linux-kernel - Re: [PATCH 4/4] add ksm kernel shared memory driver.

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Date:	Tue, 31 Mar 2009 09:37:17 -0500
From:	Anthony Liguori <anthony@...emonkey.ws>
To:	Andrea Arcangeli <aarcange@...hat.com>
CC:	Izik Eidus <ieidus@...hat.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, linux-mm@...ck.org, avi@...hat.com,
	chrisw@...hat.com, riel@...hat.com, jeremy@...p.org,
	mtosatti@...hat.com, hugh@...itas.com, corbet@....net,
	yaniv@...hat.com, dmonakhov@...nvz.org
Subject: Re: [PATCH 4/4] add ksm kernel shared memory driver.

Andrea Arcangeli wrote:

>> the ability to disable KSM.  That seems like a security concern to me since 
>> registering a memory region ought to be an unprivileged action whereas 
>> enabling/disabling KSM ought to be a privileged action.
>>     
>
> sysfs files would then only be writeable by admin, so if we want to
> allow only admin to start/stop/tune ksm it'd be enough to plug an
> admin capability check in the ioctl to provide equivalent permissions.
>   

Caps are not very granular unless you introduce a new capability.  
Furthermore, it's a bit more difficult to associate a capability with a 
user/group.

With sysfs, you use file based permissions to control the API.  It also 
fits into things like selinux a lot better.

In the very least, if you insist on not using sysfs, you should have a 
separate character device that's used for control (like /dev/ksmctl).

Regards,

Anthony Liguori

> I could imagine converting the enable/pages_to_scan/sleep_time to
> module params and tweaking them through /sys/module/ksm/parameters,
> but for "enable" to work that way, we'd need to intercept the write so
> we can at least weakup the kksmd daemon, which doesn't seem possible
> with /sys/module/ksm/parameters, so in the end if we stick to the
> ioctl for registering regions, it seems simpler to use it for
> start/stop/tune too.
>   

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