f2fs: fix dereference of stale list iterator after loop body

The list iterator variable will be a bogus pointer if no break was hit.
Dereferencing it (cur->page in this case) could load an out-of-bounds/undefined
value making it unsafe to use that in the comparision to determine if the
specific element was found.

Since 'cur->page' *can* be out-ouf-bounds it cannot be guaranteed that
by chance (or intention of an attacker) it matches the value of 'page'
even though the correct element was not found.

This is fixed by using a separate list iterator variable for the loop
and only setting the original variable if a suitable element was found.
Then determing if the element was found is simply checking if the
variable is set.

Fixes: 8c242db9 ("f2fs: fix stale ATOMIC_WRITTEN_PAGE private pointer")
Signed-off-by: Jakob Koschel <jakobkoschel@gmail.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/segment.c

@@ -355,16 +355,19 @@ void f2fs_drop_inmem_page(struct inode *inode, struct page *page)
 	struct f2fs_sb_info *sbi = F2FS_I_SB(inode);
 	struct list_head *head = &fi->inmem_pages;
 	struct inmem_pages *cur = NULL;
+	struct inmem_pages *tmp;
 
 	f2fs_bug_on(sbi, !page_private_atomic(page));
 
 	mutex_lock(&fi->inmem_lock);
-	list_for_each_entry(cur, head, list) {
-		if (cur->page == page)
+	list_for_each_entry(tmp, head, list) {
+		if (tmp->page == page) {
+			cur = tmp;
 			break;
+		}
 	}
 
-	f2fs_bug_on(sbi, list_empty(head) || cur->page != page);
+	f2fs_bug_on(sbi, !cur);
 	list_del(&cur->list);
 	mutex_unlock(&fi->inmem_lock);