netdev - [PATCH] net: kcm: use previously opened message only once

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Message-ID: <20240801130833.680962-1-dmantipov@yandex.ru>
Date: Thu,  1 Aug 2024 16:08:33 +0300
From: Dmitry Antipov <dmantipov@...dex.ru>
To: Tom Herbert <tom@...bertland.com>
Cc: Jakub Kicinski <kuba@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>,
	netdev@...r.kernel.org,
	lvc-project@...uxtesting.org,
	Dmitry Antipov <dmantipov@...dex.ru>,
	syzbot+b72d86aa5df17ce74c60@...kaller.appspotmail.com
Subject: [PATCH] net: kcm: use previously opened message only once

When syzkaller reproducer injects 'alloc_skb()' failure at line
817, 'kcm_sendmsg()' may return with partial message saved at
'kcm->seq_skb'. Next call of this function will try to build the
next message starting from the saved one, but should do it only
once. Otherwise a complete mess in skb management causes an
undefined behavior of any kind, including UAFs reported by KASAN.

Fixes: ab7ac4eb9832 ("kcm: Kernel Connection Multiplexor module")
Reported-by: syzbot+b72d86aa5df17ce74c60@...kaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=b72d86aa5df17ce74c60
Signed-off-by: Dmitry Antipov <dmantipov@...dex.ru>
---
 net/kcm/kcmsock.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/kcm/kcmsock.c b/net/kcm/kcmsock.c
index 2f191e50d4fc..fa5ce5c88045 100644
--- a/net/kcm/kcmsock.c
+++ b/net/kcm/kcmsock.c
@@ -766,6 +766,8 @@ static int kcm_sendmsg(struct socket *sock, struct msghdr *msg, size_t len)
 	if (kcm->seq_skb) {
 		/* Previously opened message */
 		head = kcm->seq_skb;
+		/* ...should be used only once */
+		kcm->seq_skb = NULL;
 		skb = kcm_tx_msg(head)->last_skb;
 		goto start;
 	}
-- 
2.45.2