linux-hardening - Re: [PATCH] net: sched: sch: Bounds check priority

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Date:   Sat, 28 Jan 2023 14:43:28 +0100
From:   Simon Horman <simon.horman@...igine.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Jamal Hadi Salim <jhs@...atatu.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jiri Pirko <jiri@...nulli.us>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] net: sched: sch: Bounds check priority

On Fri, Jan 27, 2023 at 02:40:37PM -0800, Kees Cook wrote:
> Nothing was explicitly bounds checking the priority index used to access
> clpriop[]. WARN and bail out early if it's pathological. Seen with GCC 13:
> 
> ../net/sched/sch_htb.c: In function 'htb_activate_prios':
> ../net/sched/sch_htb.c:437:44: warning: array subscript [0, 31] is outside array bounds of 'struct htb_prio[8]' [-Warray-bounds=]
>   437 |                         if (p->inner.clprio[prio].feed.rb_node)
>       |                             ~~~~~~~~~~~~~~~^~~~~~
> ../net/sched/sch_htb.c:131:41: note: while referencing 'clprio'
>   131 |                         struct htb_prio clprio[TC_HTB_NUMPRIO];
>       |                                         ^~~~~~
> 

...

> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
>  net/sched/sch_htb.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)

I'm not sure what will happen if we hit the 'break' case.
But I also think that warning and bailing out is an improvement on whatever
happens now if that scenario is hit.

Reviewed-by: Simon Horman <simon.horman@...igine.com>

> diff --git a/net/sched/sch_htb.c b/net/sched/sch_htb.c
> index f46643850df8..cc28e41fb745 100644
> --- a/net/sched/sch_htb.c
> +++ b/net/sched/sch_htb.c
> @@ -431,7 +431,10 @@ static void htb_activate_prios(struct htb_sched *q, struct htb_class *cl)
>  	while (cl->cmode == HTB_MAY_BORROW && p && mask) {
>  		m = mask;
>  		while (m) {
> -			int prio = ffz(~m);
> +			unsigned int prio = ffz(~m);
> +
> +			if (WARN_ON_ONCE(prio > ARRAY_SIZE(p->inner.clprio)))
> +				break;
>  			m &= ~(1 << prio);
>  
>  			if (p->inner.clprio[prio].feed.rb_node)
> -- 
> 2.34.1
>