svc: check for allocation overflow in crypto calls part 2

Without checking for overflow there is a risk of allocating a buffer
with size smaller than anticipated and as a consequence of that it might
lead to a heap based overflow with attacker controlled data written
outside the boundaries of the buffer.

Fixes: OP-TEE-2018-0011: "Integer overflow in crypto system calls (x2)"

Signed-off-by: Joakim Bech <joakim.bech@linaro.org>
Tested-by: Joakim Bech <joakim.bech@linaro.org> (QEMU v7, v8)
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
Reported-by: Riscure <inforequest@riscure.com>
Reported-by: Alyssa Milburn <a.a.milburn@vu.nl>
Acked-by: Etienne Carriere <etienne.carriere@linaro.org>
static TEE_Result op_attr_bignum_from_user(void *attr, const void *buffer,
					   size_t size)
{
	struct bignum **bn = attr;

	return crypto_bignum_bin2bn(buffer, size, *bn);
}

static TEE_Result op_attr_bignum_from_user(void *attr, const void *buffer,
					   size_t size)
{
	struct bignum **bn = attr;

	return crypto_bignum_bin2bn(buffer, size, *bn);
}
