Hi Mickaël,

On Thu, Dec 12, 2024 at 06:42:22PM +0100, Mickaël Salaün wrote:

Add a very simple script interpreter called "inc" that can evaluate two
different commands (one per line):
- "?" to initialize a counter from user's input;
- "+" to increment the counter (which is set to 0 by default).

It is enlighten to only interpret executable files according to
AT_EXECVE_CHECK and the related securebits:

  # Executing a script with RESTRICT_FILE is only allowed if the script
  # is executable:
  ./set-exec -f -- ./inc script-exec.inc # Allowed
  ./set-exec -f -- ./inc script-noexec.inc # Denied

  # Executing stdin with DENY_INTERACTIVE is only allowed if stdin is an
  # executable regular file:
  ./set-exec -i -- ./inc -i < script-exec.inc # Allowed
  ./set-exec -i -- ./inc -i < script-noexec.inc # Denied

  # However, a pipe is not executable and it is then denied:
  cat script-noexec.inc | ./set-exec -i -- ./inc -i # Denied

  # Executing raw data (e.g. command argument) with DENY_INTERACTIVE is
  # always denied.
  ./set-exec -i -- ./inc -c "+" # Denied
  ./inc -c "$(<script-ask.inc)" # Allowed

  # To directly execute a script, we can update $PATH (used by `env`):
  PATH="${PATH}:." ./script-exec.inc

  # To execute several commands passed as argument:

Add a complete test suite to check the script interpreter against all
possible execution cases:

  make TARGETS=exec kselftest-install
  ./tools/testing/selftests/kselftest_install/run_kselftest.sh

Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Kees Cook <redacted>
Cc: Paul Moore <paul@paul-moore.com>
Cc: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Link: https://lore.kernel.org/r/20241212174223.389435-8-mic@digikod.net (local)

...

...

+/* Returns 1 on error, 0 otherwise. */
+static int interpret_stream(FILE *script, char *const script_name,
+			    char *const *const envp, const bool restrict_stream)
+{
+	int err;
+	char *const script_argv[] = { script_name, NULL };
+	char buf[128] = {};
+	size_t buf_size = sizeof(buf);
+
+	/*
+	 * We pass a valid argv and envp to the kernel to emulate a native
+	 * script execution.  We must use the script file descriptor instead of
+	 * the script path name to avoid race conditions.
+	 */
+	err = execveat(fileno(script), "", script_argv, envp,
+		       AT_EMPTY_PATH | AT_EXECVE_CHECK);
+	if (err && restrict_stream) {
+		perror("ERROR: Script execution check");
+		return 1;
+	}
+
+	/* Reads script. */
+	buf_size = fread(buf, 1, buf_size - 1, script);
+	return interpret_buffer(buf, buf_size);
+}

The use of execveat() in this test case breaks the build when glibc is
less than 2.34, as that is the earliest version that has the execveat()
wrapper:

https://sourceware.org/git/?p=glibc.git;a=commit;h=19d83270fcd993cc349570164e21b06d57036704

  $ ldd --version | head -1
  ldd (Debian GLIBC 2.31-13+deb11u11) 2.31

  $ make -skj"$(nproc)" ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- mrproper allmodconfig samples/
  ...
  samples/check-exec/inc.c:81:8: error: call to undeclared function 'execveat'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
     81 |         err = execveat(fileno(script), "", script_argv, envp,
        |               ^
  samples/check-exec/inc.c:81:8: note: did you mean 'execve'?
  /usr/include/unistd.h:551:12: note: 'execve' declared here
    551 | extern int execve (const char *__path, char *const __argv[],
        |            ^
  1 error generated.
  ...

Should this just use the syscall directly?

Cheers,
Nathan