On Fri, Sep 01, 2023 at 01:21:37PM -0700, John Fastabend wrote:
> There is a race where skb's from the sk_psock_backlog can be referenced
> after userspace side has already skb_consumed() the sk_buff and its
> refcnt dropped to zer0 causing use after free.
>
> The flow is the following,
>
> while ((skb = skb_peek(&psock->ingress_skb))
> sk_psock_handle_Skb(psock, skb, ..., ingress)
> if (!ingress) ...
> sk_psock_skb_ingress
> sk_psock_skb_ingress_enqueue(skb)
> msg->skb = skb
> sk_psock_queue_msg(psock, msg)
> skb_dequeue(&psock->ingress_skb)
>
> The sk_psock_queue_msg() puts the msg on the ingress_msg queue. This is
> what the application reads when recvmsg() is called. An application can
> read this anytime after the msg is placed on the queue. The recvmsg
> hook will also read msg->skb and then after user space reads the msg
> will call consume_skb(skb) on it effectively free'ing it.
>
> But, the race is in above where backlog queue still has a reference to
> the skb and calls skb_dequeue(). If the skb_dequeue happens after the
> user reads and free's the skb we have a use after free.
>
> The !ingress case does not suffer from this problem because it uses
> sendmsg_*(sk, msg) which does not pass the sk_buff further down the
> stack.
>
> The following splat was observed with 'test_progs -t sockmap_listen':
>
> [ 1022.710250][ T2556] general protection fault, ...
> ...
> [ 1022.712830][ T2556] Workqueue: events sk_psock_backlog
> [ 1022.713262][ T2556] RIP: 0010:skb_dequeue+0x4c/0x80
> [ 1022.713653][ T2556] Code: ...
> ...
> [ 1022.720699][ T2556] Call Trace:
> [ 1022.720984][ T2556] <TASK>
> [ 1022.721254][ T2556] ? die_addr+0x32/0x80^M
> [ 1022.721589][ T2556] ? exc_general_protection+0x25a/0x4b0
> [ 1022.722026][ T2556] ? asm_exc_general_protection+0x22/0x30
> [ 1022.722489][ T2556] ? skb_dequeue+0x4c/0x80
> [ 1022.722854][ T2556] sk_psock_backlog+0x27a/0x300
> [ 1022.723243][ T2556] process_one_work+0x2a7/0x5b0
> [ 1022.723633][ T2556] worker_thread+0x4f/0x3a0
> [ 1022.723998][ T2556] ? __pfx_worker_thread+0x10/0x10
> [ 1022.724386][ T2556] kthread+0xfd/0x130
> [ 1022.724709][ T2556] ? __pfx_kthread+0x10/0x10
> [ 1022.725066][ T2556] ret_from_fork+0x2d/0x50
> [ 1022.725409][ T2556] ? __pfx_kthread+0x10/0x10
> [ 1022.725799][ T2556] ret_from_fork_asm+0x1b/0x30
> [ 1022.726201][ T2556] </TASK>
>
> To fix we add an skb_get() before passing the skb to be enqueued in
> the engress queue. This bumps the skb->users refcnt so that consume_skb
> and kfree_skb will not immediately free the sk_buff. With this we can
> be sure the skb is still around when we do the dequeue. Then we just
> need to decrement the refcnt or free the skb in the backlog case which
> we do by calling kfree_skb() on the ingress case as well as the sendmsg
> case.
>
> Before locking change from fixes tag we had the sock locked so we
> couldn't race with user and there was no issue here.
>
> Fixes: 799aa7f98d53e (skmsg: Avoid lock_sock() in sk_psock_backlog())
Hi John,
A minor nit from my side.
I think the usual format for a fixes tag is follows.
Fixes: 799aa7f98d53e ("skmsg: Avoid lock_sock() in sk_psock_backlog()")
> Reported-by: Jiri Olsa <jolsa@xxxxxxxxxx>
> Signed-off-by: John Fastabend <john.fastabend@xxxxxxxxx>
...
