linux-kernel - Re: [PATCH RFC 1/7] x86/head/64: Mark startup_gdt and startup_gdt_descr as _

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Message-ID: <77B66FD0-ED28-4D3F-8D28-467AC4FCD00D@zytor.com>
Date:   Tue, 17 Oct 2023 09:34:41 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Ingo Molnar <mingo@...nel.org>,
        Hou Wenlong <houwenlong.hwl@...group.com>
CC:     linux-kernel@...r.kernel.org,
        Lai Jiangshan <jiangshan.ljs@...group.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "maintainer:X86 ARCHITECTURE 32-BIT AND 64-BIT" <x86@...nel.org>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Mike Rapoport <rppt@...nel.org>,
        Pasha Tatashin <pasha.tatashin@...een.com>
Subject: Re: [PATCH RFC 1/7] x86/head/64: Mark startup_gdt and startup_gdt_descr as __initdata

On October 17, 2023 6:02:27 AM PDT, Ingo Molnar <mingo@...nel.org> wrote:
>
>* Hou Wenlong <houwenlong.hwl@...group.com> wrote:
>
>> Hi Ingo,
>> 
>> I have sent patch #6 separately for x86. Do you have any ideas about 
>> building the head code as PIE? Should I resend the patchset for the PIE 
>> feature?
>
>So I had a brief look, and despite reading 0/43 it was unclear to me what 
>the precise advantages of building as PIE are.
>
>Ie. could you please outline:
>
> - *Exactly* how much PIE based KASLR randomization would gain us in terms 
>   of randomization granularity and effective number of randomization bits, 
>   compared to the current status quo?
>
> - How is code generation changed at the instruction level - how does 
>   kernel size change and what are the micro-advantages/disadvantages?
>
> - Are there any other advantages/motivation than improving KASLR?
>
>Ie. before asking us to apply ~50 patches and add a whole new build mode 
>and the maintainance overhead to support it into infinity and beyond, could 
>you please offer a better list of pros and cons?
>
>Thanks,
>
>	Ingo

If the goal is better KASLR, then what we really should spend time on was Kristen Accardi's fgKASLR patches, which not only exponentially(!) increases the randomization entrophy but also *actually* avoids the "one leak and it's over" problem.

However, she gave up on it because she got no interest, despite working code.