Re: [patch 7/8] allow unprivileged mounts

[ebiederm@xmission.com (Eric W. Biederman)]

Andi Kleen <andi@firstfloor.org> writes:

> Andrew Morton <akpm@linux-foundation.org> writes:
>
>> On Fri, 20 Apr 2007 12:25:39 +0200 Miklos Szeredi <miklos@szeredi.hu> wrote:
>> 
>> > Define a new fs flag FS_SAFE, which denotes, that unprivileged
>> > mounting of this filesystem may not constitute a security problem.
>> > 
>> > Since most filesystems haven't been designed with unprivileged
>> > mounting in mind, a thorough audit is needed before setting this flag.
>> 
>> Practically speaking, is there any realistic likelihood that any filesystem
>> apart from FUSE will ever use this?
>
> If it worked for mount --bind for any fs I could see uses of this.  I haven't
> thought
> through the security implications though, so it might not work.

Binding a directory that you have access to in other was is essentially
the same thing as a symlink.  So there are no real security implications
there.  The only problem case I can think of is removal media that you
want to remove but someone has made a bind mount to.  But that is
essentially the same case as opening a file so there are no new
real issues.  Although our diagnostic tools will likely fall behind
for a bit.

We handle the security implications by assigning an owner to all mounts
and only allowing you to add additional mounts on top of a mount you
already own.

If you have the right capabilities you can create a mount owned by
another user.

For a new mount if you don't have the appropriate capabilities nodev
and nosuid will be forced.

Initial super block creation is a lot more delicate so we need the
FS_SAFE flag, to know that the kernel is prepared to deal with the
crazy things that a hostile user space is prepared to do.

Eric
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