linux-kernel - [PATCH] fs/proc/kcore.c: remove check of list iterator against head past the loop body

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Date:   Fri,  1 Apr 2022 00:37:00 +0200
From:   Jakob Koschel <jakobkoschel@...il.com>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oscar Salvador <osalvador@...e.de>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        "Brian Johannesmeyer" <bjohannesmeyer@...il.com>,
        Cristiano Giuffrida <c.giuffrida@...nl>,
        "Bos, H.J." <h.j.bos@...nl>, Jakob Koschel <jakobkoschel@...il.com>
Subject: [PATCH] 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].

Link: https://lore.kernel.org/all/CAHk-=wgRr_D8CB-D9Kg-c=EHreAsk5SqXPwr9Y7k9sA6cWXJ6w@mail.gmail.com/ [1]
Signed-off-by: Jakob Koschel <jakobkoschel@...il.com>
---
 fs/proc/kcore.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/fs/proc/kcore.c b/fs/proc/kcore.c
index 982e694aae77..344edcb2addd 100644
--- a/fs/proc/kcore.c
+++ b/fs/proc/kcore.c
@@ -316,6 +316,7 @@ read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
 	size_t page_offline_frozen = 1;
 	size_t phdrs_len, notes_len;
 	struct kcore_list *m;
+	struct kcore_list *iter;
 	size_t tsz;
 	int nphdr;
 	unsigned long start;
@@ -479,10 +480,13 @@ 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)
+			m = NULL;
+			list_for_each_entry(iter, &kclist_head, list) {
+				if (start >= iter->addr &&
+				    start < iter->addr + iter->size) {
+					m = iter;
 					break;
+				}
 			}
 		}
 
@@ -492,12 +496,11 @@ 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;
 		}
 

base-commit: f82da161ea75dc4db21b2499e4b1facd36dab275
-- 
2.25.1