crypto: ecc - Prevent ecc_digits_from_bytes from reading too many bytes

Prevent ecc_digits_from_bytes from reading too many bytes from the input
byte array in case an insufficient number of bytes is provided to fill the
output digit array of ndigits. Therefore, initialize the most significant
digits with 0 to avoid trying to read too many bytes later on. Convert the
function into a regular function since it is getting too big for an inline
function.

If too many bytes are provided on the input byte array the extra bytes
are ignored since the input variable 'ndigits' limits the number of digits
that will be filled.

Fixes: d67c96fb ("crypto: ecdsa - Convert byte arrays with key coordinates to digits")
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/ecc.c

@@ -68,6 +68,28 @@ const struct ecc_curve *ecc_get_curve(unsigned int curve_id)
 }
 EXPORT_SYMBOL(ecc_get_curve);
 
+void ecc_digits_from_bytes(const u8 *in, unsigned int nbytes,
+			   u64 *out, unsigned int ndigits)
+{
+	int diff = ndigits - DIV_ROUND_UP(nbytes, sizeof(u64));
+	unsigned int o = nbytes & 7;
+	__be64 msd = 0;
+
+	/* diff > 0: not enough input bytes: set most significant digits to 0 */
+	if (diff > 0) {
+		ndigits -= diff;
+		memset(&out[ndigits - 1], 0, diff * sizeof(u64));
+	}
+
+	if (o) {
+		memcpy((u8 *)&msd + sizeof(msd) - o, in, o);
+		out[--ndigits] = be64_to_cpu(msd);
+		in += o;
+	}
+	ecc_swap_digits(in, out, ndigits);
+}
+EXPORT_SYMBOL(ecc_digits_from_bytes);
+
 static u64 *ecc_alloc_digits_space(unsigned int ndigits)
 {
 	size_t len = ndigits * sizeof(u64);

include/crypto/internal/ecc.h

@@ -64,19 +64,8 @@ static inline void ecc_swap_digits(const void *in, u64 *out, unsigned int ndigit
  * @out       Output digits array
  * @ndigits:  Number of digits to create from byte array
  */
-static inline void ecc_digits_from_bytes(const u8 *in, unsigned int nbytes,
-					 u64 *out, unsigned int ndigits)
-{
-	unsigned int o = nbytes & 7;
-	__be64 msd = 0;
-
-	if (o) {
-		memcpy((u8 *)&msd + sizeof(msd) - o, in, o);
-		out[--ndigits] = be64_to_cpu(msd);
-		in += o;
-	}
-	ecc_swap_digits(in, out, ndigits);
-}
+void ecc_digits_from_bytes(const u8 *in, unsigned int nbytes,
+			   u64 *out, unsigned int ndigits);
 
 /**
  * ecc_is_key_valid() - Validate a given ECDH private key