Merge tag 'AT_EXECVE_CHECK-v6.14-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux

Pull AT_EXECVE_CHECK from Kees Cook:

- Implement AT_EXECVE_CHECK flag to execveat(2) (Mickaël Sala

Merge tag 'AT_EXECVE_CHECK-v6.14-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux

Pull AT_EXECVE_CHECK from Kees Cook:

- Implement AT_EXECVE_CHECK flag to execveat(2) (Mickaël Salaün)

- Implement EXEC_RESTRICT_FILE and EXEC_DENY_INTERACTIVE securebits
(Mickaël Salaün)

- Add selftests and samples for AT_EXECVE_CHECK (Mickaël Salaün)

* tag 'AT_EXECVE_CHECK-v6.14-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux:
ima: instantiate the bprm_creds_for_exec() hook
samples/check-exec: Add an enlighten "inc" interpreter and 28 tests
selftests: ktap_helpers: Fix uninitialized variable
samples/check-exec: Add set-exec
selftests/landlock: Add tests for execveat + AT_EXECVE_CHECK
selftests/exec: Add 32 tests for AT_EXECVE_CHECK and exec securebits
security: Add EXEC_RESTRICT_FILE and EXEC_DENY_INTERACTIVE securebits
exec: Add a new AT_EXECVE_CHECK flag to execveat(2)

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security: Add EXEC_RESTRICT_FILE and EXEC_DENY_INTERACTIVE securebits

The new SECBIT_EXEC_RESTRICT_FILE, SECBIT_EXEC_DENY_INTERACTIVE, and
their *_LOCKED counterparts are designed to be set by proce

security: Add EXEC_RESTRICT_FILE and EXEC_DENY_INTERACTIVE securebits

The new SECBIT_EXEC_RESTRICT_FILE, SECBIT_EXEC_DENY_INTERACTIVE, and
their *_LOCKED counterparts are designed to be set by processes setting
up an execution environment, such as a user session, a container, or a
security sandbox. Unlike other securebits, these ones can be set by
unprivileged processes. Like seccomp filters or Landlock domains, the
securebits are inherited across processes.

When SECBIT_EXEC_RESTRICT_FILE is set, programs interpreting code should
control executable resources according to execveat(2) + AT_EXECVE_CHECK
(see previous commit).

When SECBIT_EXEC_DENY_INTERACTIVE is set, a process should deny
execution of user interactive commands (which excludes executable
regular files).

Being able to configure each of these securebits enables system
administrators or owner of image containers to gradually validate the
related changes and to identify potential issues (e.g. with interpreter
or audit logs).

It should be noted that unlike other security bits, the
SECBIT_EXEC_RESTRICT_FILE and SECBIT_EXEC_DENY_INTERACTIVE bits are
dedicated to user space willing to restrict itself. Because of that,
they only make sense in the context of a trusted environment (e.g.
sandbox, container, user session, full system) where the process
changing its behavior (according to these bits) and all its parent
processes are trusted. Otherwise, any parent process could just execute
its own malicious code (interpreting a script or not), or even enforce a
seccomp filter to mask these bits.

Such a secure environment can be achieved with an appropriate access
control (e.g. mount's noexec option, file access rights, LSM policy) and
an enlighten ld.so checking that libraries are allowed for execution
e.g., to protect against illegitimate use of LD_PRELOAD.

Ptrace restrictions according to these securebits would not make sense
because of the processes' trust assumption.

Scripts may need some changes to deal with untrusted data (e.g. stdin,
environment variables), but that is outside the scope of the kernel.

See chromeOS's documentation about script execution control and the
related threat model:
https://www.chromium.org/chromium-os/developer-library/guides/security/noexec-shell-scripts/

Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Paul Moore <paul@paul-moore.com>
Reviewed-by: Serge Hallyn <serge@hallyn.com>
Reviewed-by: Jeff Xu <jeffxu@chromium.org>
Tested-by: Jeff Xu <jeffxu@chromium.org>
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Link: https://lore.kernel.org/r/20241212174223.389435-3-mic@digikod.net
Signed-off-by: Kees Cook <kees@kernel.org>

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exec: Add a new AT_EXECVE_CHECK flag to execveat(2)

Add a new AT_EXECVE_CHECK flag to execveat(2) to check if a file would
be allowed for execution. The main use case is for script interpreters
and

exec: Add a new AT_EXECVE_CHECK flag to execveat(2)

Add a new AT_EXECVE_CHECK flag to execveat(2) to check if a file would
be allowed for execution. The main use case is for script interpreters
and dynamic linkers to check execution permission according to the
kernel's security policy. Another use case is to add context to access
logs e.g., which script (instead of interpreter) accessed a file. As
any executable code, scripts could also use this check [1].

This is different from faccessat(2) + X_OK which only checks a subset of
access rights (i.e. inode permission and mount options for regular
files), but not the full context (e.g. all LSM access checks). The main
use case for access(2) is for SUID processes to (partially) check access
on behalf of their caller. The main use case for execveat(2) +
AT_EXECVE_CHECK is to check if a script execution would be allowed,
according to all the different restrictions in place. Because the use
of AT_EXECVE_CHECK follows the exact kernel semantic as for a real
execution, user space gets the same error codes.

An interesting point of using execveat(2) instead of openat2(2) is that
it decouples the check from the enforcement. Indeed, the security check
can be logged (e.g. with audit) without blocking an execution
environment not yet ready to enforce a strict security policy.

LSMs can control or log execution requests with
security_bprm_creds_for_exec(). However, to enforce a consistent and
complete access control (e.g. on binary's dependencies) LSMs should
restrict file executability, or measure executed files, with
security_file_open() by checking file->f_flags & __FMODE_EXEC.

Because AT_EXECVE_CHECK is dedicated to user space interpreters, it
doesn't make sense for the kernel to parse the checked files, look for
interpreters known to the kernel (e.g. ELF, shebang), and return ENOEXEC
if the format is unknown. Because of that, security_bprm_check() is
never called when AT_EXECVE_CHECK is used.

It should be noted that script interpreters cannot directly use
execveat(2) (without this new AT_EXECVE_CHECK flag) because this could
lead to unexpected behaviors e.g., `python script.sh` could lead to Bash
being executed to interpret the script. Unlike the kernel, script
interpreters may just interpret the shebang as a simple comment, which
should not change for backward compatibility reasons.

Because scripts or libraries files might not currently have the
executable permission set, or because we might want specific users to be
allowed to run arbitrary scripts, the following patch provides a dynamic
configuration mechanism with the SECBIT_EXEC_RESTRICT_FILE and
SECBIT_EXEC_DENY_INTERACTIVE securebits.

This is a redesign of the CLIP OS 4's O_MAYEXEC:
https://github.com/clipos-archive/src_platform_clip-patches/blob/f5cb330d6b684752e403b4e41b39f7004d88e561/1901_open_mayexec.patch
This patch has been used for more than a decade with customized script
interpreters. Some examples can be found here:
https://github.com/clipos-archive/clipos4_portage-overlay/search?q=O_MAYEXEC

Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Acked-by: Paul Moore <paul@paul-moore.com>
Reviewed-by: Serge Hallyn <serge@hallyn.com>
Reviewed-by: Jeff Xu <jeffxu@chromium.org>
Tested-by: Jeff Xu <jeffxu@chromium.org>
Link: https://docs.python.org/3/library/io.html#io.open_code [1]
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Link: https://lore.kernel.org/r/20241212174223.389435-2-mic@digikod.net
Signed-off-by: Kees Cook <kees@kernel.org>

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