ipc/shm: fix shmat() nil address after round-down when remapping

shmat()'s SHM_REMAP option forbids passing a nil address for; this is in
fact the very first thing we check for.  Andrea reported that for
SHM_RND|SHM_REMAP cases we can end up bypassing the initial addr check,
but we need to check again if the address was rounded down to nil.  As
of this patch, such cases will return -EINVAL.

Link: http://lkml.kernel.org/r/20180503204934.kk63josdu6u53fbd@linux-n805
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Reported-by: Andrea Arcangeli <aarcange@redhat.com>
Cc: Joe Lawrence <joe.lawrence@redhat.com>
Cc: Manfred Spraul <manfred@colorfullife.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

ipc/shm.c

@@ -1363,9 +1363,17 @@ long do_shmat(int shmid, char __user *shmaddr, int shmflg,
 
 	if (addr) {
 		if (addr & (shmlba - 1)) {
-			if (shmflg & SHM_RND)
+			if (shmflg & SHM_RND) {
 				addr &= ~(shmlba - 1);  /* round down */
-			else
+
+				/*
+				 * Ensure that the round-down is non-nil
+				 * when remapping. This can happen for
+				 * cases when addr < shmlba.
+				 */
+				if (!addr && (shmflg & SHM_REMAP))
+					goto out;
+			} else
 #ifndef __ARCH_FORCE_SHMLBA
 				if (addr & ~PAGE_MASK)
 #endif