crypto: rsa-pkcs1pad - correctly get hash from source scatterlist

crypto: rsa-pkcs1pad - correctly get hash from source scatterlist

Commit c7381b012872 ("crypto: akcipher - new verify API for public key
algorithms") changed akcipher_alg::verify to take in both the signature
and the actual hash and do the signature verification, rather than just
return the hash expected by the signature as was the case before.  To do
this, it implemented a hack where the signature and hash are
concatenated with each other in one scatterlist.

Obviously, for this to work correctly, akcipher_alg::verify needs to
correctly extract the two items from the scatterlist it is given.
Unfortunately, it doesn't correctly extract the hash in the case where
the signature is longer than the RSA key size, as it assumes that the
signature's length is equal to the RSA key size.  This causes a prefix
of the hash, or even the entire hash, to be taken from the *signature*.

(Note, the case of a signature longer than the RSA key size should not
be allowed in the first place; a separate patch will fix that.)

It is unclear whether the resulting scheme has any useful security
properties.

Fix this by correctly extracting the hash from the scatterlist.

Fixes: c7381b012872 ("crypto: akcipher - new verify API for public key algorithms")
Cc: <stable@vger.kernel.org> # v5.2+
Reviewed-by: Vitaly Chikunov <vt@altlinux.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>