crypto: ecdsa - Harden against integer overflows in DIV_ROUND_UP()

commit b16510a upstream.

Herbert notes that DIV_ROUND_UP() may overflow unnecessarily if an ecdsa
implementation's ->key_size() callback returns an unusually large value.
Herbert instead suggests (for a division by 8):

  X / 8 + !!(X & 7)

Based on this formula, introduce a generic DIV_ROUND_UP_POW2() macro and
use it in lieu of DIV_ROUND_UP() for ->key_size() return values.

Additionally, use the macro in ecc_digits_from_bytes(), whose "nbytes"
parameter is a ->key_size() return value in some instances, or a
user-specified ASN.1 length in the case of ecdsa_get_signature_rs().

Link: https://lore.kernel.org/r/Z3iElsILmoSu6FuC@gondor.apana.org.au/
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-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: l1k

crypto/ecc.c

@@ -69,7 +69,7 @@ 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));
+	int diff = ndigits - DIV_ROUND_UP_POW2(nbytes, sizeof(u64));
 	unsigned int o = nbytes & 7;
 	__be64 msd = 0;
 

include/linux/math.h

@@ -34,6 +34,18 @@
  */
 #define round_down(x, y) ((x) & ~__round_mask(x, y))
 
+/**
+ * DIV_ROUND_UP_POW2 - divide and round up
+ * @n: numerator
+ * @d: denominator (must be a power of 2)
+ *
+ * Divides @n by @d and rounds up to next multiple of @d (which must be a power
+ * of 2). Avoids integer overflows that may occur with __KERNEL_DIV_ROUND_UP().
+ * Performance is roughly equivalent to __KERNEL_DIV_ROUND_UP().
+ */
+#define DIV_ROUND_UP_POW2(n, d) \
+	((n) / (d) + !!((n) & ((d) - 1)))
+
 #define DIV_ROUND_UP __KERNEL_DIV_ROUND_UP
 
 #define DIV_ROUND_DOWN_ULL(ll, d) \