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

On 2020-01-14, Al Viro [off-list ref] wrote:

On Sat, Jan 11, 2020 at 08:07:19AM +1100, Aleksa Sarai wrote:

quoted

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/>