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

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Message-ID: <20200528143957.lhyjorrfqrexjurz@wittgenstein>
Date:   Thu, 28 May 2020 16:39:57 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...nel.org>,
        Tycho Andersen <tycho@...ho.ws>,
        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 Thu, May 28, 2020 at 04:17:00PM +0200, Christian Brauner wrote:
> On Wed, May 27, 2020 at 06:59:54PM -0700, Kees Cook wrote:
> > On Thu, May 28, 2020 at 01:16:46AM +0200, Christian Brauner wrote:
> > > I'm also starting to think this isn't even possible or currently doable
> > > safely.
> > > The fdtable in the kernel would end up with a dangling pointer, I would
> > > think. Unless you backtrack all fds that still have a reference into the
> > > fdtable and refer to that file and close them all in the kernel which I
> > > don't think is possible and also sounds very dodgy. This also really
> > > seems like we would be breaking a major contract, namely that fds stay
> > > valid until userspace calls close, execve(), or exits.
> > 
> > Right, I think I was just using the wrong words? I was looking at it
> > like a pipe, or a socket, where you still have an fd, but reads return
> > 0, you might get SIGPIPE, etc. The VFS clearly knows what a
> > "disconnected" fd is, and I had assumed there was general logic for it
> > to indicate "I'm not here any more".
> > 
> > I recently did something very similar to the pstore filesystem, but I got
> > to cheat with some massive subsystem locks. In that case I needed to clear
> > all the inodes out of the tmpfs, so I unlink them all and manage the data
> > lifetimes pointing back into the (waiting to be unloaded) backend module
> > by NULLing the pointer back, which is safe because of the how the locking
> > there happens to work. Any open readers, when they close, will have the
> > last ref count dropped, at which point the record itself is released too.
> > 
> > Back to the seccomp subject: should "all tasks died" be distinguishable
> > from "I can't find that notification" in the ioctl()? (i.e. is ENOENT
> > sufficient, or does there need to be an EIO or ESRCH there?)
> 
> I personally think it's fine as it is but as it might help users if we
> reported ESRCH something like the patch below might do.
> Actual cleanup of the notifier should still happen in
> seccomp_notify_release() imho, and not in __poll_t both conceptually and
> also because f_op->release() happens on finaly fput() which punts it to
> task_work() which finishes when the task returns from kernel mode (or
> exits) - or - if the task is not alive anymore just puts it on the
> kernel global workqueue which is perfect for non-high-priority cleanup
> stuff. It's better than making __poll_t heavier than it needs to be.
> Unless there's an obvious reason not to.

Scratch the patch I posted before here; it's garbage of course.