@@ -35,29 +35,24 @@ 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);
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 <lukas@wunner.de> Reviewed-by: Stefan Berger <stefanb@linux.ibm.com> Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com> --- Changes v1 -> v2: * Add code comment explaining why vlen may be larger than bufsize (Stefan) crypto/ecdsa.c | 19 +++++++------------ 1 file changed, 7 insertions(+), 12 deletions(-)