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>