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Message-ID: <20250115.asu4ueXao3ho@digikod.net>
Date: Wed, 15 Jan 2025 16:24:33 +0100
From: Mickaël Salaün <mic@...ikod.net>
To: Nathan Chancellor <nathan@...nel.org>
Cc: Al Viro <viro@...iv.linux.org.uk>,
Christian Brauner <brauner@...nel.org>, Kees Cook <keescook@...omium.org>,
Paul Moore <paul@...l-moore.com>, Serge Hallyn <serge@...lyn.com>,
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linux-security-module@...r.kernel.org
Subject: Re: [PATCH v23 7/8] samples/check-exec: Add an enlighten "inc"
interpreter and 28 tests
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@...iv.linux.org.uk>
> > Cc: Christian Brauner <brauner@...nel.org>
> > Cc: Kees Cook <keescook@...omium.org>
> > Cc: Paul Moore <paul@...l-moore.com>
> > Cc: Serge Hallyn <serge@...lyn.com>
> > Signed-off-by: Mickaël Salaün <mic@...ikod.net>
> > Link: https://lore.kernel.org/r/20241212174223.389435-8-mic@digikod.net
> ...
> > 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@digikod.net
>
> Cheers,
> Nathan
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