ipc/shm: Fix shmat mmap nil-page protection

The issue is described here, with a nice testcase:

    https://bugzilla.kernel.org/show_bug.cgi?id=192931

The problem is that shmat() calls do_mmap_pgoff() with MAP_FIXED, and the
address rounded down to 0.  For the regular mmap case, the protection
mentioned above is that the kernel gets to generate the address --
arch_get_unmapped_area() will always check for MAP_FIXED and return that
address.  So by the time we do security_mmap_addr(0) things get funky for
shmat().

The testcase itself shows that while a regular user crashes, root will not
have a problem attaching a nil-page.  There are two possible fixes to
this.  The first, and which this patch does, is to simply allow root to
crash as well -- this is also regular mmap behavior, ie when hacking up
the testcase and adding mmap(...  |MAP_FIXED).  While this approach is the
safer option, the second alternative is to ignore SHM_RND if the rounded
address is 0, thus only having MAP_SHARED flags.  This makes the behavior
of shmat() identical to the mmap() case.  The downside of this is
obviously user visible, but does make sense in that it maintains semantics
after the round-down wrt 0 address and mmap.

Passes shm related ltp tests.

Link: http://lkml.kernel.org/r/1486050195-18629-1-git-send-email-dave@stgolabs.net
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Reported-by: Gareth Evans <gareth.evans@contextis.co.uk>
Cc: Manfred Spraul <manfred@colorfullife.com>
Cc: Michael Kerrisk <mtk.manpages@googlemail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
static void shm_open(struct vm_area_struct *vma)
{
	int err = __shm_open(vma);
	/*
	 * We raced in the idr lookup or with shm_destroy().
	 * Either way, the ID is busted.
	 */
	WARN_ON_ONCE(err);
}

static void shm_open(struct vm_area_struct *vma)
{
	int err = __shm_open(vma);
	/*
	 * We raced in the idr lookup or with shm_destroy().
	 * Either way, the ID is busted.
	 */
	WARN_ON_ONCE(err);
}
