linux-kernel - Regarding 711f8c3fb3db "Bluetooth: L2CAP: Fix accepting connection request for invalid SPSM"

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Date:   Tue, 6 Dec 2022 14:27:27 +0300
From:   Cengiz Can <cengiz.can@...onical.com>
To:     Luiz Augusto von Dentz <luiz.dentz@...il.com>
Cc:     linux-bluetooth@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, Greg KH <gregkh@...uxfoundation.org>
Subject: Regarding 711f8c3fb3db "Bluetooth: L2CAP: Fix accepting connection
 request for invalid SPSM"

Hello Luiz Augusto,

I'm by no means a bluetooth expert so please bear with me if my
questions sound dumb or pointless.

I'm trying to backport commit 711f8c3fb3db ("Bluetooth: L2CAP: Fix
accepting connection request for invalid SPSM") to v4.15.y and older
stable kernels. (CVE-2022-42896)

According to the changes to `net/bluetooth/l2cap_core.c` there are two
functions that need patching:

* l2cap_le_connect_req
* l2cap_ecred_conn_req

Only the former exists in kernels <= v4.15.y. So I decided to skip

l2cap_ecred_conn_req for older kernels.

Do you think this would be enough to mitigate the issue?

If so, older kernels also lack definitions of L2CAP_CR_LE_BAD_PSM and

L2CAP_PSM_LE_DYN_END.

I see that L2CAP_CR_LE_BAD_PSM is basically the same as
L2CAP_CR_BAD_PSM so I used it to signify an error.

I think it should be enough for the sake of a backport.

What do you think?

Also the range boundary that is defined with L2CAP_PSM_LE_DYN_END is

non-existent in older kernels, and it's hard to decide which value to
use in this expression:

`if (!psm || __le16_to_cpu(psm) > L2CAP_PSM_LE_DYN_END) {`

I can easily define L2CAP_PSM_LE_DYN_END as 0x00FF and call it a day
but I had to ask if we are absolutely sure if that's the right value.

Because the comment block states that it's from the "credit based
connection request" ranges but l2cap_le_connect_req is not credit based.

Is it?

Thank you in advance.

Cengiz Can