Fail the build if MBEDTLS_ECP_MAX_BITS is not large enough
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/ChangeLog.d/ecp_max_bits.txt b/ChangeLog.d/ecp_max_bits.txt
new file mode 100644
index 0000000..bfbe11f
--- /dev/null
+++ b/ChangeLog.d/ecp_max_bits.txt
@@ -0,0 +1,4 @@
+Security
+ * It was possible to configure MBEDTLS_ECP_MAX_BITS to a value that is
+ too small, leading to buffer overflows in ECC operations. Fail the build
+ in such a case.
diff --git a/include/mbedtls/ecp.h b/include/mbedtls/ecp.h
index b974e59..a4aa3a3 100644
--- a/include/mbedtls/ecp.h
+++ b/include/mbedtls/ecp.h
@@ -171,6 +171,40 @@
}
mbedtls_ecp_point;
+/* Determine the minimum safe value of MBEDTLS_ECP_MAX_BITS. */
+#if !defined(MBEDTLS_ECP_C)
+#define MBEDTLS_ECP_MAX_BITS_MIN 0
+/* Note: the curves must be listed in DECREASING size! */
+#elif defined(MBEDTLS_ECP_DP_SECP521R1_ENABLED)
+#define MBEDTLS_ECP_MAX_BITS_MIN 521
+#elif defined(MBEDTLS_ECP_DP_BP512R1_ENABLED)
+#define MBEDTLS_ECP_MAX_BITS_MIN 512
+#elif defined(MBEDTLS_ECP_DP_CURVE448_ENABLED)
+#define MBEDTLS_ECP_MAX_BITS_MIN 448
+#elif defined(MBEDTLS_ECP_DP_BP384R1_ENABLED)
+#define MBEDTLS_ECP_MAX_BITS_MIN 384
+#elif defined(MBEDTLS_ECP_DP_SECP384R1_ENABLED)
+#define MBEDTLS_ECP_MAX_BITS_MIN 384
+#elif defined(MBEDTLS_ECP_DP_BP256R1_ENABLED)
+#define MBEDTLS_ECP_MAX_BITS_MIN 256
+#elif defined(MBEDTLS_ECP_DP_SECP256K1_ENABLED)
+#define MBEDTLS_ECP_MAX_BITS_MIN 256
+#elif defined(MBEDTLS_ECP_DP_SECP256R1_ENABLED)
+#define MBEDTLS_ECP_MAX_BITS_MIN 256
+#elif defined(MBEDTLS_ECP_DP_CURVE25519_ENABLED)
+#define MBEDTLS_ECP_MAX_BITS_MIN 255
+#elif defined(MBEDTLS_ECP_DP_SECP224K1_ENABLED)
+#define MBEDTLS_ECP_MAX_BITS_MIN 225 // n is slightly above 2^224
+#elif defined(MBEDTLS_ECP_DP_SECP224R1_ENABLED)
+#define MBEDTLS_ECP_MAX_BITS_MIN 224
+#elif defined(MBEDTLS_ECP_DP_SECP192K1_ENABLED)
+#define MBEDTLS_ECP_MAX_BITS_MIN 192
+#elif defined(MBEDTLS_ECP_DP_SECP192R1_ENABLED)
+#define MBEDTLS_ECP_MAX_BITS_MIN 192
+#else
+#error "MBEDTLS_ECP_C enabled, but no curve?"
+#endif
+
#if !defined(MBEDTLS_ECP_ALT)
/*
* default mbed TLS elliptic curve arithmetic implementation
@@ -245,7 +279,13 @@
* \{
*/
-#if !defined(MBEDTLS_ECP_MAX_BITS)
+#if defined(MBEDTLS_ECP_MAX_BITS)
+
+#if MBEDTLS_ECP_MAX_BITS < MBEDTLS_ECP_MAX_BITS_MIN
+#error "MBEDTLS_ECP_MAX_BITS is smaller than the largest supported curve"
+#endif
+
+#else
/**
* The maximum size of the groups, that is, of \c N and \c P.
*/