[SRU Focal, Jammy, HWE-5.19, OEM-6.0, Lunar 1/2] net: tun_chr_open(): set sk_uid from current_fsuid()

[Cengiz Can]

From: Laszlo Ersek <lersek at redhat.com>

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 at google.com>
Cc: Lorenzo Colitti <lorenzo at google.com>
Cc: Paolo Abeni <pabeni at redhat.com>
Cc: Pietro Borrello <borrello at diag.uniroma1.it>
Cc: netdev at vger.kernel.org
Cc: stable at vger.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 at redhat.com>
Signed-off-by: David S. Miller <davem at davemloft.net>
(cherry picked from commit 9bc3047374d5bec163e83e743709e23753376f0c)
CVE-2023-4194
Signed-off-by: Cengiz Can <cengiz.can at canonical.com>
---
 drivers/net/tun.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index a4e44f98fbc3..309a0dd16bdc 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -3534,7 +3534,7 @@ static int tun_chr_open(struct inode *inode, struct file * file)
 	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;
-- 
2.39.2