linux-kernel - Re: dm snap : add sanity checks to snapshot

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Message-ID: <20210104202749.GA3721@redhat.com>
Date:   Mon, 4 Jan 2021 15:27:50 -0500
From:   Mike Snitzer <snitzer@...hat.com>
To:     Defang Bo <bodefang@....com>
Cc:     agk@...hat.com, linux-kernel@...r.kernel.org, dm-devel@...hat.com
Subject: Re: dm snap : add sanity checks to snapshot_ctr

On Fri, Dec 25 2020 at  1:48am -0500,
Defang Bo <bodefang@....com> wrote:

> Similar to commit<70de2cbd>,there should be a check for argc and argv to prevent Null pointer dereferencing
> when the dm_get_device invoked twice on the same device path with differnt mode.
> 
> Signed-off-by: Defang Bo <bodefang@....com>
> ---
>  drivers/md/dm-snap.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/drivers/md/dm-snap.c b/drivers/md/dm-snap.c
> index 4668b2c..dccce8b 100644
> --- a/drivers/md/dm-snap.c
> +++ b/drivers/md/dm-snap.c
> @@ -1258,6 +1258,13 @@ static int snapshot_ctr(struct dm_target *ti, unsigned int argc, char **argv)
>  
>  	as.argc = argc;
>  	as.argv = argv;
> +
> +	if (!strcmp(argv[0], argv[1])) {
> +		ti->error = "Error setting metadata or data device";
> +		r = -EINVAL;
> +		goto bad;
> +	}
> +
>  	dm_consume_args(&as, 4);
>  	r = parse_snapshot_features(&as, s, ti);
>  	if (r)
> -- 
> 2.7.4
> 

We already have this later in snapshot_ctr:

        if (cow_dev && cow_dev == origin_dev) {
                ti->error = "COW device cannot be the same as origin device";
                r = -EINVAL;
                goto bad_cow;
        }

Which happens before the 2nd dm_get_device() for the cow device.  So
I'm not seeing how you could experience the NULL pointer you say is
possible.

Mike