SUNRPC: Fix a crash in gss_krb5_checksum()

Anna says:
> KASAN reports [...] a slab-out-of-bounds in gss_krb5_checksum(),
> and it can cause my client to panic when running cthon basic
> tests with krb5p.

> Running faddr2line gives me:
>
> gss_krb5_checksum+0x4b6/0x630:
> ahash_request_free at
> /home/anna/Programs/linux-nfs.git/./include/crypto/hash.h:619
> (inlined by) gss_krb5_checksum at
> /home/anna/Programs/linux-nfs.git/net/sunrpc/auth_gss/gss_krb5_crypto.c:358

My diagnosis is that the memcpy() at the end of gss_krb5_checksum()
reads past the end of the buffer containing the checksum data
because the callers have ignored gss_krb5_checksum()'s API contract:

 * Caller provides the truncation length of the output token (h) in
 * cksumout.len.

Instead they provide the fixed length of the hmac buffer. This
length happens to be larger than the value returned by
crypto_ahash_digestsize().

Change these errant callers to work like krb5_etm_{en,de}crypt().
As a defensive measure, bound the length of the byte copy at the
end of gss_krb5_checksum().

Kunit sez:
Testing complete. Ran 68 tests: passed: 68
Elapsed time: 81.680s total, 5.875s configuring, 75.610s building, 0.103s running
Reported-by: Anna Schumaker <schumaker.anna@gmail.com>
Fixes: 8270dbfc ("SUNRPC: Obscure Kerberos integrity keys")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

net/sunrpc/auth_gss/gss_krb5_crypto.c

@@ -353,7 +353,9 @@ gss_krb5_checksum(struct crypto_ahash *tfm, char *header, int hdrlen,
 	err = crypto_ahash_final(req);
 	if (err)
 		goto out_free_ahash;
-	memcpy(cksumout->data, checksumdata, cksumout->len);
+
+	memcpy(cksumout->data, checksumdata,
+	       min_t(int, cksumout->len, crypto_ahash_digestsize(tfm)));
 
 out_free_ahash:
 	ahash_request_free(req);
@@ -809,8 +811,7 @@ gss_krb5_aes_encrypt(struct krb5_ctx *kctx, u32 offset,
 	buf->tail[0].iov_len += GSS_KRB5_TOK_HDR_LEN;
 	buf->len += GSS_KRB5_TOK_HDR_LEN;
 
-	/* Do the HMAC */
-	hmac.len = GSS_KRB5_MAX_CKSUM_LEN;
+	hmac.len = kctx->gk5e->cksumlength;
 	hmac.data = buf->tail[0].iov_base + buf->tail[0].iov_len;
 
 	/*
@@ -873,8 +874,7 @@ gss_krb5_aes_decrypt(struct krb5_ctx *kctx, u32 offset, u32 len,
 	if (ret)
 		goto out_err;
 
-	/* Calculate our hmac over the plaintext data */
-	our_hmac_obj.len = sizeof(our_hmac);
+	our_hmac_obj.len = kctx->gk5e->cksumlength;
 	our_hmac_obj.data = our_hmac;
 	ret = gss_krb5_checksum(ahash, NULL, 0, &subbuf, 0, &our_hmac_obj);
 	if (ret)