crypto: ecdsa - Avoid signed integer overflow on signature decoding

[ Upstream commit 3b0565c ]

When extracting a signature component r or s from an ASN.1-encoded
integer, ecdsa_get_signature_rs() subtracts the expected length
"bufsize" from the ASN.1 length "vlen" (both of unsigned type size_t)
and stores the result in "diff" (of signed type ssize_t).

This results in a signed integer overflow if vlen > SSIZE_MAX + bufsize.

The kernel is compiled with -fno-strict-overflow, which implies -fwrapv,
meaning signed integer overflow is not undefined behavior.  And the
function does check for overflow:

       if (-diff >= bufsize)
               return -EINVAL;

So the code is fine in principle but not very obvious.  In the future it
might trigger a false-positive with CONFIG_UBSAN_SIGNED_WRAP=y.

Avoid by comparing the two unsigned variables directly and erroring out
if "vlen" is too large.

Signed-off-by: Lukas Wunner <[email protected]>
Reviewed-by: Stefan Berger <[email protected]>
Reviewed-by: Jonathan Cameron <[email protected]>
Signed-off-by: Herbert Xu <[email protected]>
Signed-off-by: Sasha Levin <[email protected]>

crypto/ecdsa.c

@@ -36,29 +36,24 @@ static int ecdsa_get_signature_rs(u64 *dest, size_t hdrlen, unsigned char tag,
 				  const void *value, size_t vlen, unsigned int ndigits)
 {
 	size_t bufsize = ndigits * sizeof(u64);
-	ssize_t diff = vlen - bufsize;
 	const char *d = value;
 
-	if (!value || !vlen)
+	if (!value || !vlen || vlen > bufsize + 1)
 		return -EINVAL;
 
-	/* diff = 0: 'value' has exacly the right size
-	 * diff > 0: 'value' has too many bytes; one leading zero is allowed that
-	 *           makes the value a positive integer; error on more
-	 * diff < 0: 'value' is missing leading zeros
+	/*
+	 * vlen may be 1 byte larger than bufsize due to a leading zero byte
+	 * (necessary if the most significant bit of the integer is set).
 	 */
-	if (diff > 0) {
+	if (vlen > bufsize) {
 		/* skip over leading zeros that make 'value' a positive int */
 		if (*d == 0) {
 			vlen -= 1;
-			diff--;
 			d++;
-		}
-		if (diff)
+		} else {
 			return -EINVAL;
+		}
 	}
-	if (-diff >= bufsize)
-		return -EINVAL;
 
 	ecc_digits_from_bytes(d, vlen, dest, ndigits);