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

Davide Libenzi
2007-06-29T05:21:00Z

On Thu, 28 Jun 2007, Kyle Moffett wrote:

> On Jun 28, 2007, at 14:49:24, Davide Libenzi wrote:
> > I was using oprofile to sample some userspace code I am working on, and I
> > was continuosly noticing clear_page in the top three entries of the
> > oprofile logs.
> >
> > Also, a simple kernel build, in my Dual Opteron with 8GB of RAM, shows
> > clear_page as the first kernel entry, second only to the userspace the cc1
> > and as. Most of the userspace code uses malloc() (and anonymous mappings)
> > in such a way that the memory returned via kernel->glibc is immediately
> > written soon after. The POSIX malloc() definition itself also, does not
> > require the returned memory to be zeroed (as calloc() does).
> >
> > So I implemented a rather quick hack that introduces a new mmap() flag
> > MAP_NOZERO (only valid for anonymous mappings) and the vma counter-part
> > VM_NOZERO. Also, a new sys_brk2() has been introduced to accept a new flags
> > parameter. A brief description of the patches follows in the next emails.
>
> Hmm, sounds like this would also need a "MAP_NOREUSE" flag of some kind for
> security sensitive applications. Basically, I wouldn't want my ssh-agent
> pages holding private SSH keys to be reused by my web browser which then gets
> exploited :-D

Well, if your web browser and your ssh session are running under the same
user, and your web browser gets hacked, someone is basically logged in in
your system under your user. It can ptrace-attach your ssh-agent and take
it from there. It can also read your .ssh directory for what it matter and
more simply ;)

> It would also be a massive information leak under SELinux. To
> fix it properly according to the SELinux model you would need to tag each page
> with a label immediately after it's freed and then do an access-vector-check
> against the old page and the new process before allowing reuse. On the other
> hand, that would probably be at least as expensive as zeroing the page.

SeLinux could use a simple hook and disable the feature per-task and
globally. Just assign an invalid UID to mm->owner_uid and pages will never
be used. It could also hook mmap and clear off MAP_NOZERO.

- Davide