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

Kyle Moffett

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