linux-kernel - Re: [PATCH] x86: combine memmove FSRM and ERMS alternatives

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Message-ID: <Y8MVPcgWLZspXJxq@zn.tnic>
Date:   Sat, 14 Jan 2023 21:49:01 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Daniel Verkamp <dverkamp@...omium.org>
Cc:     x86@...nel.org, Tony Luck <tony.luck@...el.com>,
        Jiri Slaby <jirislaby@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: combine memmove FSRM and ERMS alternatives

On Sat, Jan 14, 2023 at 05:17:28PM +0100, Borislav Petkov wrote:
> On Sat, Jan 14, 2023 at 11:42:13AM +0100, Borislav Petkov wrote:
> > Or, altenatively (pun intended), you can do what copy_user_generic() does and
> > move all that logic into C and inline asm. Which I'd prefer, actually, instead of
> > doing ugly asm hacks. Depends on how ugly it gets...
> 
> Alternatively #2, you can do the below as a minimal fix for stable along with
> explaining what we're doing there and why and then do the other things I
> suggested - if you'd like, that is - later and with no pressure.
> 
> Thx.
> 
> ---
> diff --git a/arch/x86/lib/memmove_64.S b/arch/x86/lib/memmove_64.S
> index 02661861e5dd..d6ffb4164cdb 100644
> --- a/arch/x86/lib/memmove_64.S
> +++ b/arch/x86/lib/memmove_64.S
> @@ -38,10 +38,9 @@ SYM_FUNC_START(__memmove)
>  	cmp %rdi, %r8
>  	jg 2f
>  
> -	/* FSRM implies ERMS => no length checks, do the copy directly */
>  .Lmemmove_begin_forward:
>  	ALTERNATIVE "cmp $0x20, %rdx; jb 1f", "", X86_FEATURE_FSRM
> -	ALTERNATIVE "", "jmp .Lmemmove_erms", X86_FEATURE_ERMS
> +	ALTERNATIVE "cmp $0x20, %rdx; jb 1f", "jmp .Lmemmove_erms", X86_FEATURE_ERMS

Forget what I said - now that I think of it this is bull.

The more and more I think about it, the more I like the copy_user_generic() idea
but lemme see how ugly it gets...


-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette