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

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Date:   Wed, 27 May 2020 16:56:00 -0600
From:   Tycho Andersen <tycho@...ho.ws>
To:     Kees Cook <keescook@...omium.org>
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 03:36:09PM -0700, Kees Cook wrote:
> 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.

It exists because Andy asked for it :)

I agree that there's no technical reason for it to be there. I think
it's just that the semantics were potentially confusing, and it wasn't
a requirement anyone had to have multiples attached.

Tycho