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Message-ID: <CALCETrWpk5Es9GPoAdDD=m_vgSePm=cA16zCor_aJV0EPXBw1A@mail.gmail.com>
Date: Sat, 20 Jul 2024 10:06:28 +0800
From: Andy Lutomirski <luto@...nel.org>
To: Mickaël Salaün <mic@...ikod.net>
Cc: Al Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>,
Kees Cook <keescook@...omium.org>, Linus Torvalds <torvalds@...ux-foundation.org>,
Paul Moore <paul@...l-moore.com>, "Theodore Ts'o" <tytso@....edu>,
Alejandro Colomar <alx.manpages@...il.com>, Aleksa Sarai <cyphar@...har.com>,
Andrew Morton <akpm@...ux-foundation.org>, Andy Lutomirski <luto@...nel.org>,
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Vincent Strubel <vincent.strubel@....gouv.fr>, Xiaoming Ni <nixiaoming@...wei.com>,
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linux-security-module@...r.kernel.org
Subject: Re: [RFC PATCH v19 2/5] security: Add new SHOULD_EXEC_CHECK and
SHOULD_EXEC_RESTRICT securebits
On Fri, Jul 5, 2024 at 3:02 AM Mickaël Salaün <mic@...ikod.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
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