Re: [PATCH v3 09/10] virtiofsd: Optionally fill lo_inode.fhandle

[Vivek Goyal <vgoyal@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