netdev - [PATCH net] llc: fix data loss when reading from a socket in llc_ui

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Message-ID: <20250515122014.1475447-1-Ilia.Gavrilov@infotecs.ru>
Date: Thu, 15 May 2025 12:20:15 +0000
From: Ilia Gavrilov <Ilia.Gavrilov@...otecs.ru>
To: "David S. Miller" <davem@...emloft.net>
CC: Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, "Michal
 Luczaj" <mhal@...x.co>, Arnaldo Carvalho de Melo <acme@...driva.com>,
	"Stephen Hemminger" <stephen@...workplumber.org>, "netdev@...r.kernel.org"
	<netdev@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "lvc-project@...uxtesting.org"
	<lvc-project@...uxtesting.org>, "stable@...r.kernel.org"
	<stable@...r.kernel.org>
Subject: [PATCH net] llc: fix data loss when reading from a socket in
 llc_ui_recvmsg()

For SOCK_STREAM sockets, if user buffer size (len) is less
than skb size (skb->len), the remaining data from skb
will be lost after calling kfree_skb().

To fix this, move the statement for partial reading
above skb deletion.

Found by InfoTeCS on behalf of Linux Verification Center (linuxtesting.org)

Fixes: 30a584d944fb ("[LLX]: SOCK_DGRAM interface fixes")
Cc: stable@...r.kernel.org
Signed-off-by: Ilia Gavrilov <Ilia.Gavrilov@...otecs.ru>
---
 net/llc/af_llc.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/net/llc/af_llc.c b/net/llc/af_llc.c
index 0259cde394ba..cc77ec5769d8 100644
--- a/net/llc/af_llc.c
+++ b/net/llc/af_llc.c
@@ -887,15 +887,15 @@ static int llc_ui_recvmsg(struct socket *sock, struct msghdr *msg, size_t len,
 		if (sk->sk_type != SOCK_STREAM)
 			goto copy_uaddr;
 
+		/* Partial read */
+		if (used + offset < skb_len)
+			continue;
+
 		if (!(flags & MSG_PEEK)) {
 			skb_unlink(skb, &sk->sk_receive_queue);
 			kfree_skb(skb);
 			*seq = 0;
 		}
-
-		/* Partial read */
-		if (used + offset < skb_len)
-			continue;
 	} while (len > 0);
 
 out:
-- 
2.39.5