linux-kernel - Re: [PATCH v5 00/10] x86/alternative: text

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Date:   Tue, 20 Nov 2018 18:52:25 +0000
From:   Nadav Amit <namit@...are.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     Ingo Molnar <mingo@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "linux_dti@...oud.com" <linux_dti@...oud.com>,
        "linux-integrity@...r.kernel.org" <linux-integrity@...r.kernel.org>,
        "linux-security-module@...r.kernel.org" 
        <linux-security-module@...r.kernel.org>
Subject: Re: [PATCH v5 00/10] x86/alternative: text_poke() fixes

> On Nov 20, 2018, at 4:42 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> On Tue, Nov 13, 2018 at 05:07:20AM -0800, Nadav Amit wrote:
>> v4->v5:
>> - Fix Xen breakage [Damian Tometzki]
>> - BUG_ON() when poking_mm initialization fails [PeterZ]
>> - Better comments on "x86/mm: temporary mm struct"
>> - Cleaner removal of the custom poker
> 
> I'll re-iterate my position: it is impossible for the text not to match,
> and if it somehow does not match, something went sideways in an
> unrecoverably fashion.
> 
> text_poke() must not fail, ever. If it does, our text is inconsistent
> and we must abort/panic/bug.
> 
> The only way I will accept anything else is if someone can come up with
> a sensible scenario of text_poke() failing and recovering from it.
> AFAICT there is no possible way to gracefully recover.
> 
> Consider a jump label with multiple patch sites; we patch the first,
> then fail. In order to restore to a sane state, we must undo the
> patching of the first, but undoing text_poke() fails again. Then
> what?
> 
> Allowing text_poke() to fail only creates an unfixable mess. Esp. since
> there is no sane scenario under which is can fail.

Ok, ok... I tried to stand my ground, but I guess I failed. I don’t feel
that strongly about this assertion to argue with you. I’m just the “chicken”
kind of guy.

Yet, take into consideration that I will need to use you as my “vest” once I
get being “shot” for adding BUG_ON(). ;-)

I will send another version tonight, assuming no new issues are raised.

Regards,
NAdav