Re: [PATCH v23 7/8] samples/check-exec: Add an enlighten "inc" interpreter and 28 tests

[Mickaël Salaün]

On Tue, Jan 14, 2025 at 01:56:45PM -0700, Nathan Chancellor wrote:
> 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@xxxxxxxxxxxxxxxxxx>
> > Cc: Christian Brauner <brauner@xxxxxxxxxx>
> > Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> > Cc: Paul Moore <paul@xxxxxxxxxxxxxx>
> > Cc: Serge Hallyn <serge@xxxxxxxxxx>
> > Signed-off-by: Mickaël Salaün <mic@xxxxxxxxxxx>
> > Link: https://lore.kernel.org/r/20241212174223.389435-8-mic@xxxxxxxxxxx
> ...
> > diff --git a/samples/check-exec/inc.c b/samples/check-exec/inc.c
> > new file mode 100644
> > index 000000000000..94b87569d2a2
> > --- /dev/null
> > +++ b/samples/check-exec/inc.c
> ...
> > +/* 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?

Thanks for the report, I sent a fix:
https://lore.kernel.org/r/20250115144753.311152-1-mic@xxxxxxxxxxx

> 
> Cheers,
> Nathan