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

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Message-ID: <20200114051248.GX8904@ZenIV.linux.org.uk>
Date:   Tue, 14 Jan 2020 05:12:48 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Aleksa Sarai <cyphar@...har.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        David Howells <dhowells@...hat.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        stable <stable@...r.kernel.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Serge Hallyn <serge@...lyn.com>, dev@...ncontainers.org,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Linux API <linux-api@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ian Kent <raven@...maw.net>
Subject: Re: [PATCH RFC 0/1] mount: universally disallow mounting over
 symlinks

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...