linux-kernel - [PATCH 2/2] cifs: remove check of list iterator against head past the loop body

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Date:   Thu, 31 Mar 2022 23:55:41 +0200
From:   Jakob Koschel <jakobkoschel@...il.com>
To:     Steve French <sfrench@...ba.org>
Cc:     linux-cifs@...r.kernel.org, samba-technical@...ts.samba.org,
        linux-kernel@...r.kernel.org, Mike Rapoport <rppt@...nel.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 2/2] cifs: 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/cifs/smb2misc.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/fs/cifs/smb2misc.c b/fs/cifs/smb2misc.c
index b25623e3fe3d..2d862291fab9 100644
--- a/fs/cifs/smb2misc.c
+++ b/fs/cifs/smb2misc.c
@@ -150,16 +150,18 @@ smb2_check_message(char *buf, unsigned int len, struct TCP_Server_Info *srvr)
 		struct smb2_transform_hdr *thdr =
 			(struct smb2_transform_hdr *)buf;
 		struct cifs_ses *ses = NULL;
+		struct cifs_ses *iter;
 
 		/* decrypt frame now that it is completely read in */
 		spin_lock(&cifs_tcp_ses_lock);
-		list_for_each_entry(ses, &srvr->smb_ses_list, smb_ses_list) {
-			if (ses->Suid == le64_to_cpu(thdr->SessionId))
+		list_for_each_entry(iter, &srvr->smb_ses_list, smb_ses_list) {
+			if (iter->Suid == le64_to_cpu(thdr->SessionId)) {
+				ses = iter;
 				break;
+			}
 		}
 		spin_unlock(&cifs_tcp_ses_lock);
-		if (list_entry_is_head(ses, &srvr->smb_ses_list,
-				       smb_ses_list)) {
+		if (!ses) {
 			cifs_dbg(VFS, "no decryption - session id not found\n");
 			return 1;
 		}
-- 
2.25.1