linux-kernel - Re: [PATCH v1 05/12] x86, alternatives: Use optimized NOPs for padding

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <54D12342.6050909@amacapital.net>
Date:	Tue, 03 Feb 2015 11:36:34 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Borislav Petkov <bp@...en8.de>, X86 ML <x86@...nel.org>
CC:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 05/12] x86, alternatives: Use optimized NOPs for padding

On 02/03/2015 10:16 AM, Borislav Petkov wrote:
> From: Borislav Petkov <bp@...e.de>
>
> Alternatives allow now for empty old instruction. In this case we go
> and pad the space with NOPs at assembly time. However, there are the
> optimal, longer NOPs which should be used. Do that at patching time.
>
> Signed-off-by: Borislav Petkov <bp@...e.de>
> ---
>   arch/x86/kernel/alternative.c | 20 +++++++++++++++++++-
>   1 file changed, 19 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index 715af37bf008..dd0cdb6b179c 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -323,6 +323,21 @@ done:
>   		n_dspl, (unsigned long)orig_insn + n_dspl + repl_len);
>   }
>
> +static void __init_or_module optimize_nops(u8 *instr, u8 max_len)
> +{
> +	int i = 0;
> +
> +	while (instr[i] == 0x90 && i < max_len)
> +		i++;
> +
> +	if (!i)
> +		return;
> +
> +	add_nops(instr, i);
> +
> +	DUMP_BYTES(instr, i, "%p: optimized NOPs: ", instr);
> +}
> +
>   /*
>    * Replace instructions with better alternatives for this CPU type. This runs
>    * before SMP is initialized to avoid SMP problems with self modifying code.
> @@ -354,8 +369,11 @@ void __init_or_module apply_alternatives(struct alt_instr *start,
>   		replacement = (u8 *)&a->repl_offset + a->repl_offset;
>   		BUG_ON(a->instrlen > sizeof(insnbuf));
>   		BUG_ON(a->cpuid >= (NCAPINTS + NBUGINTS) * 32);
> -		if (!boot_cpu_has(a->cpuid))
> +		if (!boot_cpu_has(a->cpuid)) {
> +			if (instr[0] == 0x90)
> +				optimize_nops(instr, a->instrlen);
>   			continue;
> +		}

I'm a bit confused here.  Shouldn't NOPs after a non-NOP in the old 
instruction also be optimized?

--Andy
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/