lib/test_hmm: avoid accessing uninitialized pages

If make_device_exclusive_range() fails or returns pages marked for
exclusive access less than required, remaining fields of pages will left
uninitialized.  So dmirror_atomic_map() will access those yet
uninitialized fields of pages.  To fix it, do dmirror_atomic_map() iff all
pages are marked for exclusive access (we will break if mapped is less
than required anyway) so we won't access those uninitialized fields of
pages.

Link: https://lkml.kernel.org/r/20220609130835.35110-1-linmiaohe@huawei.com
Fixes: b659baea ("mm: selftests for exclusive device memory")
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Ralph Campbell <rcampbell@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

lib/test_hmm.c

@@ -732,7 +732,7 @@ static int dmirror_exclusive(struct dmirror *dmirror,
 
 	mmap_read_lock(mm);
 	for (addr = start; addr < end; addr = next) {
-		unsigned long mapped;
+		unsigned long mapped = 0;
 		int i;
 
 		if (end < addr + (ARRAY_SIZE(pages) << PAGE_SHIFT))
@@ -741,7 +741,13 @@ static int dmirror_exclusive(struct dmirror *dmirror,
 			next = addr + (ARRAY_SIZE(pages) << PAGE_SHIFT);
 
 		ret = make_device_exclusive_range(mm, addr, next, pages, NULL);
-		mapped = dmirror_atomic_map(addr, next, pages, dmirror);
+		/*
+		 * Do dmirror_atomic_map() iff all pages are marked for
+		 * exclusive access to avoid accessing uninitialized
+		 * fields of pages.
+		 */
+		if (ret == (next - addr) >> PAGE_SHIFT)
+			mapped = dmirror_atomic_map(addr, next, pages, dmirror);
 		for (i = 0; i < ret; i++) {
 			if (pages[i]) {
 				unlock_page(pages[i]);