linux-kernel - Re: [PATCH] mptsas: fix undefined behaviour of a shift of an int by more than 31 places

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Message-ID: <1557416545.4268.22.camel@HansenPartnership.com>
Date:   Thu, 09 May 2019 08:42:25 -0700
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     Hannes Reinecke <hare@...e.de>,
        Colin Ian King <colin.king@...onical.com>,
        Sathya Prakash <sathya.prakash@...adcom.com>,
        Chaitra P B <chaitra.basappa@...adcom.com>,
        Suganath Prabu Subramani 
        <suganath-prabu.subramani@...adcom.com>,
        MPT-FusionLinux.pdl@...adcom.com, linux-scsi@...r.kernel.org
Cc:     kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mptsas: fix undefined behaviour of a shift of an int by
 more than 31 places

On Thu, 2019-05-09 at 17:30 +0200, Hannes Reinecke wrote:
> On 5/8/19 4:24 PM, James Bottomley wrote:
> > On Wed, 2019-05-08 at 14:07 +0100, Colin Ian King wrote:
> > > On 05/05/2019 04:34, James Bottomley wrote:
> > > > On Sat, 2019-05-04 at 17:40 +0100, Colin King wrote:
> > > > > From: Colin Ian King <colin.king@...onical.com>
> > > > > 
> > > > > Currently the shift of int value 1 by more than 31 places can
> > > > > result in undefined behaviour. Fix this by making the 1 a ULL
> > > > > value before the shift operation.
> > > > 
> > > > Fusion SAS is pretty ancient.  I thought the largest one ever
> > > > produced had four phys, so how did you produce the overflow?
> > > 
> > > This was an issue found by static analysis with Coverity; so I
> > > guess won't happen in the wild, in which case the patch could be
> > > ignored.
> > 
> > The point I was more making is that if we thought this could ever
> > happen in practice, we'd need more error handling than simply this:
> > we'd be setting the phy_bitmap to zero which would be every bit as
> > bad as some random illegal value.
> > 
> 
> Thing is, mptsas is used as the default emulation in VMWare, and
> that does allow you to do some pretty weird configurations (I've
> found myself  fixing a bug with SATA hotplug on mptsas once ...).
> So I wouldn't discard this issue out of hand.

I'm not, I'm just saying the proposed fix is no fix at all since it
would just produce undefined behaviour in the driver.  I thought the
issue might have been coming from VMWare, which is why I asked how the
bug was seen.  The proper fix is probably to fail attachment if the phy
number goes over a fixed value (16 sounds reasonable) but if it's never
a problem in the field, I'm happy doing nothing because we have no real
idea what the reasonable value is.

James