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

Al Viro
2020-01-14T05:13:02Z

On Tue, Jan 14, 2020 at 04:57:33AM +0000, Al Viro 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.
>
> 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...

PS: one thing that might be interesting is exposing LOOKUP_DOWN via
AT_... flag - it would allow to request mount traversals at the starting
point explicitly. Pretty much all code needed for that is already there;
all it would take is checking the flag in path_openat() and path_parentat()
and having handle_lookup_down() called there, same as in path_lookupat().

A tricky question is whether such flag should affect absolute symlinks -
i.e.

chdir /foo
ln -s /bar barf
overmount /
do lookup with that flag for /bar/splat
do lookup with that flag for barf/splat

Do we want the same results in both calls? The first one would
traverse mounts on / and walk into /bar/splat in overmounting;
the second - see no mounts whatsoever on current directory (/foo
in old root), see the symlink to "/bar", jump to process' root
and proceed from there, first for "bar", then "splat" in it...