linux-kernel - Re: [patch 2/2] x86/alternatives: Disable interrupts and sync when optimizing NOPs in place

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Date:   Fri, 08 Dec 2023 14:37:45 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Borislav Petkov <bp@...en8.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, paul.gortmaker@...driver.com,
        x86@...nel.org, regressions@...mhuis.info,
        richard.purdie@...uxfoundation.org, regressions@...ts.linux.dev
Subject: Re: [patch 2/2] x86/alternatives: Disable interrupts and sync when
 optimizing NOPs in place

On Fri, Dec 08 2023 at 14:22, Borislav Petkov wrote:
> On Thu, Dec 07, 2023 at 08:49:26PM +0100, Thomas Gleixner wrote:
>> +static void __init_or_module noinline optimize_nops_inplace(u8 *instr, size_t len)
>> +{
>> +	unsigned long flags;
>> +
>> +	local_irq_save(flags);
>> +	optimize_nops(instr, len);
>> +	sync_core();
>> +	local_irq_restore(flags);
>> +}
>> +
>>  /*
>>   * In this context, "source" is where the instructions are placed in the
>>   * section .altinstr_replacement, for example during kernel build by the
>> @@ -438,7 +448,7 @@ void __init_or_module noinline apply_alt
>>  		 *   patch if feature is *NOT* present.
>>  		 */
>>  		if (!boot_cpu_has(a->cpuid) == !(a->flags & ALT_FLAG_NOT)) {
>> -			optimize_nops(instr, a->instrlen);
>> +			optimize_nops_inplace(instr, a->instrlen);
>>  			continue;
>>  		}
>
> Arguably, the proper thing to do here would be to convert the NOP
> optimizing to the same 2-stage process as normal patching: write insns
> into a buffer and text_poke* it.
>
> VS what we currently do: operating straight on kernel memory.

Well, apply_alternatives() results in text_poke_early() which is nothing
else than a memcpy() with interrupts disabled :)