From: Vivek Goyal <vgoyal@redhat.com>
To: Hanna Reitz <hreitz@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>,
qemu-devel@nongnu.org,
"Dr . David Alan Gilbert" <dgilbert@redhat.com>,
virtio-fs@redhat.com, Ioannis Angelakopoulos <jaggel@bu.edu>,
Max Reitz <mreitz@redhat.com>
Subject: Re: [PATCH v3 09/10] virtiofsd: Optionally fill lo_inode.fhandle
Date: Tue, 17 Aug 2021 20:14:46 -0400 [thread overview]
Message-ID: <YRxQ9rClxWux/UHs@redhat.com> (raw)
In-Reply-To: <YRwRz8aZGq6QLpx/@redhat.com>
On Tue, Aug 17, 2021 at 03:45:19PM -0400, Vivek Goyal wrote:
> On Tue, Aug 17, 2021 at 10:27:16AM +0200, Hanna Reitz wrote:
> > On 16.08.21 21:44, Vivek Goyal wrote:
> > > On Wed, Aug 11, 2021 at 08:41:18AM +0200, Hanna Reitz wrote:
> > >
> > > [..]
> > > > > > But given the inotify complications, there’s really a good reason we should
> > > > > > use mountinfo.
> > > > > >
> > > > > > > > It’s a bit tricky because our sandboxing prevents easy access to mountinfo,
> > > > > > > > but if that’s the only way...
> > > > > > > yes. We already have lo->proc_self_fd. Maybe we need to keep
> > > > > > > /proc/self/mountinfo open in lo->proc_self_mountinfo. I am assuming
> > > > > > > that any mount table changes will still be visible despite the fact
> > > > > > > I have fd open (and don't have to open new fd to notice new mount/unmount
> > > > > > > changes).
> > > > > > Well, yes, that was my idea. Unfortunately, I wasn’t quite successful yet;
> > > > > > when I tried keeping the fd open, reading from it would just return 0
> > > > > > bytes. Perhaps that’s because we bind-mount /proc/self/fd to /proc so that
> > > > > > nothing else in /proc is visible. Perhaps we need to bind-mount
> > > > > > /proc/self/mountinfo into /proc/self/fd before that...
> > > > > Or perhaps open /proc/self/mountinfo and save fd in lo->proc_mountinfo
> > > > > before /proc/self/fd is bind mounted on /proc?
> > > > Yes, I tried that, and then reading would just return 0 bytes.
> > > Hi Hanna,
> > >
> > > I tried this simple patch and I can read /proc/self/mountinfo before
> > > bind mounting /proc/self/fd and after bind mounting /proc/self/fd. Am
> > > I missing something.
> >
> > Yes, but I tried reading it in the main loop (where we’d actually need it).
> > It looks like the umount2(".", MNT_DETACH) in setup_mounts() breaks it.
>
> Good point. I modified my code and notice too that after umoutn2() it
> always reads 0 bytes. I can understand that all the other mount points
> could go away but new rootfs mount point of virtiofsd should still be
> visible, IIUC. I don't understand why.
>
> Anyway, I tried re-opening /proc/self/mountinfo file after umount2(".",
> MNT_DETACH), and that seems to work and it shows root mount point. I
> created a bind mount and it shows that too.
>
> So looks like quick fix can be that we re-open /proc/self/mountinfo. But
> that means we can't bind /proc/self/fd on /proc/. We could bind mount
> /proc/self on /proc. Not sure is it safe enough.
Or may be I can do this.
- Open O_PATH fd for /proc/self
proc_self = open("/proc/self");
- Bind mount /proc/self/fd on /proc
- pivot_root() and umount() stuff
- Openat(proc_self, "mountinfo")
- close(proc_self)
If this works, then we don't have the security issue and we managed
to open mountinfo after pivot_root() and umount(). Will give it a
try and see if it works tomorrow.
Vivek
next prev parent reply other threads:[~2021-08-18 0:16 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-07-30 15:01 [PATCH v3 00/10] virtiofsd: Allow using file handles instead of O_PATH FDs Max Reitz
2021-07-30 15:01 ` [PATCH v3 01/10] virtiofsd: Limit setxattr()'s creds-dropped region Max Reitz
2021-08-06 14:16 ` Vivek Goyal
2021-08-09 10:30 ` Max Reitz
2021-07-30 15:01 ` [PATCH v3 02/10] virtiofsd: Add TempFd structure Max Reitz
2021-08-06 14:41 ` Vivek Goyal
2021-08-09 10:44 ` Max Reitz
2021-07-30 15:01 ` [PATCH v3 03/10] virtiofsd: Use lo_inode_open() instead of openat() Max Reitz
2021-08-06 15:42 ` Vivek Goyal
2021-07-30 15:01 ` [PATCH v3 04/10] virtiofsd: Add lo_inode_fd() helper Max Reitz
2021-08-06 18:25 ` Vivek Goyal
2021-08-09 10:48 ` Max Reitz
2021-07-30 15:01 ` [PATCH v3 05/10] virtiofsd: Let lo_fd() return a TempFd Max Reitz
2021-07-30 15:01 ` [PATCH v3 06/10] virtiofsd: Let lo_inode_open() " Max Reitz
2021-08-06 19:55 ` Vivek Goyal
2021-08-09 13:40 ` Max Reitz
2021-07-30 15:01 ` [PATCH v3 07/10] virtiofsd: Add lo_inode.fhandle Max Reitz
2021-08-09 15:21 ` Vivek Goyal
2021-08-09 16:41 ` Hanna Reitz
2021-07-30 15:01 ` [PATCH v3 08/10] virtiofsd: Add inodes_by_handle hash table Max Reitz
2021-08-09 16:10 ` Vivek Goyal
2021-08-09 16:47 ` Hanna Reitz
2021-08-10 14:07 ` Vivek Goyal
2021-08-10 14:13 ` Hanna Reitz
2021-08-10 17:51 ` Vivek Goyal
2021-07-30 15:01 ` [PATCH v3 09/10] virtiofsd: Optionally fill lo_inode.fhandle Max Reitz
2021-08-09 18:41 ` Vivek Goyal
2021-08-10 8:32 ` Hanna Reitz
2021-08-10 15:23 ` Vivek Goyal
2021-08-10 15:26 ` Hanna Reitz
2021-08-10 15:57 ` Vivek Goyal
2021-08-11 6:41 ` Hanna Reitz
2021-08-16 19:44 ` Vivek Goyal
2021-08-17 8:27 ` Hanna Reitz
2021-08-17 19:45 ` Vivek Goyal
2021-08-18 0:14 ` Vivek Goyal [this message]
2021-08-18 13:32 ` Vivek Goyal
2021-08-18 13:48 ` Hanna Reitz
2021-08-19 16:38 ` Dr. David Alan Gilbert
2021-07-30 15:01 ` [PATCH v3 10/10] virtiofsd: Add lazy lo_do_find() Max Reitz
2021-08-09 19:08 ` Vivek Goyal
2021-08-10 8:38 ` Hanna Reitz
2021-08-10 14:12 ` Vivek Goyal
2021-08-10 14:17 ` Hanna Reitz
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