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Date: Sat, 19 Jun 2021 13:23:24 -0300
From: Thadeu Lima de Souza Cascardo <>
Cc: Oliver Hartkopp <>,
	Marc Kleine-Budde <>,
	Norbert Slusarek <>
Subject: Re: CVE-2021-3609: Race condition in net/can/bcm.c leads to local
 privilege escalation

On Sat, Jun 19, 2021 at 04:40:53PM +0200, Norbert Slusarek wrote:
> Hello,
> this is an announcement for the recently reported bug (CVE-2021-3609)
> in the CAN BCM networking protocol in the Linux kernel ranging from
> version 2.6.25 to mainline 5.13-rc6.
> The vulnerability is a race condition in net/can/bcm.c allowing for local
> privilege escalation to root. The issue was initially reported by syzbot and
> proven to be exploitable by Norbert Slusarek.
> The CAN BCM networking protocol allows to register a CAN message receiver for a
> specified socket. The function bcm_rx_handler() is run for incoming CAN messages.
> Simultaneously to running this function, the socket can be closed and
> bcm_release() will be called. Inside bcm_release(), struct bcm_op and
> struct bcm_sock are freed while bcm_rx_handler() is still running,
> finally leading to multiple use-after-free's.
> Reproduction
> ------------
> - setup unprivileged user namespace
> - setup vcan network interface
> - open two CAN BCM sockets and connect each to the interface
> - call sendmsg() on socket 1 with RX_SETUP to setup CAN receiver
> - call sendmsg() on socket 2 to send message to socket 1
> Here comes the race condition:
> - bcm_rx_handler() is run automatically for socket 1 to receive the message
> - call close() -> bcm_release() on socket 1 to free struct bcm_op and struct bcm_sock
> => bcm_rx_handler() is still running and will access struct bcm_op and struct
>    bcm_sock which were previously freed
> Exploitation
> ------------
> My exploitation attempt concentrates on kernels with version >= 5.4-rc1
> since commit bf74aa86e111 ("can: bcm: switch timer to HRTIMER_MODE_SOFT and
> remove hrtimer_tasklet"). I didn't investigate into exploiting kernels older
> than 5.4-rc1 which used tasklets, nevertheless exploitation on older kernels
> looks feasible as well. My specific exploitation approach was adjusted to work
> with Ubuntu 20.04.02 LTS but other known distributions could also be targeted.
> More exploitation details can be found at
> or in the attachments (plain text and attached image).
> Regards,
> Norbert Slusarek

And here is the proposed fix:

Thadeu Cascardo.

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