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Date: Thu, 3 Sep 2015 08:43:31 -0400
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Subject: Re: [BUG 4.2-rc8] Interrupt occurs while apply_alternatives() is
 patching the handler
From: Josh Boyer <jwboyer@fedoraproject.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Borislav Petkov <bp@alien8.de>, "Richard W.M. Jones" <rjones@redhat.com>,
        Chuck Ebbert <cebbert.lkml@gmail.com>,
        "Linux-Kernel@Vger. Kernel. Org" <linux-kernel@vger.kernel.org>,
        x86 <x86@kernel.org>, Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>
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On Thu, Sep 3, 2015 at 6:41 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
> On Thu, 3 Sep 2015, Borislav Petkov wrote:
>> On Wed, Sep 02, 2015 at 11:11:55AM +0200, Thomas Gleixner wrote:
>> >  static void __init_or_module add_nops(void *insns, unsigned int len)
>> >  {
>> > +   unsigned long flags;
>> > +
>> > +   local_irq_save(flags);
>> >     while (len > 0) {
>>
>> I guess you want to optimize the len==0 case to not disable interrupts
>> needlessly:
>>
>>       if (!len)
>>               return;
>>
>>       local_irq_save(flags);
>>       while (len > 0)
>>               ...
>
> Nah. I rather put the local_irq_save into optimize_nops(). All other
> callers of add_nops() are operating on a buffer and use text_poke
> after that. Aside of that optimize_nops() is missing a sync_core().
>
> Updated patch below.
>
> Thanks,
>
>         tglx
>
> ----------------->
> Subject: x86/alternatives: Make optimize_nops() interrupt safe and synced
> From: Thomas Gleixner <tglx@linutronix.de>
> Date: Thu, 03 Sep 2015 12:34:55 +0200
>
> optimize_nops() is buggy in two aspects:
>
> - It's not disabling interrupts across the modification
> - It's lacking a sync_core() call
>
> Fixes: 4fd4b6e5537c 'x86/alternatives: Use optimized NOPs for padding'

Possibly CC: stable since it fixes a commit in 4.1 and 4.2?

josh

> Reported-by: "Richard W.M. Jones" <rjones@redhat.com>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> ---
>  arch/x86/kernel/alternative.c |    5 +++++
>  1 file changed, 5 insertions(+)
>
> Index: tip/arch/x86/kernel/alternative.c
> ===================================================================
> --- tip.orig/arch/x86/kernel/alternative.c
> +++ tip/arch/x86/kernel/alternative.c
> @@ -338,10 +338,15 @@ done:
>
>  static void __init_or_module optimize_nops(struct alt_instr *a, u8 *instr)
>  {
> +       unsigned long flags;
> +
>         if (instr[0] != 0x90)
>                 return;
>
> +       local_irq_save(flags);
>         add_nops(instr + (a->instrlen - a->padlen), a->padlen);
> +       sync_core();
> +       local_irq_restore(flags);
>
>         DUMP_BYTES(instr, a->instrlen, "%p: [%d:%d) optimized NOPs: ",
>                    instr, a->instrlen - a->padlen, a->padlen);
>
>
> --
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