linux-kernel - Re: Chroot bug (was: sys_chroot+sys

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Date:	Tue, 25 Sep 2007 17:20:25 +0200 (CEST)
From:	Jan Engelhardt <jengelh@...putergmbh.de>
To:	David Newall <david@...idnewall.com>
cc:	"Serge E. Hallyn" <serge@...lyn.com>,
	Bill Davidsen <davidsen@....com>,
	Philipp Marek <philipp@...ek.priv.at>, 7eggert@....de,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	majkls <majkls@...pere.com>, bunk@...tum.de,
	linux-kernel@...r.kernel.org
Subject: Re: Chroot bug (was: sys_chroot+sys_fchdir Fix)


On Sep 26 2007 00:40, David Newall wrote:
>
> Miloslav Semler pointed out that a root process can chdir("..") out of its
> chroot.  Although this is documented in the man page, it conflicts with the
> essential function, which is to change the root directory of the process.  In
> addition to any creative uses, for example Philipp Marek's loading dynamic
> libraries, it seems clear that the prime purpose of chroot is to aid security.
> Being able to cd your way out is handy for the bad guys, but the good guys
> don't need it; there are a thousand better, safer solutions.

So what? Just do this: chdir into the root after chroot.

It won't conform to SVR4/4.4BSD anymore, but hey, let Linux set some
sane standard ain't bad either. I doubt anyone really relies on the
fact that after chroot, your cwd might be outside the root.

Signed-off-by: Jan Engelhardt <jengelh@....de>

---
 fs/open.c |    1 +
 1 file changed, 1 insertion(+)

Index: linux-2.6.23/fs/open.c
===================================================================
--- linux-2.6.23.orig/fs/open.c
+++ linux-2.6.23/fs/open.c
@@ -547,6 +547,7 @@ asmlinkage long sys_chroot(const char __
 
 	set_fs_root(current->fs, nd.mnt, nd.dentry);
 	set_fs_altroot();
+	set_fs_pwd(current->fs, nd.mnt, nd.dentry);
 	error = 0;
 dput_and_out:
 	path_release(&nd);
-
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