Re: [RFC PATCH v19 2/5] security: Add new SHOULD_EXEC_CHECK and SHOULD_EXEC_RESTRICT securebits

[Andy Lutomirski <luto@kernel.org>]

On Fri, Jul 5, 2024 at 3:02 AM Mickaël Salaün <mic@digikod.net> wrote:
>
> These new SECBIT_SHOULD_EXEC_CHECK, SECBIT_SHOULD_EXEC_RESTRICT, 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.  Like seccomp filters or Landlock domains, the
> securebits are inherited across proceses.
>
> When SECBIT_SHOULD_EXEC_CHECK is set, programs interpreting code should
> check executable resources with execveat(2) + AT_CHECK (see previous
> patch).
>
> When SECBIT_SHOULD_EXEC_RESTRICT is set, a process should only allow
> execution of approved resources, if any (see SECBIT_SHOULD_EXEC_CHECK).

I read this twice, slept on it, read them again, and I *still* can't
understand it.  See below...

> The only restriction enforced by the kernel is the right to ptrace
> another process.  Processes are denied to ptrace less restricted ones,
> unless the tracer has CAP_SYS_PTRACE.  This is mainly a safeguard to
> avoid trivial privilege escalations e.g., by a debugging process being
> abused with a confused deputy attack.

What's the actual issue?  And why can't I, as root, do, in a carefully
checked, CHECK'd and RESTRICT'd environment, # gdb -p <pid>?  Adding
weird restrictions to ptrace can substantially *weaken* security
because it forces people to do utterly daft things to work around the
restrictions.

...

> +/*
> + * When SECBIT_SHOULD_EXEC_CHECK is set, a process should check all executable
> + * files with execveat(2) + AT_CHECK.  However, such check should only be
> + * performed if all to-be-executed code only comes from regular files.  For
> + * instance, if a script interpreter is called with both a script snipped as

s/snipped/snippet/

> + * argument and a regular file, the interpreter should not check any file.
> + * Doing otherwise would mislead the kernel to think that only the script file
> + * is being executed, which could for instance lead to unexpected permission
> + * change and break current use cases.

This is IMO not nearly clear enough to result in multiple user
implementations and a kernel implementation and multiple LSM
implementations and LSM policy authors actually agreeing as to what
this means.

I also think it's wrong to give user code instructions about what
kernel checks it should do.  Have the user code call the kernel and
have the kernel implement the policy.

> +/*
> + * When SECBIT_SHOULD_EXEC_RESTRICT is set, a process should only allow
> + * execution of approved files, if any (see SECBIT_SHOULD_EXEC_CHECK).  For
> + * instance, script interpreters called with a script snippet as argument
> + * should always deny such execution if SECBIT_SHOULD_EXEC_RESTRICT is set.
> + * However, if a script interpreter is called with both
> + * SECBIT_SHOULD_EXEC_CHECK and SECBIT_SHOULD_EXEC_RESTRICT, they should
> + * interpret the provided script files if no unchecked code is also provided
> + * (e.g. directly as argument).

I think you're trying to say that this is like (the inverse of)
Content-Security-Policy: unsafe-inline.  In other words, you're saying
that, if RESTRICT is set, then programs should not execute code-like
text that didn't come from a file.  Is that right?

I feel like it would be worth looking at the state of the art of
Content-Security-Policy and all the lessons people have learned from
it.  Whatever the result is should be at least as comprehensible and
at least as carefully engineered as Content-Security-Policy.

--Andy