Syzbot reports a slab out of bounds access in squashfs_readpage_block().

This is caused by an attempt to read page index 0x2000000000.  This value
(start_index) is stored in an integer loop variable which overflows
producing a value of 0.  This causes a loop which iterates over pages
start_index -> end_index to iterate over 0 -> end_index, which ultimately
causes an out of bounds page array access.

Fix by changing variable to a loff_t, and rename to index to
make it clearer it is a page index, and not a loop count.

Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>
Reported-by: "Lai, Yi" <yi1.lai@linux.intel.com>
Closes: https://lore.kernel.org/all/ZwzcnCAosIPqQ9Ie@ly-workstation/
---
 fs/squashfs/file_direct.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/fs/squashfs/file_direct.c b/fs/squashfs/file_direct.c
index 22251743fadf..d19d4db74af8 100644
--- a/fs/squashfs/file_direct.c
+++ b/fs/squashfs/file_direct.c
@@ -30,7 +30,8 @@ int squashfs_readpage_block(struct page *target_page, u64 block, int bsize,
 	int mask = (1 << (msblk->block_log - PAGE_SHIFT)) - 1;
 	loff_t start_index = folio->index & ~mask;
 	loff_t end_index = start_index | mask;
-	int i, n, pages, bytes, res = -ENOMEM;
+	loff_t index;
+	int i, pages, bytes, res = -ENOMEM;
 	struct page **page, *last_page;
 	struct squashfs_page_actor *actor;
 	void *pageaddr;
@@ -45,9 +46,9 @@ int squashfs_readpage_block(struct page *target_page, u64 block, int bsize,
 		return res;
 
 	/* Try to grab all the pages covered by the Squashfs block */
-	for (i = 0, n = start_index; n <= end_index; n++) {
-		page[i] = (n == folio->index) ? target_page :
-			grab_cache_page_nowait(target_page->mapping, n);
+	for (i = 0, index = start_index; index <= end_index; index++) {
+		page[i] = (index == folio->index) ? target_page :
+			grab_cache_page_nowait(target_page->mapping, index);
 
 		if (page[i] == NULL)
 			continue;
-- 
2.39.2