linux-kernel - Re: [PATCH v2 3/5] nvme-fabrics: introduce ref counting for nvmf_ctrl

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Date: Mon, 11 Mar 2024 20:28:58 +0100
From: Hannes Reinecke <hare@...e.de>
To: Daniel Wagner <dwagner@...e.de>, Sagi Grimberg <sagi@...mberg.me>
Cc: James Smart <james.smart@...adcom.com>, Keith Busch <kbusch@...nel.org>,
 Christoph Hellwig <hch@....de>, linux-nvme@...ts.infradead.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/5] nvme-fabrics: introduce ref counting for
 nvmf_ctrl_options

On 3/11/24 18:36, Daniel Wagner wrote:
> On Thu, Mar 07, 2024 at 12:27:43PM +0200, Sagi Grimberg wrote:
>> Why do we need a refcount for an object that has the same exact lifetime
>> as the ctrl itself? It just feels like unneeded complication.
> 
> My claim the UAF is also possible with the current code is not correct.
> Or at least not easy to reproduce. I've re-tested a lot and I couldn't
> reproduce it.
> 
> Though, the UAF is very simple to reproduce with the sync connect patch
> applied (nvme-fc: wait for initial connect attempt to finish) together
> with Hannes' patch (nvme: authentication error are always
> non-retryable):
> 
> In this case, the initial connect fails and the resources are removed,
> while we are waiting in
> 
> +       if (!opts->connect_async) {
> +               enum nvme_ctrl_state state;
> +
> +               wait_for_completion(&ctrl->connect_completion);
> +               state = nvme_ctrl_state(&ctrl->ctrl);
> +               nvme_fc_ctrl_put(ctrl);
> +
> +               if (state != NVME_CTRL_LIVE) {
> +                       /* Cleanup is handled by the connect state machine */
> +                       return ERR_PTR(-EIO);
> +               }
> +       }
> 
> This opens up the race window. While we are waiting here for the
> completion, the ctrl entry in sysfs is still reachable. Unfortunately,
> we also fire an uevent which starts another instance of nvme-cli. And
> the new instance of nvme-cli iterates over sysfs and reads the already
> freed options object.
> 
Curiously enough, I had been digging into better error reporting for 
nvme-fabrics. And the one thing I came up with is to make the controller
_options_ as a private pointer to seq_file.
With that we can allocate and initialize the options during open(),
and then have write() do the parsing and calling create_ctrl() as usual.
But read() would then always have access to the option structure, and
we can use this structure to pass any errors. EG parsing errors could
be reported by an 'err_mask' field and so on.

That would allow us to report errors back to nvme-cli, and,
incidentally, also require reference counting.
Two stones with a bird and all that.

Patch is in testing, and I'll be posting it once I get confirmation.


Cheers,

Hannes
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@...e.de                                +49 911 74053 688
SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg
HRB 36809 (AG Nürnberg), GF: I. Totev, A. McDonald, W. Knoblich