netdev - Re: [PATCH v2 1/4] Landlock: Add signal control

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Message-ID: <CAG48ez3o9fmqz5FkFh3YoJs_jMdtDq=Jjj-qMj7v=CxFROq+Ew@mail.gmail.com>
Date: Tue, 6 Aug 2024 20:56:15 +0200
From: Jann Horn <jannh@...gle.com>
To: Tahera Fahimi <fahimitahera@...il.com>
Cc: outreachy@...ts.linux.dev, mic@...ikod.net, gnoack@...gle.com, 
	paul@...l-moore.com, jmorris@...ei.org, serge@...lyn.com, 
	linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org, 
	bjorn3_gh@...tonmail.com, netdev@...r.kernel.org
Subject: Re: [PATCH v2 1/4] Landlock: Add signal control

On Tue, Aug 6, 2024 at 8:11 PM Tahera Fahimi <fahimitahera@...il.com> wrote:
> Currently, a sandbox process is not restricted to send a signal
> (e.g. SIGKILL) to a process outside of the sandbox environment.
> Ability to sending a signal for a sandboxed process should be
> scoped the same way abstract unix sockets are scoped. Therefore,
> we extend "scoped" field in a ruleset with
> "LANDLOCK_SCOPED_SIGNAL" to specify that a ruleset will deny
> sending any signal from within a sandbox process to its
> parent(i.e. any parent sandbox or non-sandboxed procsses).
[...]
> diff --git a/security/landlock/task.c b/security/landlock/task.c
> index 7e8579ebae83..a73cff27bb91 100644
> --- a/security/landlock/task.c
> +++ b/security/landlock/task.c
> @@ -261,11 +261,54 @@ static int hook_unix_may_send(struct socket *const sock,
>         return -EPERM;
>  }
>
> +static int hook_task_kill(struct task_struct *const p,
> +                         struct kernel_siginfo *const info, const int sig,
> +                         const struct cred *const cred)
> +{
> +       bool is_scoped;
> +       const struct landlock_ruleset *target_dom;
> +
> +       /* rcu is already locked */
> +       target_dom = landlock_get_task_domain(p);
> +       if (cred)
> +               /* dealing with USB IO */
> +               is_scoped = domain_IPC_scope(landlock_cred(cred)->domain,
> +                                            target_dom,
> +                                            LANDLOCK_SCOPED_SIGNAL);
> +       else
> +               is_scoped = domain_IPC_scope(landlock_get_current_domain(),
> +                                            target_dom,
> +                                            LANDLOCK_SCOPED_SIGNAL);

This might be a bit more concise if you turn it into something like:

/* only USB IO supplies creds */
cred = cred ?: current_cred();
is_scoped = domain_IPC_scope(landlock_cred(cred)->domain,
    target_dom, LANDLOCK_SCOPED_SIGNAL);

but that's just a question of style, feel free to keep it as-is
depending on what you prefer.

> +       if (is_scoped)
> +               return 0;
> +
> +       return -EPERM;
> +}
> +
> +static int hook_file_send_sigiotask(struct task_struct *tsk,
> +                                   struct fown_struct *fown, int signum)
> +{
> +       bool is_scoped;
> +       const struct landlock_ruleset *dom, *target_dom;
> +       struct task_struct *result = get_pid_task(fown->pid, fown->pid_type);

I'm not an expert on how the fowner stuff works, but I think this will
probably give you "result = NULL" if the file owner PID has already
exited, and then the following landlock_get_task_domain() would
probably crash? But I'm not entirely sure about how this works.

I think the intended way to use this hook would be to instead use the
"file_set_fowner" hook to record the owning domain (though the setup
for that is going to be kind of a pain...), see the Smack and SELinux
definitions of that hook. Or alternatively maybe it would be even
nicer to change the fown_struct to record a cred* instead of a uid and
euid and then use the domain from those credentials for this hook...
I'm not sure which of those would be easier.

> +       /* rcu is already locked! */
> +       dom = landlock_get_task_domain(result);
> +       target_dom = landlock_get_task_domain(tsk);
> +       is_scoped = domain_IPC_scope(dom, target_dom, LANDLOCK_SCOPED_SIGNAL);
> +       put_task_struct(result);
> +       if (is_scoped)
> +               return 0;
> +       return -EPERM;
> +}