Subject: Re: [PATCH bpf] bpf: sockmap, fix skb refcnt race after locking changes

From: Simon Horman <[email-masked]>
Date: Sat, 2 Sep 2023 11:00:32 +0200

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  <[email-masked]>
> Signed-off-by: John Fastabend <[email-masked]>

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