int ndigits)
{
/* t1 = X1, t2 = Y1, t3 = X2, t4 = Y2 */
- u64 t5[ndigits];
+ u64 t5[ECC_MAX_DIGITS];

/* t5 = x2 - x1 */
vli_mod_sub(t5, x2, x1, curve_prime, ndigits);
@@ -791,9 +791,9 @@ static void xycz_add_c(u64 *x1, u64 *y1, u64 *x2, u64 *y2, u64 *curve_prime,
unsigned int ndigits)
{
/* t1 = X1, t2 = Y1, t3 = X2, t4 = Y2 */
- u64 t5[ndigits];
- u64 t6[ndigits];
- u64 t7[ndigits];
+ u64 t5[ECC_MAX_DIGITS];
+ u64 t6[ECC_MAX_DIGITS];
+ u64 t7[ECC_MAX_DIGITS];

/* t5 = x2 - x1 */
vli_mod_sub(t5, x2, x1, curve_prime, ndigits);
@@ -846,9 +846,9 @@ static void ecc_point_mult(struct ecc_point *result,
unsigned int ndigits)
{
/* R0 and R1 */
- u64 rx[2][ndigits];
- u64 ry[2][ndigits];
- u64 z[ndigits];
+ u64 rx[2][ECC_MAX_DIGITS];
+ u64 ry[2][ECC_MAX_DIGITS];
+ u64 z[ECC_MAX_DIGITS];
int i, nb;
int num_bits = vli_num_bits(scalar, ndigits);

@@ -943,13 +943,13 @@ int ecc_is_key_valid(unsigned int curve_id, unsigned int ndigits,
int ecc_gen_privkey(unsigned int curve_id, unsigned int ndigits, u64 *privkey)
{
const struct ecc_curve *curve = ecc_get_curve(curve_id);
- u64 priv[ndigits];
+ u64 priv[ECC_MAX_DIGITS];
unsigned int nbytes = ndigits << ECC_DIGITS_TO_BYTES_SHIFT;
unsigned int nbits = vli_num_bits(curve->n, ndigits);
int err;

/* Check that N is included in Table 1 of FIPS 186-4, section 6.1.1 */
- if (nbits < 160)
+ if (nbits < 160 || ndigits > ARRAY_SIZE(priv))
return -EINVAL;

/*
@@ -988,10 +988,10 @@ int ecc_make_pub_key(unsigned int curve_id, unsigned int ndigits,
{
int ret = 0;
struct ecc_point *pk;
- u64 priv[ndigits];
+ u64 priv[ECC_MAX_DIGITS];
const struct ecc_curve *curve = ecc_get_curve(curve_id);

- if (!private_key || !curve) {
+ if (!private_key || !curve || ndigits > ARRAY_SIZE(priv)) {
ret = -EINVAL;
goto out;
}
@@ -1025,12 +1025,13 @@ 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];
+ u64 priv[ECC_MAX_DIGITS];
+ u64 rand_z[ECC_MAX_DIGITS];
unsigned int nbytes;
const struct ecc_curve *curve = ecc_get_curve(curve_id);

- if (!private_key || !public_key || !curve) {
+ if (!private_key || !public_key || !curve ||
+ ndigits > ARRAY_SIZE(priv) || ndigits > ARRAY_SIZE(rand_z)) {
ret = -EINVAL;
goto out;
}
diff --git a/crypto/ecc.h b/crypto/ecc.h
index e4fd4492c765..f75a86baa3bd 100644
--- a/crypto/ecc.h
+++ b/crypto/ecc.h
@@ -26,7 +26,9 @@
#ifndef _CRYPTO_ECC_H
#define _CRYPTO_ECC_H

-#define ECC_MAX_DIGITS 4 /* 256 */
+#define ECC_CURVE_NIST_P192_DIGITS 3
+#define ECC_CURVE_NIST_P256_DIGITS 4
+#define ECC_MAX_DIGITS ECC_CURVE_NIST_P256_DIGITS

#define ECC_DIGITS_TO_BYTES_SHIFT 3

diff --git a/crypto/ecdh.c b/crypto/ecdh.c
index 3aca0933ec44..3f91ef13c8c6 100644
--- a/crypto/ecdh.c
+++ b/crypto/ecdh.c
@@ -30,8 +30,8 @@ static inline struct ecdh_ctx *ecdh_get_ctx(struct crypto_kpp *tfm)
static unsigned int ecdh_supported_curve(unsigned int curve_id)
{
switch (curve_id) {
- case ECC_CURVE_NIST_P192: return 3;
- case ECC_CURVE_NIST_P256: return 4;
+ case ECC_CURVE_NIST_P192: return ECC_CURVE_NIST_P192_DIGITS;
+ case ECC_CURVE_NIST_P256: return ECC_CURVE_NIST_P256_DIGITS;
default: return 0;
}
}
--
2.7.4

--
Kees Cook
Pixel Security

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