linux-kernel - Re: [PATCH] sched/fair: Fix shift-out-of-bounds in load

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Date:   Thu, 25 Feb 2021 23:21:17 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     linux-kernel@...r.kernel.org,
        syzbot+d7581744d5fd27c9fbe1@...kaller.appspotmail.com,
        Andrew Morton <akpm@...ux-foundation.org>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, luto@...nel.org,
        Ingo Molnar <mingo@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        syzkaller-bugs@...glegroups.com,
        Thomas Gleixner <tglx@...utronix.de>, x86 <x86@...nel.org>
Subject: Re: [PATCH] sched/fair: Fix shift-out-of-bounds in load_balance()

On Thu, Feb 25, 2021 at 05:56:56PM +0000, Valentin Schneider wrote:
> Syzbot reported a handful of occurrences where an sd->nr_balance_failed can
> grow to much higher values than one would expect.
> 
> A successful load_balance() resets it to 0; a failed one increments
> it. Once it gets to sd->cache_nice_tries + 3, this *should* trigger an
> active balance, which will either set it to sd->cache_nice_tries+1 or reset
> it to 0. However, in case the to-be-active-balanced task is not allowed to
> run on env->dst_cpu, then the increment is done without any further
> modification.
> 
> This could then be repeated ad nauseam, and would explain the absurdly high
> values reported by syzbot (86, 149). VincentG noted there is value in
> letting sd->cache_nice_tries grow, so the shift itself should be
> fixed. That means preventing:
> 
>   """
>   If the value of the right operand is negative or is greater than or equal
>   to the width of the promoted left operand, the behavior is undefined.
>   """
> 
> Thus we need to cap the shift exponent to
>   BITS_PER_TYPE(typeof(lefthand)) - 1.
> 

Thanks!