Re: [RFC 2/2] prctl: PR_SET_MM -- Introduce PR_SET_MM_MAP operation

From: Cyrill Gorcunov
Date: Tue Jul 08 2014 - 18:13:43 EST

On Tue, Jul 08, 2014 at 02:38:30PM -0700, Andrew Morton wrote:
> On Tue, 8 Jul 2014 23:08:49 +0400 Cyrill Gorcunov <gorcunov@xxxxxxxxx> wrote:
> > Ping. Guys, any commens please?
> Well, allowing a process to modify pretty deep internals like this is
> always scary from a security point of view, and now we're removing
> CAP_SYS_RESOURCE protections. Yikes. Convince me we aren't handing
> out root here.
> The changelog doesn't make it clear (to me) why this is actually being
> done. criu runs unprivileged? What's the requirement here?

It comes from user-ns, we are almost ready to support c/r for user
namespaces and faced a problem -- when we create new user-namespace
we loose CAP_SYS_RESOURCE bit and criu fails to proceed. Andrew
Vagin was implementing user-ns in criu and as far as I remember
there were a talk with Eric which (again if my memory doesn't
betray me) end up in -- guys, simply do proper check for values
you read from user space instead of relying on CAP_SYS_RESOURCE.
Erik, am I right?

Probably I should walk over _every_ member we're modifying
explaning where exactly is used in kernel.

Still the good news about all this members we modify -- they are used
for statistics mostly except brk/stack related members but they
are checked very carefully to not exceed the limits (if the
limits are set).

> struct prctl_mm_map could do with a nice comment explaining its role in
> the world.

ok, i'll update

> I'm not seeing a coherent description of the proposed userspace
> interface. We'll eventually want to update the prctl manpage for this,
> so how about laying out all the needed details now, at patch review
> time so we can see what is proposed.

Sure, I'll write more descriptive comment since original "It takes
a pointer of prctl_mm_map structure which carries all members to be
updated" is too short.

> Why isn't the newly-added code under #ifdef CONFIG_CHECKPOINT_RESTORE?

Initially all prctl set-mm opcodes were CONFIG_CHECKPOINT_RESTORE
guarded but then assumed that such modification might be
needed not only for criu but other tools as well and this #ifdef
were dropped off. Now new PR_SET_MM_MAP is a part of old
interface so I'm not sure if it should be CONFIGed. That
said it is not a problem to wrap it but looks unreasonable
in this particular case.
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