linux-kernel - [PATCH net-next 2/2] tap: tap_open(): correctly initialize socket uid

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Message-Id: <20230131-tuntap-sk-uid-v1-2-af4f9f40979d@diag.uniroma1.it>
Date:   Wed, 01 Feb 2023 00:35:46 +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 2/2] tap: tap_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, tap_open() passes a `struct socket` embedded in a `struct
tap_queue` 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
padding bytes between `int vnet_hdr_sz` and `struct tap_dev __rcu
*tap` in `struct tap_queue`, which makes the uid of all tap 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/tap.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/net/tap.c b/drivers/net/tap.c
index a2be1994b389..9a287363d695 100644
--- a/drivers/net/tap.c
+++ b/drivers/net/tap.c
@@ -538,6 +538,10 @@ static int tap_open(struct inode *inode, struct file *file)
 	q->sk.sk_destruct = tap_sock_destruct;
 	q->flags = IFF_VNET_HDR | IFF_NO_PI | IFF_TAP;
 	q->vnet_hdr_sz = sizeof(struct virtio_net_hdr);
+	// sock_init_data initializes sk.sk_uid assuming q->sock is embedded in a
+	// struct socket_alloc and reading its corresponding inode. Since we pass a
+	// socket contained in a struct tap_queue we have to fix this manually
+	q->sk.sk_uid = inode->i_uid;

 	/*
 	 * so far only KVM virtio_net uses tap, enable zero copy between

-- 
2.25.1