linux-kernel - [PATCH net-next 1/2] tun: tun_chr_open(): correctly initialize socket uid

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Message-Id: <20230131-tuntap-sk-uid-v1-1-af4f9f40979d@diag.uniroma1.it>
Date:   Wed, 01 Feb 2023 00:35:45 +0000
From:   Pietro Borrello <borrello@...g.uniroma1.it>
To:     "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Lorenzo Colitti <lorenzo@...gle.com>
Cc:     Cristiano Giuffrida <c.giuffrida@...nl>,
        "Bos, H.J." <h.j.bos@...nl>, Jakob Koschel <jkl820.git@...il.com>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Pietro Borrello <borrello@...g.uniroma1.it>
Subject: [PATCH net-next 1/2] tun: tun_chr_open(): correctly initialize socket uid

sock_init_data() assumes that the `struct socket` passed in input is
contained in a `struct socket_alloc` allocated with sock_alloc().
However, tun_chr_open() passes a `struct socket` embedded in a `struct
tun_file` allocated with sk_alloc().
This causes a type confusion when issuing a container_of() with
SOCK_INODE() in sock_init_data() which results in assigning a wrong
sk_uid to the `struct sock` in input.
On default configuration, the type confused field overlaps with the
high 4 bytes of `struct tun_struct __rcu *tun` of `struct tun_file`,
NULL at the time of call, which makes the uid of all tun sockets 0,
i.e., the root one.  Fix the assignment by overriding it with the
correct uid.

Fixes: 86741ec25462 ("net: core: Add a UID field to struct sock.")
Signed-off-by: Pietro Borrello <borrello@...g.uniroma1.it>
---
 drivers/net/tun.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index a7d17c680f4a..6713fffb1488 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -3450,6 +3450,11 @@ static int tun_chr_open(struct inode *inode, struct file * file)

 	sock_init_data(&tfile->socket, &tfile->sk);

+	// sock_init_data initializes sk.sk_uid assuming tfile->socket is embedded
+	// in a struct socket_alloc and reading its corresponding inode. Since we
+	// pass a socket contained in a struct tun_file we have to fix this manually
+	tfile->sk.sk_uid = inode->i_uid;
+
 	tfile->sk.sk_write_space = tun_sock_write_space;
 	tfile->sk.sk_sndbuf = INT_MAX;

-- 
2.25.1