aio: fix race in ring buffer page lookup introduced by page migration support

Prior to the introduction of page migration support in "fs/aio: Add support
to aio ring pages migration" / 36bc08cc,
mapping of the ring buffer pages was done via get_user_pages() while
retaining mmap_sem held for write.  This avoided possible races with userland
racing an munmap() or mremap().  The page migration patch, however, switched
to using mm_populate() to prime the page mapping.  mm_populate() cannot be
called with mmap_sem held.

Instead of dropping the mmap_sem, revert to the old behaviour and simply
drop the use of mm_populate() since get_user_pages() will cause the pages to
get mapped anyways.  Thanks to Al Viro for spotting this issue.
Signed-off-by: Benjamin LaHaise <bcrl@kvack.org>

fs/aio.c

@@ -307,16 +307,25 @@ static int aio_setup_ring(struct kioctx *ctx)
 		aio_free_ring(ctx);
 		return -EAGAIN;
 	}
-	up_write(&mm->mmap_sem);
-
-	mm_populate(ctx->mmap_base, populate);
 
 	pr_debug("mmap address: 0x%08lx\n", ctx->mmap_base);
+
+	/* We must do this while still holding mmap_sem for write, as we
+	 * need to be protected against userspace attempting to mremap()
+	 * or munmap() the ring buffer.
+	 */
 	ctx->nr_pages = get_user_pages(current, mm, ctx->mmap_base, nr_pages,
 				       1, 0, ctx->ring_pages, NULL);
+
+	/* Dropping the reference here is safe as the page cache will hold
+	 * onto the pages for us.  It is also required so that page migration
+	 * can unmap the pages and get the right reference count.
+	 */
 	for (i = 0; i < ctx->nr_pages; i++)
 		put_page(ctx->ring_pages[i]);
 
+	up_write(&mm->mmap_sem);
+
 	if (unlikely(ctx->nr_pages != nr_pages)) {
 		aio_free_ring(ctx);
 		return -EAGAIN;