linux-kernel - Re: [PATCH v2 2/2] x86, refcount: Implement fast refcount overflow protection

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Message-ID: <20170501224542.bbfhs7c4x2wibomz@treble>
Date:   Mon, 1 May 2017 17:45:42 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        PaX Team <pageexec@...email.hu>, Jann Horn <jannh@...gle.com>,
        Eric Biggers <ebiggers3@...il.com>,
        Christoph Hellwig <hch@...radead.org>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Hans Liljestrand <ishkamiel@...il.com>,
        David Windsor <dwindsor@...il.com>,
        "x86@...nel.org" <x86@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "David S. Miller" <davem@...emloft.net>,
        Rik van Riel <riel@...hat.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v2 2/2] x86, refcount: Implement fast refcount overflow
 protection

On Mon, May 01, 2017 at 10:36:59AM -0700, Kees Cook wrote:
> >> +#ifdef CONFIG_FAST_REFCOUNT
> >> +static DEFINE_RATELIMIT_STATE(refcount_ratelimit, 15 * HZ, 3);
> >> +
> >> +void refcount_error_report(struct pt_regs *regs, const char *kind)
> >> +{
> >> +     do_send_sig_info(SIGKILL, SEND_SIG_FORCED, current, true);
> >> +
> >> +     if (!__ratelimit(&refcount_ratelimit))
> >> +             return;
> >> +
> >> +     pr_emerg("%s detected in: %s:%d, uid/euid: %u/%u\n",
> >> +             kind ? kind : "refcount error",
> >> +             current->comm, task_pid_nr(current),
> >> +             from_kuid_munged(&init_user_ns, current_uid()),
> >> +             from_kuid_munged(&init_user_ns, current_euid()));
> >> +     print_symbol(KERN_EMERG "refcount error occurred at: %s\n",
> >> +             instruction_pointer(regs));
> >> +     preempt_disable();
> >> +     show_regs(regs);
> >> +     preempt_enable();
> >> +}
> >
> > Why is preemption disabled before calling show_regs()?
> 
> I thought it was to avoid interleaving show_regs() output (I can't
> think of a way regs would be externally modified).

This code is running from interrupt context, so preemption shouldn't be
an issue unless I'm missing something.

-- 
Josh