Does MAP_FIXED inhibit execmem denial?

[Stephen Smalley]

On Wed, 2006-04-19 at 22:52 -0700, John Reiser wrote:
> I develop the Linux+ELF side of UPX, which compresses executable programs
> to save storage space and invocation time.  Immediately after kernel
> execve() of a compressed program, a small decompressor reconstructs
> the original PT_LOADs directly into address space; then execution proceeds
> as usual.  The decompression writes instructions which execute later,
> directly into pages with both PROT_WRITE and PROT_EXEC, so perhaps
> there should be a { denied } avc due to execmem when SELinux is in
> enforcing mode.  Reading the explanation of execmem in
>     http://people.redhat.com/drepper/selinux-mem.html
> supports this theory.
> 
> However, under all released FC5 kernels including 2.6.16-1.2096_FC5,
> I see no execmem complaints.  Strace of typical execution begins:

Hmmm...shouldn't.  

# /usr/sbin/getsebool allow_execmem
(If on, /usr/sbin/setsebool allow_execmem=0, or run your test under a
confined domain.)
# cat /selinux/checkreqprot
# execstack -q /path/to/program

-- 
Stephen Smalley
National Security Agency