netdev - Re: [PATCH net 1/1] net_sched: sch_fq: Fix out of range band computation

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Date: Wed, 13 Dec 2023 19:04:42 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Jamal Hadi Salim <jhs@...atatu.com>
Cc: Willem de Bruijn <willemb@...gle.com>, davem@...emloft.net, kuba@...nel.org, 
	pabeni@...hat.com, jiri@...nulli.us, xiyou.wangcong@...il.com, 
	netdev@...r.kernel.org, pctammela@...atatu.com, victor@...atatu.com, 
	Coverity Scan <scan-admin@...erity.com>
Subject: Re: [PATCH net 1/1] net_sched: sch_fq: Fix out of range band computation

On Wed, Dec 13, 2023 at 6:53 PM Jamal Hadi Salim <jhs@...atatu.com> wrote:
>
> On Wed, Dec 13, 2023 at 12:42 PM Eric Dumazet <edumazet@...gle.com> wrote:
> >
> > On Wed, Dec 13, 2023 at 6:29 PM Eric Dumazet <edumazet@...gle.com> wrote:
> > >
> > > On Wed, Dec 13, 2023 at 5:57 PM Jamal Hadi Salim <jhs@...atatu.com> wrote:
> > > >
> > > > It is possible to compute a band of 3. Doing so will overrun array
> > > > q->band_pkt_count[0-2] boundaries.
> > > >
> > > > Fixes: 29f834aa326e ("net_sched: sch_fq: add 3 bands and WRR scheduling")
> > > > Reported-by: Coverity Scan <scan-admin@...erity.com>
> > > > Signed-off-by: Jamal Hadi Salim <jhs@...atatu.com>
> > > > ---
> > > >  net/sched/sch_fq.c | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > >
> > > > diff --git a/net/sched/sch_fq.c b/net/sched/sch_fq.c
> > > > index 3a31c47fea9b..217c430343df 100644
> > > > --- a/net/sched/sch_fq.c
> > > > +++ b/net/sched/sch_fq.c
> > > > @@ -159,7 +159,7 @@ struct fq_sched_data {
> > > >  /* return the i-th 2-bit value ("crumb") */
> > > >  static u8 fq_prio2band(const u8 *prio2band, unsigned int prio)
> > > >  {
> > > > -       return (prio2band[prio / 4] >> (2 * (prio & 0x3))) & 0x3;
> > > > +       return (prio2band[prio / 4] >> (2 * (prio & 0x3))) % 0x3;
> > > >  }
> > > >
> > >
> > > Are you sure this is needed ?
> > >
> > > fq_load_priomap() makes sure this can not happen...
> >
> > Yeah, I am pretty sure this patch is incorrect, we need to mask to get
> > only two bits.
>
> The check in fq_load_priomap() is what makes it moot. Masking with
> b'11 could result in b'11. Definitely the modulo will guarantee
> whatever results can only be in the range 0..2. But it is not needed.
>
>


Modulo would be incorrect, since it would use high order bits.

(0x22 % 3) is different than (0x22 & 3)

Had you written:

return ((prio2band[prio / 4] >> (2 * (prio & 0x3))) & 0x3) % 3)

Then yes, the last % 3 would be "not needed"