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

[Cengiz Can]

From: Laszlo Ersek <lersek at redhat.com>

Commit 66b2c338adce initializes the "sk_uid" field in the protocol socket
(struct sock) from the "/dev/tapX" 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 "tap", the action that creates the socket is
open("/dev/tapX").

Therefore the device node's owner UID is irrelevant. In most cases,
"/dev/tapX" will be owned by root, so in practice, commit 66b2c338adce 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/tapX" 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: 66b2c338adce ("tap: tap_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 5c9241f3ceab3257abe2923a59950db0dc8bb737)
CVE-2023-4194
Signed-off-by: Cengiz Can <cengiz.can at canonical.com>
---
 drivers/net/tap.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/tap.c b/drivers/net/tap.c
index 574c17aa4b09..c299faaf4b2d 100644
--- a/drivers/net/tap.c
+++ b/drivers/net/tap.c
@@ -525,7 +525,7 @@ static int tap_open(struct inode *inode, struct file *file)
 	q->sock.state = SS_CONNECTED;
 	q->sock.file = file;
 	q->sock.ops = &tap_socket_ops;
-	sock_init_data_uid(&q->sock, &q->sk, inode->i_uid);
+	sock_init_data_uid(&q->sock, &q->sk, current_fsuid());
 	q->sk.sk_write_space = tap_sock_write_space;
 	q->sk.sk_destruct = tap_sock_destruct;
 	q->flags = IFF_VNET_HDR | IFF_NO_PI | IFF_TAP;
-- 
2.39.2