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SubjectRe: [PATCH review 3/6] userns: Recommend use of memory control groups.
Lord Glauber Costa of Sealand <glommer@parallels.com> writes:

> On 01/26/2013 06:22 AM, Eric W. Biederman wrote:
>>
>> In the help text describing user namespaces recommend use of memory
>> control groups. In many cases memory control groups are the only
>> mechanism there is to limit how much memory a user who can create
>> user namespaces can use.
>>
>> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
>> ---
>> Documentation/namespaces/resource-control.txt | 10 ++++++++++
>> init/Kconfig | 7 +++++++
>> 2 files changed, 17 insertions(+), 0 deletions(-)
>> create mode 100644 Documentation/namespaces/resource-control.txt
>>
>> diff --git a/Documentation/namespaces/resource-control.txt b/Documentation/namespaces/resource-control.txt
>> new file mode 100644
>> index 0000000..3d8178a
>> --- /dev/null
>> +++ b/Documentation/namespaces/resource-control.txt
>> @@ -0,0 +1,10 @@
>> +There are a lot of kinds of objects in the kernel that don't have
>> +individual limits or that have limits that are ineffective when a set
>> +of processes is allowed to switch user ids. With user namespaces
>> +enabled in a kernel for people who don't trust their users or their
>> +users programs to play nice this problems becomes more acute.
>> +
>> +Therefore it is recommended that memory control groups be enabled in
>> +kernels that enable user namespaces, and it is further recommended
>> +that userspace configure memory control groups to limit how much
>> +memory users they don't trust to play nice can use.
>> diff --git a/init/Kconfig b/init/Kconfig
>> index 7d30240..c8c58bd 100644
>> --- a/init/Kconfig
>> +++ b/init/Kconfig
>> @@ -1035,6 +1035,13 @@ config USER_NS
>> help
>> This allows containers, i.e. vservers, to use user namespaces
>> to provide different user info for different servers.
>> +
>> + When user namespaces are enabled in the kernel it is
>> + recommended that the MEMCG and MEMCG_KMEM options also be
>> + enabled and that user-space use the memory control groups to
>> + limit the amount of memory a memory unprivileged users can
>> + use.
>> +
>> If unsure, say N.
>
> Since this becomes an official recommendation that people will likely
> follow, are we really that much concerned about the types of abuses the
> MEMCG_KMEM will prevent? Those are mostly metadata-based abuses users
> could do in their own local disks without mounting anything extra (and
> things that look like that)
>
> Unless there is a specific concern here, shouldn't we say "... that the
> MEMCG (and possibly MEMCG_KMEM) options..." ?

There are quite a few specific concerns. The easiest to spot is
unshare(CLONE_NEWUSER), and the other namespaces. Then there are
network devices. Then there is I don't know what else.

Most distro's don't seem to care at all about limiting a users memory
so in that sense it is not a concern.

On the other hand for everyone who wants to limit a user's memory the
only way that is going to happen in a reasonable amount of
implementation time is with memory control groups, and slabs and kmalloc
are most definitely part of the memory needs to be limited.

Eric


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