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SubjectRe: [PATCH] umount: Do not allow unmounting rootfs.
Andrew Vagin <avagin@parallels.com> writes:

> On Tue, Oct 07, 2014 at 01:58:01PM -0700, Eric W. Biederman wrote:
>> Andrew Vagin <avagin@parallels.com> writes:
>>
>> > On Tue, Oct 07, 2014 at 12:27:06PM -0700, Eric W. Biederman wrote:
>> >>
>> >> Which in practice is totally uninteresting. Only the global root user can
>> >> do it, and it is just a stupid thing to do.
>> >>
>> >> However that is no excuse to allow a silly way to oops the kernel.
>> >>
>> >> We can avoid this silly problem by setting MNT_LOCKED on the rootfs
>> >> mount point and thus avoid needing any special cases in the unmount
>> >> code.
>> >
>> > I had this idea too, but it doesn't work.
>> >
>> > MNT_LOCKED isn't inherited, if the privileged user creates a new mount
>> > namespace.
>> >
>> > So "unshame -m ./nsenter" reproduces the same BUG.
>>
>> Which broken tree do you have where MNT_LOCKED is not inherited?
>
> It is Linus' tree with your patch.
>
> I commented out one line and the BUG isn't triggered any more.

Ok. That is very very weird. It works for me and not for you.

Doh! I ran your test program first on the primary mount namespace
and didn't have /proc mounted when I tried to run your test program
later.

Thanks for the hint about copy_tree that does seem to be where the logic
goes haywire. In copy_mnt_ns we want an exact copy not a partial copy.
The good news is that this bug could only affect rootfs, so it is not a
security hole with respect to mount namespaces, created with user
namespace permissions.

mount --rbind and mount propogation should not be locked to their parent
mounts so we do need to clear MNT_LOCKED in a few places.

A patch that fixes copy_tree carefully in a moment.

Eric


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