fuse and glibc?

[BJL]

Hi, new to FUSE here.  I was trying to understand how it worked, and saw
this diagram: http://fuse.sourceforge.net/fuse_structure.png

Just to be sure, this is _not_ implying that FUSE only works with glibc
right?  That is, if an application happens to be using another library (eg
bionic, dietlibc, etc), its filesystem calls are still properly directed to
the FUSE kernel module right?

Thanks in advance for any replies.
- Nescio
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What i know is, any libc should work (Mac's does). You are welcome to  
try.

Sent from my iPhone

On Nov 7, 2009, at 20:23, Nescio Nomen <nescionomen@...> wrote:

> Hi, new to FUSE here.  I was trying to understand how it worked, and  
> saw
> this diagram: http://fuse.sourceforge.net/fuse_structure.png
>
> Just to be sure, this is _not_ implying that FUSE only works with  
> glibc
> right?  That is, if an application happens to be using another  
> library (eg
> bionic, dietlibc, etc), its filesystem calls are still properly  
> directed to
> the FUSE kernel module right?
>
> Thanks in advance for any replies.
> - Nescio
> ---
> ---
> ---
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[Jeffrey 'jf' Lim]

On Sun, Nov 8, 2009 at 12:23 PM, Nescio Nomen <nescionomen@...> wrote:

> Hi, new to FUSE here.  I was trying to understand how it worked, and saw
> this diagram: http://fuse.sourceforge.net/fuse_structure.png
>
> Just to be sure, this is _not_ implying that FUSE only works with glibc
> right?  That is, if an application happens to be using another library (eg
> bionic, dietlibc, etc), its filesystem calls are still properly directed to
> the FUSE kernel module right?
>
>
if your libc uses the same interfaces that libfuse does, I dont see why not.

-jf

--
In the meantime, here is your PSA:
"It's so hard to write a graphics driver that open-sourcing it would not
help."
   -- Andrew Fear, Software Product Manager, NVIDIA Corporation
http://kerneltrap.org/node/7228
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[BJL]

Thanks Jeff (and Manuel, from earlier).  The reason I ask is because I'm
curious how easy it is to break out of a FUSE file system that has been set
as the root filesystem.  How suitable is FUSE for implementing something
like Sandboxie (http://www.sandboxie.com/)?  Has someone done this already?


On Sun, Nov 8, 2009 at 10:14 PM, Jeffrey 'jf' Lim <jfs.world@...>wrote:

> On Sun, Nov 8, 2009 at 12:23 PM, Nescio Nomen <nescionomen@...>wrote:
>
>> Hi, new to FUSE here.  I was trying to understand how it worked, and saw
>> this diagram: http://fuse.sourceforge.net/fuse_structure.png
>>
>> Just to be sure, this is _not_ implying that FUSE only works with glibc
>> right?  That is, if an application happens to be using another library (eg
>> bionic, dietlibc, etc), its filesystem calls are still properly directed
>> to
>> the FUSE kernel module right?
>>
>>
> if your libc uses the same interfaces that libfuse does, I dont see why
> not.
>
> -jf
>
> --
> In the meantime, here is your PSA:
> "It's so hard to write a graphics driver that open-sourcing it would not
> help."
>    -- Andrew Fear, Software Product Manager, NVIDIA Corporation
> http://kerneltrap.org/node/7228
>
>

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[Jeffrey 'jf' Lim]

On Mon, Nov 9, 2009 at 2:55 PM, Nescio Nomen <nescionomen@...> wrote:

> Thanks Jeff (and Manuel, from earlier).  The reason I ask is because I'm
> curious how easy it is to break out of a FUSE file system that has been set
> as the root filesystem.  How suitable is FUSE for implementing something
> like Sandboxie (http://www.sandboxie.com/)?  Has someone done this
> already?
>
>
a chroot would achieve the same thing. What would u specifically need fuse
for?

-jf

--
In the meantime, here is your PSA:
"It's so hard to write a graphics driver that open-sourcing it would not
help."
   -- Andrew Fear, Software Product Manager, NVIDIA Corporation
http://kerneltrap.org/node/7228
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[Michael Raskin-4]

Jeffrey 'jf' Lim wrote:
>> as the root filesystem.  How suitable is FUSE for implementing something
>> like Sandboxie (http://www.sandboxie.com/)?  Has someone done this
>> already?
>>
> a chroot would achieve the same thing. What would u specifically need fuse
> for?
>
> -jf

Well, it is convenient not to maintain chroots separately, but just
configure unionfs writes and maybe shadow some files to prevent reading.
Now, why unionfs is not enough is another question.

I remember preparing a Fuse-Unionfs chroot because of its runtime
configurability. I ended up with around 100 branches, though, and it had
pitiful performance (tens of stats on file access are no good..).

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[BJL]

Thanks Michael, I hadn't heard of UnionFS before actually.  Sandboxie seems
to be somewhat different though.  You can 'sandbox' an application so that
its writes are redirected to some safe area on disk, but when you unsandbox
later, you can choose to either 'commit' some subset of the writes or
discard all of them.  Or is this possible with UnionFS too?  In either case
it's more of a tool than a filesystem.

I think I went off on a bit of a tangent though.  I wasn't investigating how
to build a Sandboxie-clone (although it sounds like fun) but whether or not
it is possible to break out of a FUSE filesystem that has been set as the
root filesystem.  What options are available to a regular user (not
superuser)?  Generalizing a bit further, I am curious about the limits of
FUSE's transparency.  For instance, there is a loopback filesystem example
in the FUSE tar.  It seems to just forward all the calls to the backing
filesystem.  Let's say I change the root to be this loopback filesystem.
Would any applications break?  If so, why?

On Mon, Nov 9, 2009 at 12:30 AM, Michael Raskin <fb08af68@...> wrote:

> Jeffrey 'jf' Lim wrote:
> >> as the root filesystem.  How suitable is FUSE for implementing something
> >> like Sandboxie (http://www.sandboxie.com/)?  Has someone done this
> >> already?
> >>
> > a chroot would achieve the same thing. What would u specifically need
> fuse
> > for?
> >
> > -jf
>
> Well, it is convenient not to maintain chroots separately, but just
> configure unionfs writes and maybe shadow some files to prevent reading.
> Now, why unionfs is not enough is another question.
>
> I remember preparing a Fuse-Unionfs chroot because of its runtime
> configurability. I ended up with around 100 branches, though, and it had
> pitiful performance (tens of stats on file access are no good..).
>

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[Michael Raskin-4]

Nescio Nomen wrote:
> Thanks Michael, I hadn't heard of UnionFS before actually.  Sandboxie
> seems to be somewhat different though.  You can 'sandbox' an application
> so that its writes are redirected to some safe area on disk, but when
> you unsandbox later, you can choose to either 'commit' some subset of
> the writes or discard all of them.  Or is this possible with UnionFS
> too?  In either case it's more of a tool than a filesystem.

Well, committing may be done by copying some branches. You can make a
tool around the FS, the question is what you need from the FS.

> how to build a Sandboxie-clone (although it sounds like fun) but whether
> or not it is possible to break out of a FUSE filesystem that has been
> set as the root filesystem.  What options are available to a regular

First, there may be an exploitable bug in that specific FS.. Maybe there
are bugs in FUSE. FUSE design doesn't look like it has some specific
methods of chroot escaping hidden. But I didn't study it.

> user (not superuser)?  Generalizing a bit further, I am curious about
> the limits of FUSE's transparency.  For instance, there is a loopback

http://sourceforge.net/apps/mediawiki/fuse/index.php?title=FUSE_Development_Roadmap

There are some compatibility features.

I have run LFS system with post-boot root on FUSE. It generally works.
In Debian, though, apt-get needs mmap, so you need bind-mounting a
directory from outside.

> filesystem example in the FUSE tar.  It seems to just forward all the
> calls to the backing filesystem.  Let's say I change the root to be this
> loopback filesystem.  Would any applications break?  If so, why?



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[Goswin von Brederlow-2]

Nescio Nomen <nescionomen@...> writes:

> Thanks Jeff (and Manuel, from earlier).  The reason I ask is because I'm
> curious how easy it is to break out of a FUSE file system that has been set
> as the root filesystem.  How suitable is FUSE for implementing something
> like Sandboxie (http://www.sandboxie.com/)?  Has someone done this already?

If you mean root filesystem as in pivot_root then it is impossible for
any application to break out. There simply is no out.

They can though remain outside if they were outside before (which is
why you pivot_root before starting other things) or they can break
"in" so to speak. A chroot() call will change their personal root but
only to something they can already reach. The problem, as seen from
chroot experience, is that that is usualy the exploit. There is
somewhere inside they can reach that should be "outside" only, like
/proc/1/root.

The only risk there is is that your FUSE FS is exploitable
somehow. If you want to use something like unionfs fuse then that will
need access to the filesystems it unions. And if the unionfs fuse can
be exploited in some way then you could gain access to things that
should be hidden.


FYI unionfs fuse can't easily work as root yet. The "outside"
filesystem will need to be visible "inside" for unionfs fuse to
work. It need to be changed to use *at() calls before the "outside"
can be hidden. But if you change that it is unlikely to be exploitable
given the strict libfuse interface and small codesize. Easy to audit
to make sure.

MfG
        Goswin

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