Re: [PATCH] [RFC] mnt: add ability to clone mntns starting with the current root

[Andy Lutomirski]

On Tue, Oct 7, 2014 at 4:42 PM, Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote:
> Andy Lutomirski <luto@xxxxxxxxxxxxxx> writes:
>
>> On Tue, Oct 7, 2014 at 3:42 PM, Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote:
>>> Andy Lutomirski <luto@xxxxxxxxxxxxxx> writes:
>>>
>>>> On Tue, Oct 7, 2014 at 2:42 PM, Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote:
>>>>>
>>>>> I am squinting and looking this way and that but while I can imagine
>>>>> someone more clever than I can think up some unique property of rootfs
>>>>> that makes it a little more exploitable than just mounting a ramfs,
>>>>> but since you have to be root to exploit those properties I think the
>>>>> game is pretty much lost.
>>>>
>>>> Yes.  rootfs might not be empty, it might have totally insane
>>>> permissions, and it's globally shared, which makes it into a wonderful
>>>> channel to pass things around that shouldn't be passed around.
>>>
>>> But if only root with proc mounted can reach it...  I don't know.
>>
>> It doesn't have to be global root.  It could be userns root.
>>
>>> There might be a case for setting MNT_LOCKED when we overmount "/"
>>> as root but I don't yet see it.
>>>
>>>> Can non-root do this?  You'd need to be in a userns with a "/" that
>>>> isn't MNT_LOCKED.  Can this happen on any normal setup?
>>>>
>>>> FWIW, I think we should unconditionally MNT_LOCKED the root on userns
>>>> unshare, even if it's the only mount.
>>>
>>> To the best of my knowledge MNT_LOCKED is set uncondintially on userns
>>> unshare.
>>
>> Only if list_empty(&old->mnt_expire), whatever that means, I think.
>
> An autofs or nfs automounted mount.  Can those ever become root?

Dunno.

I thought that this code was what set MNT_LOCKED for things that
should be locked:

    /* Don't allow unprivileged users to reveal what is under a mount */
    if ((flag & CL_UNPRIVILEGED) && list_empty(&old->mnt_expire))
        mnt->mnt.mnt_flags |= MNT_LOCKED;

Now I'm confused.  Shouldn't that be checking for submounts?  Is that
what it's doing?

Anyway, I think that this should unconditionally set MNT_LOCKED on the
thing that mounted on rootfs.

--Andy
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/