Re: [PATCH RFC 0/1] mount: universally disallow mounting over symlinks

[Aleksa Sarai <cyphar-gVpy/LI/lHzQT0dZR+AlfA@public.gmane.org>]

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On 2020-01-14, Al Viro <viro-RmSDqhL/yNMiFSDQTTA3OLVCufUGDwFn@public.gmane.org> wrote:
> On Sat, Jan 11, 2020 at 08:07:19AM +1100, Aleksa Sarai wrote:
> 
> > If I'm understanding this proposal correctly, this would be a problem
> > for the libpathrs use-case -- if this is done then there's no way to
> > avoid a TOCTOU with someone mounting and the userspace program checking
> > whether something is a mountpoint (unless you have Linux >5.6 and
> > RESOLVE_NO_XDEV). Today, you can (in theory) do it with MNT_EXPIRE:
> > 
> >   1. Open the candidate directory.
> >   2. umount2(MNT_EXPIRE) the fd.
> >     * -EINVAL means it wasn't a mountpoint when we got the fd, and the
> > 	  fd is a stable handle to the underlying directory.
> > 	* -EAGAIN or -EBUSY means that it was a mountpoint or became a
> > 	  mountpoint after the fd was opened (we don't care about that, but
> > 	  fail-safe is better here).
> >   3. Use the fd from (1) for all operations.
> 
> ... except that foo/../bar *WILL* cross into the covering mount, on any
> kernel that supports ...at(2) at all, so I would be very cautious about
> any kind "hardening" claims in that case.

In the use-case I have, we would have full control over what the path
being opened is (and thus you wouldn't open "foo/../bar"). But I agree
that generally the MNT_EXPIRE solution is really non-ideal anyway.

Not to mention that we're still screwed when it comes to using
magic-links (because if someone bind-mounts a magic-link over a
magic-link there's absolutely no race-free way to be sure that we're
traversing the right magic-link -- for that we'll need to have a
different solution).

> I'm not sure about Linus' proposal - it looks rather convoluted and we
> get a hard to describe twist of semantics in an area (procfs symlinks
> vs. mount traversal) on top of everything else in there...

Yeah, I agree.

> 1) do you see any problems on your testcases with the current #fixes?
> That's commit 7a955b7363b8 as branch tip.

I will take a quick look later today, but I'm currently at a conference.

> 2) do you have any updates you would like to fold into stuff in
> #work.openat2?  Right now I have a local variant of #work.namei (with
> fairly cosmetical change compared to vfs.git one) that merges clean
> with #work.openat2; I would like to do any updates/fold-ins/etc.
> of #work.openat2 *before* doing a merge and continuing to work on
> top of the merge results...

Yes, there were two patches I sent a while ago[1]. I can re-send them if
you like. The second patch switches open_how->mode to a u64, but I'm
still on the fence about whether that makes sense to do...

[1]: https://lore.kernel.org/lkml/20191219105533.12508-1-cyphar-gVpy/LI/lHzQT0dZR+AlfA@public.gmane.org/

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

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