crypto: ecdsa - Avoid signed integer overflow on signature decoding

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>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

diff --git a/crypto/ecdsa.c b/crypto/ecdsa.c
index 3b9873f56b0a64c47fbd63acbab9b59482f1dbd1..4a0ca93c99eaaaa14366d26065761489d35eb7b9 100644 (file)
--- a/crypto/ecdsa.c
+++ b/crypto/ecdsa.c
@@ -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);