linux-kernel - Re: [PATCH 09/31] x86/entry/32: Leave the kernel via trampoline stack

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: <4a047ea5-7717-d089-48bf-597434be7c4c@redhat.com>
Date:   Fri, 9 Feb 2018 20:30:55 +0100
From:   Denys Vlasenko <dvlasenk@...hat.com>
To:     Joerg Roedel <jroedel@...e.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>, Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>, Jiri Kosina <jkosina@...e.cz>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Brian Gerst <brgerst@...il.com>,
        David Laight <David.Laight@...lab.com>,
        Eduardo Valentin <eduval@...zon.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Will Deacon <will.deacon@....com>,
        "Liguori, Anthony" <aliguori@...zon.com>,
        Daniel Gruss <daniel.gruss@...k.tugraz.at>,
        Hugh Dickins <hughd@...gle.com>,
        Kees Cook <keescook@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Waiman Long <llong@...hat.com>, Pavel Machek <pavel@....cz>
Subject: Re: [PATCH 09/31] x86/entry/32: Leave the kernel via trampoline stack

On 02/09/2018 08:02 PM, Joerg Roedel wrote:
> On Fri, Feb 09, 2018 at 09:05:02AM -0800, Linus Torvalds wrote:
>> On Fri, Feb 9, 2018 at 1:25 AM, Joerg Roedel <joro@...tes.org> wrote:
>>> +
>>> +       /* Copy over the stack-frame */
>>> +       cld
>>> +       rep movsb
>>
>> Ugh. This is going to be horrendous. Maybe not noticeable on modern
>> CPU's, but the whole 32-bit code is kind of pointless on a modern CPU.
>>
>> At least use "rep movsl". If the kernel stack isn't 4-byte aligned,
>> you have issues.
>
> Okay, I used movsb because I remembered that being the recommendation
> for the most efficient memcpy, and it safes me an instruction. But that
> is probably only true on modern CPUs.

It's fast (copies data with full-width loads and stores,
up to 64-byte wide on latest Intel CPUs), but this kicks in only for
largish blocks. In your case, you are copying less than 100 bytes.