[PATCH review 3/6] userns: Recommend use of memory control groups.

[Lord Glauber Costa of Sealand]

On 01/28/2013 11:37 AM, Lord Glauber Costa of Sealand wrote:
> 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 at 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..." ?
> 
> 
I just saw in a later patch of yours that your concern here seems not
limited to backed ram by tmpfs, but with things like the internal
structures for userns , to avoid patterns in the form: 'for (;;)
unshare(...)'

Humm, it does seem sensible. The kernel memory controller aims to
prevent exactly things like that. But they all exist already before
userns: there are destructive patterns like that with sockets, dentries,
processes, and pretty much every other resource in the kernel. So
Although the recommendation per-se makes sense, I am wondering if it is
worth it to mention anything in the user_ns config?