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

From: Kyle Moffett
Date: Thu Jun 28 2007 - 22:57:18 EST


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.

Cheers,
Kyle Moffett



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