crypto: ecc - Fix carry overflow in vli multiplication

commit 27b536a upstream.

The carry flag calculation fails when r01.m_high is saturated
(0xFFFFFFFFFFFFFFFF) and addition of lower bits overflows.

The condition (r01.m_high < product.m_high) doesn't handle the case
where r01.m_high == product.m_high and an additional carry exists
from lower-bit overflow.

When commit 3c4b239 ("crypto: ecdh - Add ECDH software support")
introduced crypto/ecc.c, it split the muladd() function in the
micro-ecc library into separate mul_64_64() and add_128_128() helpers.
It seems the check got lost in translation.

Add proper handling for this boundary by accounting for the carry
from the lower addition.

Fixes: 3c4b239 ("crypto: ecdh - Add ECDH software support")
Signed-off-by: Anastasia Tishchenko <sv3iry@gmail.com>
Cc: stable@vger.kernel.org # v4.8+
Reviewed-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: sv3iry

crypto/ecc.c

@@ -402,14 +402,26 @@ static uint128_t mul_64_64(u64 left, u64 right)
 	return result;
 }
 
-static uint128_t add_128_128(uint128_t a, uint128_t b)
+/* Calculate addition with overflow checking. Returns true on wrap-around,
+ * false otherwise.
+ */
+static bool check_add_128_128_overflow(uint128_t *result, uint128_t a,
+				       uint128_t b)
 {
-	uint128_t result;
+	bool carry;
 
-	result.m_low = a.m_low + b.m_low;
-	result.m_high = a.m_high + b.m_high + (result.m_low < a.m_low);
+	result->m_low = a.m_low + b.m_low;
+	carry = (result->m_low < a.m_low);
 
-	return result;
+	result->m_high = a.m_high + b.m_high + carry;
+
+	/* Using constant-time bitwise arithmetic to prevent timing
+	 * side-channels.
+	 */
+	carry = (result->m_high < a.m_high) |
+		((result->m_high == a.m_high) & carry);
+
+	return carry;
 }
 
 static void vli_mult(u64 *result, const u64 *left, const u64 *right,
@@ -434,9 +446,7 @@ static void vli_mult(u64 *result, const u64 *left, const u64 *right,
 			uint128_t product;
 
 			product = mul_64_64(left[i], right[k - i]);
-
-			r01 = add_128_128(r01, product);
-			r2 += (r01.m_high < product.m_high);
+			r2 += check_add_128_128_overflow(&r01, r01, product);
 		}
 
 		result[k] = r01.m_low;
@@ -459,7 +469,7 @@ static void vli_umult(u64 *result, const u64 *left, u32 right,
 		uint128_t product;
 
 		product = mul_64_64(left[k], right);
-		r01 = add_128_128(r01, product);
+		check_add_128_128_overflow(&r01, r01, product);
 		/* no carry */
 		result[k] = r01.m_low;
 		r01.m_low = r01.m_high;
@@ -496,8 +506,7 @@ static void vli_square(u64 *result, const u64 *left, unsigned int ndigits)
 				product.m_low <<= 1;
 			}
 
-			r01 = add_128_128(r01, product);
-			r2 += (r01.m_high < product.m_high);
+			r2 += check_add_128_128_overflow(&r01, r01, product);
 		}
 
 		result[k] = r01.m_low;