Re: [patch 0/4] MAP_NOZERO v2 - VM_NOZERO/MAP_NOZERO early summer madness

Davide Libenzi
2007-06-30T19:03:19Z

On Fri, 29 Jun 2007, Kyle Moffett wrote:

> Well I would be very interested in actually being able to use this feature
> under SELinux, I think that just the underlying "can-I-use-this-page" logic
> needs modification. Maybe "MAP_REUSABLE"? That would both imply that we can
> accept reused (IE: nonzeroed) memory *AND* that the current page may be reused
> (IE: remapped without zeroing), although you could conceivably have one flag
> for each case. The userspace allocator should be able to (when prompted by
> MAP_REUSABLE) look in a page "pool" of sorts before falling back to a zeroed
> page. The pool would be created for a given "key" the first time it unmaps
> MAP_REUSABLE pages, possibly using the memory freed by said unmap.

Hmm, why would you need MAP_REUSABLE? If a page is visible at any time for
a given UID, and you have a login under such UID, you can fetch the content
of the page at any time (ie, ptrace_attach, gdb, ...). And if you are
not under a UID login, you'll never get to see that page. ATM not even the
classical "root can see everything" rule is applied.
I think the focus should be to find a case where under the currently
implemented policy for MAP_NOZERO, MAP_NOZERO represent a loss of security
WRT no MAP_NOZERO. I have not been able to find one yet, although Andy
found a potential one in the setuid+exec/ptrace race (fixed by a patch
that should IMO go in in any case).
The more ppl think about breaking it, the better it is.

> The real trick is how to define the "key". The default, without LSMs, should
> be something like the UID. SELinux, on the other hand, would probably want to
> use some kind of hash of the label as the "key", (and store the label in each
> pool, as well). That way SELinux could have a simple access-vector check for
> process:reusepage, as well as an access-vector check and type transition for
> "freereusablepage". Then a policy could allow most user processes to
> unconditionally reuse pages (which would end up in the access-vector-cache and
> therefore be fast), while security-sensitive processes like ssh-agent could
> neither reuse pages nor have their pages reused, even if they request it.

It is very easy, assuming a simple unsigned long cookie is enough for
SeLinux, to fit in the current MAP_NOZERO. Well, we have to change
something in the current struct page _mapcount reuse, but that doable.
There is one line to change, that is the line where the cookie is assigned
to the mm_struct. From there on, it's all handled the same way.
If the hash is any longer than unsigned long, I don't really think is ever
gonna fly, being it stored inside a struct page.
Also, if you start introducing "keys" whose domain is wider than a single
user, then you'll run for sure in some sort of problem. This is why the
current code does not even try to go into the "group" policies.
IMO all this may have some sense if 1) it is very simple 2) limits code
and data structures bloat to very little or nothing 3) stays all the way
to the safe side, at the cost of losing some possible valid pages to be
recycled. After all, MAP_NOZERO is an hint and not a requirement.
I think that the current method (either UID or KEY based) is simple, does
not add extra management pools and, *so far*, is not showing security
leaks.

- Davide