linux-kernel - Re: [syzbot] KASAN: use-after-free Read in addr

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Date:   Tue, 5 Oct 2021 09:23:41 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Hillf Danton <hdanton@...a.com>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        syzbot <syzbot+dc3dfba010d7671e05f5@...kaller.appspotmail.com>,
        dledford@...hat.com, leon@...nel.org, linux-kernel@...r.kernel.org,
        linux-rdma@...r.kernel.org, syzkaller-bugs@...glegroups.com,
        Aleksandr Nogikh <nogikh@...gle.com>
Subject: Re: [syzbot] KASAN: use-after-free Read in addr_handler (4)

On Tue, Oct 05, 2021 at 11:29:01AM +0800, Hillf Danton wrote:
> On Mon, 20 Sep 2021 10:13:10 +0200 Dmitry Vyukov wrote:
> >On Thu, 16 Sept 2021 at 18:28, Jason Gunthorpe <jgg@...pe.ca> wrote:
> >>
> >> On Thu, Sep 16, 2021 at 04:45:27PM +0200, Dmitry Vyukov wrote:
> >>
> >> > Answering your question re what was running concurrently with what.
> >> > Each of the syscalls in these programs can run up to 2 times and
> >> > ultimately any of these calls can race with any. Potentially syzkaller
> >> > can predict values kernel will return (e.g. id's) before kernel
> >> > actually returned them. I guess this does not restrict search area for
> >> > the bug a lot...
> >>
> >> I have a reasonable theory now..
> >>
> >> Based on the ops you provided this FSM sequence is possible
> >>
> >> RDMA_USER_CM_CMD_RESOLVE_IP
> >>   RDMA_CM_IDLE -> RDMA_CM_ADDR_QUERY
> >>   does rdma_resolve_ip(addr_handler)
> >>
> >>                           addr_handler
> >>                             RDMA_CM_ADDR_QUERY -> RDMA_CM_ADDR_BOUND
> >>                             [.. handler still running ..]
> >>
> >> RDMA_USER_CM_CMD_RESOLVE_IP
> >>   RDMA_CM_ADDR_BOUND -> RDMA_CM_ADDR_QUERY
> >>   does rdma_resolve_ip(addr_handler)
> >>
> >> RDMA_DESTROY_ID
> >>   rdma_addr_cancel()
> >>
> >> Which, if it happens fast enough, could trigger a situation where the
> >> '&id_priv->id.route.addr.dev_addr' "handle" is in the req_list twice
> >> beacause the addr_handler work queue hasn't yet got to the point of
> >> deleting it from the req_list before the the 2nd one is added.
> >>
> >> The issue is rdma_addr_cancel() has to be called rdma_resolve_ip() can
> >> be called again.
> >>
> >> Skipping it will cause 'req_list' to have two items in the internal
> >> linked list with the same key and it will not cancel the newest one
> >> with the active timer. This would cause the use after free syndrome
> >> like this trace is showing.
> >>
> >> I can make a patch, but have no way to know if it is any good :\
> >
> >Good detective work!
> >
> >But if you have a theory of what happens, it's usually easy to write a
> >reproducer that aims at triggering this exact scenario.
> 
> Greate to know the gadgets on the syzkaller side!
> 
> In the scenario derived from the log of 2ee9bf346fbf
> ("RDMA/addr: Fix race with netevent_callback()/rdma_addr_cancel()"),
> 
>  CPU1			CPU2			CPU3
>  netevent_callback()	rdma_addr_cancel()	process_one_req()
>  
>  spin_lock_bh()
>  set_timeout()					req->callback()
>    mod_delayed_work(addr_wq,
>            &req->work, delay);
>  spin_unlock_bh()
>  			spin_lock_bh()
>  			list_del_init(&req->list)
>  			spin_unlock_bh()
>  			cancel_delayed_work_sync(&req->work)
>  			kfree(req)
>  						req->callback = NULL
>  
> the chance for uaf on CPU3 is not zero, given that canceling of the requeued
> work will not wait for the worker running the callback to complete.

I'm not sure what you are trying to say

Jason