Package: emacs;
Reported by: Stefan Monnier <monnier <at> iro.umontreal.ca>
Date: Sat, 12 Dec 2020 18:20:02 UTC
Severity: normal
Tags: patch
Found in version 28.0.50
Message #257 received at 45198 <at> debbugs.gnu.org (full text, mbox):
From: Philipp <p.stephani2 <at> gmail.com> To: Mattias Engdegård <mattiase <at> acm.org> Cc: alan <at> idiocy.org, 45198 <at> debbugs.gnu.org, stefan <at> marxist.se, Stefan Monnier <monnier <at> iro.umontreal.ca>, Eli Zaretskii <eliz <at> gnu.org>, João Távora <joaotavora <at> gmail.com> Subject: Re: bug#45198: 28.0.50; Sandbox mode Date: Sat, 17 Apr 2021 21:42:31 +0200
> Am 17.04.2021 um 20:59 schrieb Mattias Engdegård <mattiase <at> acm.org>:
>
> 17 apr. 2021 kl. 20.21 skrev Stefan Monnier <monnier <at> iro.umontreal.ca>:
>
>>> Very reasonable. Or would you prefer having the sandboxing in flymake?
>>
>> AFAICT this question refers to a minor implementation detail ;-)
>
> Of course, sorry.
>
> Looks like I forgot to attach the updated patch in a previous message. Here it is.
>
> <0001-Add-macOS-sandboxing-bug-45198.patch>
Looks generally fine, just a few minor comments.
> diff --git a/lisp/subr.el b/lisp/subr.el
> index c2be26a15f..196512c0c6 100644
> --- a/lisp/subr.el
> +++ b/lisp/subr.el
Should this maybe be in a platform-specific file such as ns-fns.el (like dos-fns.el, w32-fns.el)?
> @@ -6262,4 +6262,22 @@ internal--format-docstring-line
> This is intended for internal use only."
> (internal--fill-string-single-line (apply #'format string objects)))
>
> +(when (eq system-type 'darwin)
> + (defun darwin-sandbox-enter (dirs)
I don't think we use the "darwin-" prefix anywhere else in Emacs. Maybe use a "ns-" prefix.
Also I think such internal functions commonly have an "internal" piece somewhere in their name, e.g. "ns-enter-sandbox-internal".
> + "Enter a sandbox only permitting reading files under DIRS.
> +DIRS is a list of directory names. Most other operations such as
> +writing files and network access are disallowed.
> +Existing open descriptors can still be used freely.
> +
> +This is not a supported interface and is for internal use only."
> + (darwin-sandbox-init
Can you also refer to the documents listed below, so that readers aren't wondering what this syntax means?
> + (concat "(version 1)\n"
Since this uses Lisp syntax, maybe use (prin1-to-string `(...))?
> + "(deny default)\n"
> + ;; Emacs seems to need /dev/null; allowing it does no harm.
> + "(allow file-read* (path \"/dev/null\"))\n"
> + (mapconcat (lambda (dir)
> + (format "(allow file-read* (subpath %S))\n" dir))
Are you sure that the string quoting syntaxes are compatible? We really need to avoid injection attacks here.
> + dirs ""))))
> + )
> +
> ;;; subr.el ends here
> diff --git a/src/sysdep.c b/src/sysdep.c
> index d940acc4e0..44e8b82bc6 100644
> --- a/src/sysdep.c
> +++ b/src/sysdep.c
> @@ -4286,8 +4286,40 @@ str_collate (Lisp_Object s1, Lisp_Object s2,
> }
> #endif /* WINDOWSNT */
>
> +#ifdef DARWIN_OS
> +
> +/* This function prototype is not in the platform header files.
> + See https://reverse.put.as/wp-content/uploads/2011/09/Apple-Sandbox-Guide-v1.0.pdf
> + and https://chromium.googlesource.com/chromium/src/+/master/sandbox/mac/seatbelt_sandbox_design.md */
Thanks, I'd expect at least the Chromium reference to stick around.
> +int sandbox_init_with_parameters(const char *profile,
> + uint64_t flags,
> + const char *const parameters[],
> + char **errorbuf);
> +
> +DEFUN ("darwin-sandbox-init", Fdarwin_sandbox_init, Sdarwin_sandbox_init,
Similar comments about naming here; maybe call this ns-init-sandbox-internal or so.
> + 1, 1, 0,
> + doc: /* Enter a sandbox whose permitted access is curtailed by PROFILE.
> +Already open descriptors can be used freely.
> +PROFILE is a string in the macOS sandbox profile language,
> +a set of rules in a Lisp-like syntax.
I'd also refer to the Chromium document here, otherwise C-h f for this function won't be very useful.
> +
> +This is not a supported interface and is for internal use only. */)
> + (Lisp_Object profile)
> +{
> + CHECK_STRING (profile);
> + char *err = NULL;
> + if (sandbox_init_with_parameters (SSDATA (profile), 0, NULL, &err) != 0)
If you're using SSDATA, better check that the string doesn't contain embedded null bytes.
Also does this need to be encoded somehow? What happens if the string contains non-Unicode characters like raw bytes?
> + error ("sandbox error: %s", err);
This looks like an error that clients could handle, so I'd signal a specific error symbol here.
> + return Qnil;
> +}
> +
> +#endif /* DARWIN_OS */
> +
> void
> syms_of_sysdep (void)
> {
> defsubr (&Sget_internal_run_time);
> +#ifdef DARWIN_OS
> + defsubr (&Sdarwin_sandbox_init);
> +#endif
> }
> diff --git a/test/src/emacs-tests.el b/test/src/emacs-tests.el
This should probably be in subr-tests.el since it tests a function in subr.el.
> index 09f9a248ef..c1a741c359 100644
> --- a/test/src/emacs-tests.el
> +++ b/test/src/emacs-tests.el
> @@ -210,4 +210,74 @@ emacs-tests/bwrap/allows-stdout
> (should (eql status 0)))
> (should (equal (string-trim (buffer-string)) "Hi"))))))
>
> +(defun emacs-tests--darwin-run-sandboxed-emacs (sandbox-dirs body)
> + "Run Emacs and evaluate BODY, only allowing reads from SANDBOX-DIRS.
> +If SANDBOX-DIRS is `no-sandbox', then run without sandbox.
> +Return (EXIT-STATUS . OUTPUT), where OUTPUT is stderr and stdout."
> + (let ((emacs (expand-file-name invocation-name invocation-directory))
> + (process-environment nil))
> + (with-temp-buffer
> + (let* ((prog `(progn
> + ,@(and (not (eq sandbox-dirs 'no-sandbox))
> + `((darwin-sandbox-enter ',sandbox-dirs)))
> + ,@body))
> + (res (call-process emacs nil t nil
> + "--quick" "--batch"
> + (format "--eval=%S" prog))))
> + (cons res (buffer-string))))))
This is more of a personal style, feel free to ignore:
I like tests that are somewhat repetitive but more decoupled and easier to read better than tests with factored-out assertions. See e.g. the arguments in https://www.maaikebrinkhof.nl/2016/03/repetition-in-testing/ and https://stackoverflow.com/a/130038.
> +
> +(ert-deftest emacs-tests/darwin-sandbox ()
> + (skip-unless (eq system-type 'darwin))
> + (emacs-tests--with-temp-file test-file ("test")
> + (let ((some-text "abcdef")
> + (other-text "ghijkl")
> + (test-file (file-truename test-file))) ; resolve symlinks
> + (with-temp-buffer
> + (insert some-text)
> + (write-file test-file))
> +
> + ;; Read the file without allowing its directory -- should fail.
> + (let ((res-out (emacs-tests--darwin-run-sandboxed-emacs
> + nil
> + `((find-file-literally ,test-file)
> + (message "OK: %s" (buffer-string))))))
> + (ert-info ((cdr res-out) :prefix "output: ")
> + (should-not (equal (car res-out) 0))
> + (should-not (string-search some-text (cdr res-out)))))
> +
> + ;; Read the file allowing its directory -- should succeed.
> + (let ((res-out (emacs-tests--darwin-run-sandboxed-emacs
> + (list (file-name-directory test-file))
> + `((find-file-literally ,test-file)
> + (message "OK: %s" (buffer-string))))))
> + (should (equal res-out (cons 0 (format "OK: %s\n" some-text)))))
> +
> + ;; Write to the file allowing directory reads -- should fail.
> + (let ((res-out (emacs-tests--darwin-run-sandboxed-emacs
> + (list (file-name-directory test-file))
> + `((with-temp-buffer
> + (insert ,other-text)
> + (write-file ,test-file))))))
> + (ert-info ((cdr res-out) :prefix "output: ")
> + (should-not (equal (car res-out) 0))
> + ;; The file should be unchanged.
> + (let ((contents (with-temp-buffer
> + (insert-file-contents-literally test-file)
> + (buffer-string))))
> + (should (equal contents some-text)))))
> +
> + ;; Write to the file without sandbox -- should succeed.
> + (let ((res-out (emacs-tests--darwin-run-sandboxed-emacs
> + 'no-sandbox
> + `((with-temp-buffer
> + (insert ,other-text)
> + (write-file ,test-file))))))
> + (ert-info ((cdr res-out) :prefix "output: ")
> + (should (equal (car res-out) 0))
> + ;; The file should be changed.
> + (let ((contents (with-temp-buffer
> + (insert-file-contents-literally test-file)
> + (buffer-string))))
> + (should (equal contents other-text))))))))
These should be four separate tests, as they test four separate things.
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