This patch adds a reqprot parameter to the security_file_mmap and
security_file_mprotect hooks that is the original requested protection
value prior to any modification for read-implies-exec, and changes the
SELinux module to allow a mode of operation (controllable via a
checkreqprot setting) where it applies checks based on that protection
value rather than the protection that will be applied by the kernel,
effectively restoring SELinux's original behavior prior to the introduction
of the read-implies-exec logic in the mainline kernel.  The patch also
disables execmem and execmod checking entirely on PPC32, as the PPC32 ELF
ABI presently requires RWE segments per Ulrich Drepper.

At present, the read-implies-exec logic causes SELinux to see every
mmap/mprotect read request by legacy binaries or binaries marked with
PT_GNU_STACK RWE as a read|execute request, which tends to distort policy
even if it reflects what is ultimately possible.  The checkreqprot setting
allows one to set the desired behavior for SELinux, so either the current
behavior or the original behavior is possible.  The checkreqprot value has
a compile-time configurable default value and can also be set via boot
parameter or at runtime via /selinux/checkreqprot if allowed by policy. 
Thanks to Chris Wright, James Morris, and Colin Walters for comments on an
earlier version of the patch.
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: James Morris <jmorris@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>