netdev - [PATCH 5.4 127/154] net: tun_chr_open(): set sk_uid from current

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Message-ID: <20230809103641.099103532@linuxfoundation.org>
Date: Wed,  9 Aug 2023 12:42:38 +0200
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: stable@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	patches@...ts.linux.dev,
	Eric Dumazet <edumazet@...gle.com>,
	Lorenzo Colitti <lorenzo@...gle.com>,
	Paolo Abeni <pabeni@...hat.com>,
	Pietro Borrello <borrello@...g.uniroma1.it>,
	netdev@...r.kernel.org,
	Laszlo Ersek <lersek@...hat.com>,
	"David S. Miller" <davem@...emloft.net>
Subject: [PATCH 5.4 127/154] net: tun_chr_open(): set sk_uid from current_fsuid()

From: Laszlo Ersek <lersek@...hat.com>

commit 9bc3047374d5bec163e83e743709e23753376f0c upstream.

Commit a096ccca6e50 initializes the "sk_uid" field in the protocol socket
(struct sock) from the "/dev/net/tun" device node's owner UID. Per
original commit 86741ec25462 ("net: core: Add a UID field to struct
sock.", 2016-11-04), that's wrong: the idea is to cache the UID of the
userspace process that creates the socket. Commit 86741ec25462 mentions
socket() and accept(); with "tun", the action that creates the socket is
open("/dev/net/tun").

Therefore the device node's owner UID is irrelevant. In most cases,
"/dev/net/tun" will be owned by root, so in practice, commit a096ccca6e50
has no observable effect:

- before, "sk_uid" would be zero, due to undefined behavior
  (CVE-2023-1076),

- after, "sk_uid" would be zero, due to "/dev/net/tun" being owned by root.

What matters is the (fs)UID of the process performing the open(), so cache
that in "sk_uid".

Cc: Eric Dumazet <edumazet@...gle.com>
Cc: Lorenzo Colitti <lorenzo@...gle.com>
Cc: Paolo Abeni <pabeni@...hat.com>
Cc: Pietro Borrello <borrello@...g.uniroma1.it>
Cc: netdev@...r.kernel.org
Cc: stable@...r.kernel.org
Fixes: a096ccca6e50 ("tun: tun_chr_open(): correctly initialize socket uid")
Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2173435
Signed-off-by: Laszlo Ersek <lersek@...hat.com>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 drivers/net/tun.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -3534,7 +3534,7 @@ static int tun_chr_open(struct inode *in
 	tfile->socket.file = file;
 	tfile->socket.ops = &tun_socket_ops;
 
-	sock_init_data_uid(&tfile->socket, &tfile->sk, inode->i_uid);
+	sock_init_data_uid(&tfile->socket, &tfile->sk, current_fsuid());
 
 	tfile->sk.sk_write_space = tun_sock_write_space;
 	tfile->sk.sk_sndbuf = INT_MAX;