From: Winston Wen <wentao@uniontech.com>
Date: Mon, 26 Jun 2023 11:42:55 +0800
Subject: [PATCH] cifs: fix session state transition to avoid use-after-free
 issue
Git-commit: ff7d80a9f2711bf3d9fe1cfb70b3fd15c50584b7
References: bsc#1193629
Patch-mainline: v6.5-rc1

We switch session state to SES_EXITING without cifs_tcp_ses_lock now,
it may lead to potential use-after-free issue.

Consider the following execution processes:

Thread 1:
__cifs_put_smb_ses()
    spin_lock(&cifs_tcp_ses_lock)
    if (--ses->ses_count > 0)
        spin_unlock(&cifs_tcp_ses_lock)
        return
    spin_unlock(&cifs_tcp_ses_lock)
        ---> **GAP**
    spin_lock(&ses->ses_lock)
    if (ses->ses_status == SES_GOOD)
        ses->ses_status = SES_EXITING
    spin_unlock(&ses->ses_lock)

Thread 2:
cifs_find_smb_ses()
    spin_lock(&cifs_tcp_ses_lock)
    list_for_each_entry(ses, ...)
        spin_lock(&ses->ses_lock)
        if (ses->ses_status == SES_EXITING)
            spin_unlock(&ses->ses_lock)
            continue
        ...
        spin_unlock(&ses->ses_lock)
    if (ret)
        cifs_smb_ses_inc_refcount(ret)
    spin_unlock(&cifs_tcp_ses_lock)

If thread 1 is preempted in the gap and thread 2 start executing, thread 2
will get the session, and soon thread 1 will switch the session state to
SES_EXITING and start releasing it, even though thread 1 had increased the
session's refcount and still uses it.

So switch session state under cifs_tcp_ses_lock to eliminate this gap.

Signed-off-by: Winston Wen <wentao@uniontech.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Acked-by: Paulo Alcantara <palcantara@suse.de>
---
 fs/smb/client/connect.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/fs/smb/client/connect.c b/fs/smb/client/connect.c
index dab7bc876507..85dd1b373974 100644
--- a/fs/smb/client/connect.c
+++ b/fs/smb/client/connect.c
@@ -1967,15 +1967,16 @@ void __cifs_put_smb_ses(struct cifs_ses *ses)
 		spin_unlock(&cifs_tcp_ses_lock);
 		return;
 	}
-	spin_unlock(&cifs_tcp_ses_lock);
-
-	/* ses_count can never go negative */
-	WARN_ON(ses->ses_count < 0);
-
 	spin_lock(&ses->ses_lock);
 	if (ses->ses_status == SES_GOOD)
 		ses->ses_status = SES_EXITING;
+	spin_unlock(&ses->ses_lock);
+	spin_unlock(&cifs_tcp_ses_lock);
 
+	/* ses_count can never go negative */
+	WARN_ON(ses->ses_count < 0);
+
+	spin_lock(&ses->ses_lock);
 	if (ses->ses_status == SES_EXITING && server->ops->logoff) {
 		spin_unlock(&ses->ses_lock);
 		cifs_free_ipc(ses);
-- 
2.41.0