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SubjectRe: [lxc-devel] Detecting if you are running in a container
Michael Tokarev <> writes:

> [Replying to an oldish email...]
> On 12.10.2011 20:59, Kay Sievers wrote:
>> On Mon, Oct 10, 2011 at 23:41, Lennart Poettering <> wrote:
>>> On Mon, 10.10.11 13:59, Eric W. Biederman ( wrote:
>>>> - udev. All of the kernel interfaces for udev should be supported in
>>>> current kernels. However I believe udev is useless because container
>>>> start drops CAP_MKNOD so we can't do evil things. So I would
>>>> recommend basing the startup of udev on presence of CAP_MKNOD.
>>> Using CAP_MKNOD as test here is indeed a good idea. I'll make sure udev
>>> in a systemd world makes use of that.
>> Done.
> Maybe CAP_MKNOD isn't actually a good idea, having in mind devtmpfs?
> Without CAP_MKNOD, is devtmpfs still being populated internally by
> the kernel, so that udev only needs to change ownership/permissions
> and maintain symlinks in response to device changes, and perform
> other duties (reacting to other types of events) normally?
> In other words, provided devtmpfs works even without CAP_MKNOD,
> I can easily imagine a whole system running without this capability
> from the very early boot, with all functionality in place, including
> udev and what not...

Agreed devtmpfs does pretty much make dropping CAP_MKNOD useless. I
expect we should verify that whoever mounts devtmpfs has CAP_MKNOD.

> And having CAP_MKNOD in container may not be that bad either, while
> cgroup device.permission is set correctly - some nodes may need to
> be created still, even in an unprivileged containers. Who filters
> out CAP_MKNOD during container startup (I don't see it in the code,
> which only removes CAP_SYS_BOOT, and even that due to current
> limitation), and which evil things can be done if it is not filtered?

If you don't filter which device nodes you a process can read/write then
that process can access any device on the system. Steal the keyboard,
the X display, access any filesystem, directly access memory. Basically
the process can escalate that permission to full control of the system
without needing any kernel bugs to help it.


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