linux-kernel - Re: dcache: remove trylock loops (was Re: [BUG] lock_parent() breakage when used from shrink_dentry

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20180312203916.GQ30522@ZenIV.linux.org.uk>
Date:   Mon, 12 Mar 2018 20:39:17 +0000
From:   Al Viro <viro@...IV.linux.org.uk>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     John Ogness <john.ogness@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Christoph Hellwig <hch@....de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: dcache: remove trylock loops (was Re: [BUG] lock_parent()
 breakage when used from shrink_dentry_list())

On Mon, Mar 12, 2018 at 03:23:44PM -0500, Eric W. Biederman wrote:

> Of the two code paths you are concert about:
> 
> For path path_connected looking at s_root is a heuristic to avoid
> calling is_subdir every time we need to do that check.  If the heuristic
> fails we still have is_subdir which should remain accurate.  If
> is_subdir fails the path is genuinely not connected at that moment
> and failing is the correct thing to do.

Umm...  That might be not good enough - the logics is "everything's
reachable from ->s_root anyway, so we might as well not bother checking".
For NFS it's simply not true.

We can mount server:/foo/bar/baz on /tmp/a, then server:/foo on /tmp/b
and we'll have ->s_root pointing to a subtree of what's reachable at
/tmp/b.  Play with renames under /tmp/b and you just might end up with
a problem.  And mount on /tmp/a will be (mistakenly) considered to
be safe, since it satisfies the heuristics in path_connected().