linux-kernel - Re: [RESEND PATCH v3 2/2] sysctl: handle overflow for file-max

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Date:   Thu, 10 Jan 2019 16:00:52 +0100
From:   Christian Brauner <christian@...uner.io>
To:     Dominik Brodowski <linux@...inikbrodowski.net>
Cc:     akpm@...ux-foundation.org, keescook@...omium.org,
        linux-kernel@...r.kernel.org, ebiederm@...ssion.com,
        mcgrof@...nel.org, joe.lawrence@...hat.com, longman@...hat.com,
        viro@...iv.linux.org.uk, adobriyan@...il.com,
        linux-api@...r.kernel.org
Subject: Re: [RESEND PATCH v3 2/2] sysctl: handle overflow for file-max

On Thu, Jan 10, 2019 at 03:55:59PM +0100, Dominik Brodowski wrote:
> On Thu, Jan 10, 2019 at 03:50:05PM +0100, Christian Brauner wrote:
> > On Tue, Jan 08, 2019 at 08:01:10AM +0100, Dominik Brodowski wrote:
> > > On Mon, Jan 07, 2019 at 11:27:00PM +0100, Christian Brauner wrote:
> > > > @@ -2833,6 +2836,10 @@ static int __do_proc_doulongvec_minmax(void *data, struct ctl_table *table, int
> > > >  				break;
> > > >  			if (neg)
> > > >  				continue;
> > > > +			if ((max && val > *max) || (min && val < *min)) {
> > > > +				err = -EINVAL;
> > > > +				break;
> > > > +			}
> > > >  			val = convmul * val / convdiv;
> > > >  			if ((min && val < *min) || (max && val > *max))
> > > >  				continue;
> > > 
> > > This is a generic change which affects all users of
> > > do_proc_doulongvec_minmax() that have extra1 or extra2 set. In sysctl.c, I
> > > do not see another user of proc_doulongvec_minmax() that has extra1 or
> > > extra2 set. However, have you verified whether your patch changes the
> > > behaviour for other files that make use of proc_doulongvec_minmax() or
> > > proc_doulongvec_ms_jiffies_minmax(), and not only of the file-max sysctl?
> > 
> > Sorry for the delayed reply. I did look at the callers. The functions
> > that are of interest afaict are:
> > 
> > proc_doulongvec_ms_jiffies_minmax
> > proc_doulongvec_minmax
> > 
> > So this could be visible when users write values that would overflow the
> > type used in the kernel.
> >
> > I guess your point is whether we are venturing into userspace break
> > territory. Hm... We should probably make sure that we're not regressing
> > anyone else! What do you think if instead of the above patch we did:
> 
> Hm, I prefer the original patch -- as the same (valid) reasons which apply
> for the file-max sysctl might also apply to other users of this function
> where extra1 and/or2 extra2 are set.

In that case we should erorr out on:

val = convmul * val / convdiv;
if ((min && val < *min) || (max && val > *max)) {
        err = -EINVAL;
        break;
}

I fear that erroring out before might break *_jiffies since they are the
only caller that request a convmul/convdiv value that is not 1l.

Christian