Secure the lisp reader in spirit of Let Over Lambda. See section "Reader Security" on www.letoverlambda.com to get the initial idea.
CL-USER> (ql:quickload 'cl-secure-read) CL-USER> (in-package cl-secure-read) ;; Define a function DEFAULT-RFS, which is a restricted version of READ-FROM-STRING CL-SECURE-READ> (define-secure-read-from-string default-rfs :fail-value "caboom!") CL-SECURE-READ> (default-rfs "123") ; this will read in number 123, as expected ... ;; ... and this will hopefully just return "caboom!", ;; not executing the removal shell-command. CL-SECURE-READ> (default-rfs "#.(shell-eval \"rm -rf ./\"")
Now exports 4 macro:
- DEFINE-SECURE-READ-FROM-STRING - defines a function, which acts exactly like READ-FROM-STRING, only some macro-characters and dispatch-macro-characters (such as read-eval sequence #.) are disabled.
- DEFINE-SECURE-READ - same for READ or READ-PRESERVING-WHITESPACE
- SECURE-READ-FROM-STRING-LAMBDA - do not define READ-FROM-STRING-like function globally, but return a lambda instead
- SECURE-READ-LAMBDA - same for READ and READ-PRESERVING-WHITESPACE
Here are some notable parameters to macro, which control the behavior of resulting restricted reader:
- :READTABLE keyword, which allows you to specify, which readtable should your restricted reader-function use. Default is to take standard readtable.
:BLACKLIST/:WHITELIST keywords, which specify, what macro-characters should be disabled/enabled.
lisp ;; In this function read-eval is enabled, as well as comments (define-secure-read-from-string my-rfs :whitelist (#\; (#\# #\.) :allow-read-eval))Note, that black/white-list may contain sublists and keywords. Meaning of these will be explained below.
Default black/white-list pair forces standard-io-syntax, disabled read-eval and disables all macro-characters except #' #, #( and #` (thus allowing only special syntax for construction of lists).
SAFE-READ-FROM-STRING-WHITELIST and SAFE-READ-FROM-STRING-BLACKLIST variables can be used instead to specify whitelist and blacklist, by wrapping call to macro in LET.
lisp ;; Same behavior, as in the previous example (let ((safe-read-from-string-whitelist '(#\; (#\# #\.) :allow-read-eval))) (define-secure-read-from-string my-rfs))
:FAIL-VALUE is used to specify, what to return, when input contains disabled characters, default is to return NIL
Here is a full-fledged example, using most of the described features
;; use readtable :clesh, allow comments, special clesh bang-syntax, allow read-eval, ;; do not force standard-io-syntax, in case of failure return string "caboom!" (let ((safe-read-from-string-whitelist '(#\; #\! (#\# #\.) :allow-read-eval :keep-io-syntax))) (define-secure-read-from-string not-so-strict-read-from-string :readtable :clesh :fail-value "caboom!")) (not-so-strict-read-from-string "asdf") ; this will read-in symbol ASDF (not-so-strict-read-from-string "#(1 2 3)") ; and this will return "caboom!" ;; since we've requested not to force io-syntax, we may control read-eval dynamically. ;; Here returns "caboom!", even though *READ-EVAL* was enabled in the definition (let (*read-eval*) (not-so-strict-read-from-string "#.(1 2 3)"))
Syntax of black/white-lists
Black/white list may contain:
- characters, which are interpreted as macro-characters to deny/allow
- lists of characters, which are interpreted as (macro-char ,@sub-macro-chars), where sub-macro-chars is a list of dispatch-macro-chars to deny/allow, which correspond to the given macro-char (usually ##)
- special keywords, which for now are the following :allow-read-eval - do not bind READ-EVAL to NIL explicitly :keep-io-syntax - do not wrap a call to READ-FROM-STRING into WITH-STANDARD-IO-SYNTAX :lists - allow/deny #) and #( :quotes - allow/deny #` #' and #, All other keywords are ignored.
If BLACKLIST is NIL, all the macro-characters and dispatching macro-characters of the readtable are disabled, unless they are explicitly enabled in the WHITELIST. To actually enable all the macrocharacters in the readtable, use something like
- Alexander Popolitov <email@example.com>
- language extension