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Date: Sun, 23 Sep 2012 07:30:38 +0100
From: Al Viro <viro@ZenIV.linux.org.uk>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Kay Sievers <kay.sievers@vrfy.org>,
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        Linus Torvalds <torvalds@linux-foundation.org>,
        Alan Cox <alan@lxorguk.ukuu.org.uk>,
        Serge Hallyn <serge.hallyn@canonical.com>
Subject: Re: [PATCH 3/4] devpts: Make the newinstance option historical
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On Sat, Sep 22, 2012 at 10:59:04PM -0700, Eric W. Biederman wrote:

> The test:
> >> +	if (filp->f_vfsmnt->mnt_root == filp->f_dentry)
> kicks in and no redirection is performed.

Umm...  OK, after the first round of recursion.  Unless you bind /something/pts
on /something.  Or simply create a symlink.  Hell, if static /dev is on the
same fs as /tmp, it can even be done by unpriveleged attacker -
mkdir /tmp/pts
ln /dev/ptmx /tmp
ln -s /tmp/ptmx /tmp/pts/ptmx
exec </tmp/ptmx
and enjoy the stack overflow in kernel mode.  It's not particulary common
setup, of course, but I think it demonstrates that you are playing with
fire...