linux-kernel - RE: [char-misc-next 2/8] mei: hbm: validate client index is not exceeding allocated array size

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Message-ID: <5B8DA87D05A7694D9FA63FD143655C1B1AEFB95D@HASMSX106.ger.corp.intel.com>
Date:	Wed, 30 Oct 2013 07:31:06 +0000
From:	"Winkler, Tomas" <tomas.winkler@...el.com>
To:	Greg KH <gregkh@...uxfoundation.org>
CC:	"arnd@...db.de" <arnd@...db.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [char-misc-next 2/8] mei: hbm: validate client index is not
 exceeding allocated array size



> > ---
> >  drivers/misc/mei/hbm.c | 6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/misc/mei/hbm.c b/drivers/misc/mei/hbm.c
> > index 9b3a0fb..0f5e8ca 100644
> > --- a/drivers/misc/mei/hbm.c
> > +++ b/drivers/misc/mei/hbm.c
> > @@ -228,8 +228,6 @@ static int mei_hbm_prop_req(struct mei_device *dev)
> >  	unsigned long client_num;
> >
> >
> > -	client_num = dev->me_client_presentation_num;
> > -
> >  	next_client_index = find_next_bit(dev->me_clients_map,
> MEI_CLIENTS_MAX,
> >  					  dev->me_client_index);
> >
> > @@ -241,6 +239,10 @@ static int mei_hbm_prop_req(struct mei_device *dev)
> >  		return 0;
> >  	}
> >
> > +	client_num = dev->me_client_presentation_num;
> > +	if (WARN_ON(dev->me_clients_num <= client_num))
> > +		return -EIO;
> 
> How can this happen?  Why is spitting out a huge warning in the syslog
> going to help anything?  If a user can do this, then great, now you can
> DoS your syslog :(
> 
> If a user can't do this, then why tell them, it's your driver's bug that
> you should just fix.

This somehow should guard buffer overflow allocated of size dev->me_clients_num
In theory this can happen only if something go wrong in hardware initialization or there is some other security hole that can change client_num.

After this happen you probably won't be able to use the driver anyhow so I do not expect DoS on the syslog, but we can drop the WARN_ON
but I would stick with the check

Thanks
Tomas



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