NFSD: fix nfsd4_listxattr_validate_cookie

If LISTXATTRS is sent with a correct cookie but a small maxcount,
this could lead function nfsd4_listxattr_validate_cookie to
return NFS4ERR_BAD_COOKIE. If maxcount = 20, then second check
on function gives RHS = 3 thus any cookie larger than 3 returns
NFS4ERR_BAD_COOKIE.

There is no need to validate the cookie on the return XDR buffer
since attribute referenced by cookie will be the first in the
return buffer.

Fixes: 23e50fe3 ("nfsd: implement the xattr functions and en/decode logic")
Signed-off-by: Jorge Mora <mora@netapp.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/nfs4xdr.c

@@ -5386,16 +5386,11 @@ nfsd4_listxattr_validate_cookie(struct nfsd4_listxattrs *listxattrs,
 
 	/*
 	 * If the cookie is larger than the maximum number we can fit
-	 * in either the buffer we just got back from vfs_listxattr, or,
-	 * XDR-encoded, in the return buffer, it's invalid.
+	 * in the buffer we just got back from vfs_listxattr, it's invalid.
 	 */
 	if (cookie > (listxattrs->lsxa_len) / (XATTR_USER_PREFIX_LEN + 2))
 		return nfserr_badcookie;
 
-	if (cookie > (listxattrs->lsxa_maxcount /
-		      (XDR_QUADLEN(XATTR_USER_PREFIX_LEN + 2) + 4)))
-		return nfserr_badcookie;
-
 	*offsetp = (u32)cookie;
 	return 0;
 }