linux-kernel - Re: procfs: open("/proc/self/fd/...") allows bypassing O

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Date:   Thu, 26 May 2022 23:03:55 +1000
From:   Aleksa Sarai <cyphar@...har.com>
To:     Christian Brauner <brauner@...nel.org>
Cc:     Miklos Szeredi <miklos@...redi.hu>,
        Amir Goldstein <amir73il@...il.com>,
        Simon Ser <contact@...rsion.fr>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: procfs: open("/proc/self/fd/...") allows bypassing O_RDONLY

On 2022-05-13, Christian Brauner <brauner@...nel.org> wrote:
> On Thu, May 12, 2022 at 02:56:22PM +0200, Miklos Szeredi wrote:
> > On Thu, 12 May 2022 at 14:41, Simon Ser <contact@...rsion.fr> wrote:
> > >
> > > On Thursday, May 12th, 2022 at 12:37, Simon Ser <contact@...rsion.fr> wrote:
> > >
> > > > what would be a good way to share a FD to another
> > > > process without allowing it to write to the underlying file?
> > >
> > > (I'm reminded that memfd + seals exist for this purpose. Still, I'd be
> > > interested to know whether that O_RDONLY/O_RDWR behavior is intended,
> > > because it's pretty surprising. The motivation for using O_RDONLY over
> > > memfd seals is that it isn't Linux-specific.)
> > 
> > Yes, this is intended.   The /proc/$PID/fd/$FD file represents the
> > inode pointed to by $FD.   So the open flags for $FD are irrelevant
> > when operating on the proc fd file.
> 
> Fwiw, the original openat2() patchset contained upgrade masks which we
> decided to split it out into a separate patchset.
> 
> The idea is that struct open_how would be extended with an upgrade mask
> field which allows the opener to specify with which permissions a file
> descriptor is allowed to be re-opened. This has quite a lot of
> use-cases, especially in container runtimes. So one could open an fd and
> restrict it from being re-opened with O_WRONLY. For container runtimes
> this is a huge security win and for userspace in general it would
> provide a backwards compatible way of e.g., making O_PATH fds
> non-upgradable. The plan is to resend the extension at some point in the
> not too distant future.

I am currently working on reviving this patchset.

The main issue at the moment is that the semantics for how we should
deal with directories is a little difficult to define (we want to ignore
modes for directories because of *at(2) semantics but there's no fmode_t
bits at the moment representing that the flip is a directory), but I'm
working on it.

This is going to be included along with the O_EMPTYPATH feature (since
making this safe is IMHO a pre-requisite for O_EMPTYPATH).

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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