linux-kernel - Re: [PATCH 1/2] seccomp: notify user trap about unused filter

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Date:   Wed, 27 May 2020 15:36:09 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Tycho Andersen <tycho@...ho.ws>
Cc:     Christian Brauner <christian.brauner@...ntu.com>,
        linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...nel.org>,
        Matt Denton <mpdenton@...gle.com>,
        Sargun Dhillon <sargun@...gun.me>,
        Jann Horn <jannh@...gle.com>, Chris Palmer <palmer@...gle.com>,
        Aleksa Sarai <cyphar@...har.com>,
        Robert Sesek <rsesek@...gle.com>,
        Jeffrey Vander Stoep <jeffv@...gle.com>,
        Linux Containers <containers@...ts.linux-foundation.org>
Subject: Re: [PATCH 1/2] seccomp: notify user trap about unused filter

On Wed, May 27, 2020 at 03:52:03PM -0600, Tycho Andersen wrote:
> On Wed, May 27, 2020 at 02:43:49PM -0700, Kees Cook wrote:
> > (While I'm here -- why can there be only one listener per task? The
> > notifications are filter-specific, not task-specific?)
> 
> Not sure what you mean here?

tatic struct file *init_listener(struct seccomp_filter *filter)
{
        struct file *ret = ERR_PTR(-EBUSY);
        struct seccomp_filter *cur;

        for (cur = current->seccomp.filter; cur; cur = cur->prev) {
                if (cur->notif)
                        goto out;
        }

...

        /* Installing a second listener in the chain should EBUSY */
        EXPECT_EQ(user_trap_syscall(__NR_getpid,
                                    SECCOMP_FILTER_FLAG_NEW_LISTENER),
                  -1);
        EXPECT_EQ(errno, EBUSY);

Why does this limit exist? Since the fd is tied to a specific filter,
I don't see conflicts about having multiple USER_NOTIF filters on one
task -- the monitor's response will either fake it or continue it, so
there is no "composition" needed? I must be missing something.

> Maybe it's best to decouple them entirely, and have usage go back to
> just being the number of tasks, and introduce a new counter for
> notification fds.

But isn't that already tracked by the VFS? i.e. there is a one-to-one
mapping from the "struct file *" returned by "anon_inode_getfile()" and
the "struct filter" (though we do not presently save it in the filter)
and the VFS tracks how many userspace fds are attached to that struct
file via ->f_count (i.e. f_count reaching zero is what triggers calling
seccomp_notify_release()).

In trying to write up an example patch for this, though, yeah, I don't
see how to do the locking. There is the "file" part, which is effectively
held by both any task's fd table and by the seccomp filter.

I suspect the issue is that the private_data can't be the
seccomp_filter. The "validity" of the mapping between kernel and user
needs to be tracked externally:

struct seccomp_notification_pipe
{
	struct filter *filter;
	struct file *file;
};

But I still can't see where to put the lock or refcount....

-- 
Kees Cook