CHROMIUM: mmap, sysctl: add sysctl for controlling VM_MAYEXEC taint

CHROMIUM: mmap, sysctl: add sysctl for controlling VM_MAYEXEC taint

This patch proposes a sysctl knob that allows a privileged user to
disable ~VM_MAYEXEC tainting when mapping in a vma from a MNT_NOEXEC
mountpoint.  It does not alter the normal behavior resulting from
attempting to directly mmap(PROT_EXEC) a vma (-EPERM) nor the behavior
of any other subsystems checking MNT_NOEXEC.

It is motivated by a common /dev/shm, /tmp usecase. There are few
facilities for creating a shared memory segment that can be remapped in
the same process address space with different permissions.  Often, a
file in /tmp provides this functionality.  However, on distributions
that are more restrictive/paranoid, world-writeable directories are
often mounted "noexec".  The only workaround to support software that
needs this behavior is to either not use that software or remount /tmp
exec.  (E.g., https://bugs.gentoo.org/350336?id=350336)  Given that
the only recourse is using SysV IPC, the application programmer loses
many of the useful ABI features that they get using a mmap'd file.

With this patch, it would be possible to change the sysctl variable
such that mprotect(PROT_EXEC) would succeed.  In cases like the example
above, an additional userspace mmap-wrapper would be needed, but in
other cases, like how code.google.com/p/nativeclient mmap()s then
mprotect()s, the behavior would be unaffected.

The tradeoff is a loss of defense in depth, but it seems reasonable when
the alternative is frequently to disable the defense entirely.

(There are many other ways to approach this problem, but this seemed to
 be the most practical and feel the least like a hack or a major change.)

Signed-off-by: Will Drewry <wad@chromium.org>
BUG=chromium-os:19221,native-client:1883
TEST=built x86, booted,
     ran sysctl -w vm.mmap_noexec_taint=0
     run mmap_tester.c from issue such that it uses mprotect.

Change-Id: I1e13adc0d9e8fdb1a0027a2e715557f4aff1b283
Reviewed-on: http://gerrit.chromium.org/gerrit/6007
Reviewed-by: <krasin@google.com>
Reviewed-by: Roland McGrath <mcgrathr@chromium.org>
Reviewed-by: Mandeep Singh Baines <msb@chromium.org>
Tested-by: Will Drewry <wad@chromium.org>