CVE-2009-1895 kernel: personality: fix PER_CLEAR_ON_SETID

[Eugene Teo <eugene () redhat com>]

Reported by Julien Tinnes.

"We have found that the current PER_CLEAR_ON_SETID mask on Linux doesn't
include neither ADDR_COMPAT_LAYOUT, nor MMAP_PAGE_ZERO.

The current mask is READ_IMPLIES_EXEC|ADDR_NO_RANDOMIZE.

We believe it is important to add MMAP_PAGE_ZERO, because by using this
personality it is possible to have the first page mapped inside a
process running as setuid root.  This could be used in those scenarios:

- Exploiting a NULL pointer dereference issue in a setuid root binary
- Bypassing the mmap_min_addr restrictions of the Linux kernel: by
running a setuid binary that would drop privileges before giving us
control back (for instance by loading a user-supplied library), we could
get the first page mapped in a process we control.  By further using
mremap and mprotect on this mapping, we can then completely bypass the
mmap_min_addr restrictions.

Less importantly, we believe ADDR_COMPAT_LAYOUT should also be added
since on x86 32bits it will in practice disable most of the address
space layout randomization (only the stack will remain randomized)."

Upstream commit:
http://git.kernel.org/linus/f9fabcb58a6d26d6efde842d1703ac7cfa9427b6

References:
https://bugzilla.redhat.com/show_bug.cgi?id=CVE-2009-1895
http://blog.cr0.org/2009/06/bypassing-linux-null-pointer.html
http://patchwork.kernel.org/patch/32598/
http://marc.info/?l=linux-security-module&m=124724852000951&w=2

Thanks, Eugene