fs/proc/kcore.c: remove check of list iterator against head past the loop body

When list_for_each_entry() completes the iteration over the whole list
without breaking the loop, the iterator value will be a bogus pointer
computed based on the head element.

While it is safe to use the pointer to determine if it was computed based
on the head element, either with list_entry_is_head() or &pos->member ==
head, using the iterator variable after the loop should be avoided.

In preparation to limit the scope of a list iterator to the list traversal
loop, use a dedicated pointer to point to the found element [1].

[akpm@linux-foundation.org: reduce scope of `iter']
Link: https://lore.kernel.org/all/CAHk-=wgRr_D8CB-D9Kg-c=EHreAsk5SqXPwr9Y7k9sA6cWXJ6w@mail.gmail.com/ [1]
Link: https://lkml.kernel.org/r/20220331223700.902556-1-jakobkoschel@gmail.comSigned-off-by: Jakob Koschel <jakobkoschel@gmail.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: "Brian Johannesmeyer" <bjohannesmeyer@gmail.com>
Cc: Cristiano Giuffrida <c.giuffrida@vu.nl>
Cc: "Bos, H.J." <h.j.bos@vu.nl>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/proc/kcore.c

@@ -479,10 +479,15 @@ read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
 		 * the previous entry, search for a matching entry.
 		 */
 		if (!m || start < m->addr || start >= m->addr + m->size) {
-			list_for_each_entry(m, &kclist_head, list) {
-				if (start >= m->addr &&
-				    start < m->addr + m->size)
+			struct kcore_list *iter;
+
+			m = NULL;
+			list_for_each_entry(iter, &kclist_head, list) {
+				if (start >= iter->addr &&
+				    start < iter->addr + iter->size) {
+					m = iter;
 					break;
+				}
 			}
 		}
 
@@ -492,12 +497,11 @@ read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
 			page_offline_freeze();
 		}
 
-		if (&m->list == &kclist_head) {
+		if (!m) {
 			if (clear_user(buffer, tsz)) {
 				ret = -EFAULT;
 				goto out;
 			}
-			m = NULL;	/* skip the list anchor */
 			goto skip;
 		}