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() and tun_chr_open() pass a `struct socket` embedded
in a `struct tap_queue` and `struct tun_file` respectively, both
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.

Due to the type confusion, both sockets happen to have their uid set
to 0, i.e. root.
While it will be often correct, as tuntap devices require
CAP_NET_ADMIN, it may not always be the case.
Not sure how widespread is the impact of this, it seems the socket uid
may be used for network filtering and routing, thus tuntap sockets may
be incorrectly managed.
Additionally, it seems a socket with an incorrect uid may be returned
to the vhost driver when issuing a get_socket() on a tuntap device in
vhost_net_set_backend().

Fix the bugs by adding and using sock_init_data_uid(), which 
explicitly takes a uid as argument.

Signed-off-by: Pietro Borrello <borrello@diag.uniroma1.it>
---
Changes in v3:
- Fix the bug by defining and using sock_init_data_uid()
- Link to v2: https://lore.kernel.org/r/20230131-tuntap-sk-uid-v2-0-29ec15592813@diag.uniroma1.it

Changes in v2:
- Shorten and format comments
- Link to v1: https://lore.kernel.org/r/20230131-tuntap-sk-uid-v1-0-af4f9f40979d@diag.uniroma1.it

---
Pietro Borrello (3):
      net: add sock_init_data_uid()
      tun: tun_chr_open(): correctly initialize socket uid
      tap: tap_open(): correctly initialize socket uid

 drivers/net/tap.c  |  2 +-
 drivers/net/tun.c  |  2 +-
 include/net/sock.h |  7 ++++++-
 net/core/sock.c    | 15 ++++++++++++---
 4 files changed, 20 insertions(+), 6 deletions(-)
---
base-commit: 6d796c50f84ca79f1722bb131799e5a5710c4700
change-id: 20230131-tuntap-sk-uid-78efc80f4b82

Best regards,
-- 
Pietro Borrello <borrello@diag.uniroma1.it>

Hello:

This series was applied to netdev/net-next.git (master)
by David S. Miller <davem@davemloft.net>:

On Sat, 04 Feb 2023 17:39:19 +0000 you wrote:
> 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() and tun_chr_open() pass a `struct socket` embedded
> in a `struct tap_queue` and `struct tun_file` respectively, both
> 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.
> 
> [...]

Here is the summary with links:
  - [net-next,v3,1/3] net: add sock_init_data_uid()
    https://git.kernel.org/netdev/net-next/c/584f3742890e
  - [net-next,v3,2/3] tun: tun_chr_open(): correctly initialize socket uid
    https://git.kernel.org/netdev/net-next/c/a096ccca6e50
  - [net-next,v3,3/3] tap: tap_open(): correctly initialize socket uid
    https://git.kernel.org/netdev/net-next/c/66b2c338adce

You are awesome, thank you!
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