linux-kernel - Re: [PATCH 8/8] xen/hvc: replace BUG

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Date:   Thu, 13 May 2021 12:35:19 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     xen-devel@...ts.xenproject.org, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, Jiri Slaby <jirislaby@...nel.org>
Subject: Re: [PATCH 8/8] xen/hvc: replace BUG_ON() with negative return value

On 13.05.21 12:25, Greg Kroah-Hartman wrote:
> 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@...e.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?

Failed to delete it, sorry.

> 
> 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.

It can be triggered by the console backend, but I agree a WARN isn't the
way to go here.


Juergen

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