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From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Tue, 3 Mar 2020 11:13:50 +0100
Message-ID: <CAJfpegu_O=wQsewDWdM39dhkrEoMPG4ZBkTQOsWTgFnYmvrLeA@mail.gmail.com>
Subject: Re: [PATCH 00/17] VFS: Filesystem information and notifications [ver #17]
To:     Christian Brauner <christian.brauner@ubuntu.com>
Cc:     David Howells <dhowells@redhat.com>, Ian Kent <raven@themaw.net>,
        James Bottomley <James.Bottomley@hansenpartnership.com>,
        Steven Whitehouse <swhiteho@redhat.com>,
        Miklos Szeredi <mszeredi@redhat.com>,
        viro <viro@zeniv.linux.org.uk>,
        Christian Brauner <christian@brauner.io>,
        Jann Horn <jannh@google.com>,
        "Darrick J. Wong" <darrick.wong@oracle.com>,
        Linux API <linux-api@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        lkml <linux-kernel@vger.kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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On Tue, Mar 3, 2020 at 11:00 AM Christian Brauner
<christian.brauner@ubuntu.com> wrote:
>
> On Tue, Mar 03, 2020 at 10:26:21AM +0100, Miklos Szeredi wrote:
> > On Tue, Mar 3, 2020 at 10:13 AM David Howells <dhowells@redhat.com> wrote:
> > >
> > > Miklos Szeredi <miklos@szeredi.hu> wrote:
> > >
> > > > I'm doing a patch.   Let's see how it fares in the face of all these
> > > > preconceptions.
> > >
> > > Don't forget the efficiency criterion.  One reason for going with fsinfo(2) is
> > > that scanning /proc/mounts when there are a lot of mounts in the system is
> > > slow (not to mention the global lock that is held during the read).
> > >
> > > Now, going with sysfs files on top of procfs links might avoid the global
> > > lock, and you can avoid rereading the options string if you export a change
> > > notification, but you're going to end up injecting a whole lot of pathwalk
> > > latency into the system.
> >
> > Completely irrelevant.  Cached lookup is so much optimized, that you
> > won't be able to see any of it.
> >
> > No, I don't think this is going to be a performance issue at all, but
> > if anything we could introduce a syscall
> >
> >   ssize_t readfile(int dfd, const char *path, char *buf, size_t
> > bufsize, int flags);
> >
> > that is basically the equivalent of open + read + close, or even a
> > vectored variant that reads multiple files.  But that's off topic
> > again, since I don't think there's going to be any performance issue
> > even with plain I/O syscalls.
> >
> > >
> > > On top of that, it isn't going to help with the case that I'm working towards
> > > implementing where a container manager can monitor for mounts taking place
> > > inside the container and supervise them.  What I'm proposing is that during
> > > the action phase (eg. FSCONFIG_CMD_CREATE), fsconfig() would hand an fd
> > > referring to the context under construction to the manager, which would then
> > > be able to call fsinfo() to query it and fsconfig() to adjust it, reject it or
> > > permit it.  Something like:
> > >
> > >         fd = receive_context_to_supervise();
> > >         struct fsinfo_params params = {
> > >                 .flags          = FSINFO_FLAGS_QUERY_FSCONTEXT,
> > >                 .request        = FSINFO_ATTR_SB_OPTIONS,
> > >         };
> > >         fsinfo(fd, NULL, &params, sizeof(params), buffer, sizeof(buffer));
> > >         supervise_parameters(buffer);
> > >         fsconfig(fd, FSCONFIG_SET_FLAG, "hard", NULL, 0);
> > >         fsconfig(fd, FSCONFIG_SET_STRING, "vers", "4.2", 0);
> > >         fsconfig(fd, FSCONFIG_CMD_SUPERVISE_CREATE, NULL, NULL, 0);
> > >         struct fsinfo_params params = {
> > >                 .flags          = FSINFO_FLAGS_QUERY_FSCONTEXT,
> > >                 .request        = FSINFO_ATTR_SB_NOTIFICATIONS,
> > >         };
> > >         struct fsinfo_sb_notifications sbnotify;
> > >         fsinfo(fd, NULL, &params, sizeof(params), &sbnotify, sizeof(sbnotify));
> > >         watch_super(fd, "", AT_EMPTY_PATH, watch_fd, 0x03);
> > >         fsconfig(fd, FSCONFIG_CMD_SUPERVISE_PERMIT, NULL, NULL, 0);
> > >         close(fd);
> > >
> > > However, the supervised mount may be happening in a completely different set
> > > of namespaces, in which case the supervisor presumably wouldn't be able to see
> > > the links in procfs and the relevant portions of sysfs.
> >
> > It would be a "jump" link to the otherwise invisible directory.
>
> More magic links to beam you around sounds like a bad idea. We had a
> bunch of CVEs around them in containers and they were one of the major
> reasons behind us pushing for openat2(). That's why it has a
> RESOLVE_NO_MAGICLINKS flag.

No, that link wouldn't beam you around at all, it would end up in an
internally mounted instance of a mountfs, a safe place where no
dangerous CVE's roam.

Thanks,
Miklos