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Date:   Thu, 13 May 2021 12:25:44 +0200
From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     Juergen Gross <jgross@suse.com>
Cc:     xen-devel@lists.xenproject.org, linuxppc-dev@lists.ozlabs.org,
        linux-kernel@vger.kernel.org, Jiri Slaby <jirislaby@kernel.org>
Subject: Re: [PATCH 8/8] xen/hvc: replace BUG_ON() with negative return value
Message-ID: <YJz+qK8snI64/TKh@kroah.com>
References: <20210513100302.22027-1-jgross@suse.com>
 <20210513100302.22027-9-jgross@suse.com>
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On Thu, May 13, 2021 at 12:03:02PM +0200, Juergen Gross wrote:
> Xen frontends shouldn't BUG() in case of illegal data received from
> their backends. So replace the BUG_ON()s when reading illegal data from
> the ring page with negative return values.
> 
> Signed-off-by: Juergen Gross <jgross@suse.com>
> ---
>  drivers/tty/hvc/hvc_xen.c | 15 +++++++++++++--
>  1 file changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/tty/hvc/hvc_xen.c b/drivers/tty/hvc/hvc_xen.c
> index 92c9a476defc..30d7ffb1e04c 100644
> --- a/drivers/tty/hvc/hvc_xen.c
> +++ b/drivers/tty/hvc/hvc_xen.c
> @@ -86,6 +86,11 @@ static int __write_console(struct xencons_info *xencons,
>  	cons = intf->out_cons;
>  	prod = intf->out_prod;
>  	mb();			/* update queue values before going on */
> +
> +	if (WARN_ONCE((prod - cons) > sizeof(intf->out),
> +		      "Illegal ring page indices"))
> +		return -EINVAL;

How nice, you just rebooted on panic-on-warn systems :(

> +
>  	BUG_ON((prod - cons) > sizeof(intf->out));

Why keep this line?

Please just fix this up properly, if userspace can trigger this, then
both the WARN_ON() and BUG_ON() are not correct and need to be correctly
handled.


>  
>  	while ((sent < len) && ((prod - cons) < sizeof(intf->out)))
> @@ -114,7 +119,10 @@ static int domU_write_console(uint32_t vtermno, const char *data, int len)
>  	 */
>  	while (len) {
>  		int sent = __write_console(cons, data, len);
> -		
> +
> +		if (sent < 0)
> +			return sent;
> +
>  		data += sent;
>  		len -= sent;
>  
> @@ -138,7 +146,10 @@ static int domU_read_console(uint32_t vtermno, char *buf, int len)
>  	cons = intf->in_cons;
>  	prod = intf->in_prod;
>  	mb();			/* get pointers before reading ring */
> -	BUG_ON((prod - cons) > sizeof(intf->in));
> +
> +	if (WARN_ONCE((prod - cons) > sizeof(intf->in),
> +		      "Illegal ring page indices"))
> +		return -EINVAL;

Same here, you still just paniced a machine :(

thanks,

greg k-h