linux-kernel - Re: Linux 6.11-rc1

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Message-ID: <e15a45c4-77a0-4eec-84b3-d09ba1e8b681@roeck-us.net>
Date: Wed, 31 Jul 2024 09:49:12 -0700
From: Guenter Roeck <linux@...ck-us.net>
To: Linus Torvalds <torvalds@...ux-foundation.org>,
 Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>, Jens Axboe <axboe@...nel.dk>,
 Andy Lutomirski <luto@...nel.org>, Ingo Molnar <mingo@...hat.com>,
 Peter Anvin <hpa@...or.com>,
 Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
 the arch/x86 maintainers <x86@...nel.org>
Subject: Re: Linux 6.11-rc1

On 7/31/24 09:17, Linus Torvalds wrote:
> On Wed, 31 Jul 2024 at 08:55, Peter Zijlstra <peterz@...radead.org> wrote:
>>
>> Right, so Thomas found that i386-pti fails to map the entire entry text.
>> Specifically pti_clone_pgtable() hard relies -- and does not verify --
>> that the start address is aligned to the given granularity.
>>
>> Now, i386 does not align __entry_text_start, and so the termination
>> condition goes sideways and pte_clone_entry() does not always work right
>> and it becomes a games of code layout roulette.
> 
> Lovely.
> 
>> Also, should we just kill PTI on 32bit perhaps?
> 
> I don't think there's much technical reason to keep it - I can't
> imagine any security-conscious people actually use 32-bit x86 any more
> - but apart from fixing this bug I wonder how much of a maintenance
> burden it is? I think most of the code is shared with 64-bit, isn't
> it? The 32-bit case in many ways is simpler, even if it happened to
> hit this odd alignment issue because it's obviously also a lot less
> tested.
> 
> I'd rather kill highmem and X86_PAE, but I also suspect that horror
> has a much larger chance of still being used.
> 

I guess there is at least one user - me with my annoying boot tests ;-).

But seriously the question is: How likely is it for that code to find
potential problems in the 64-bit code ? pti_clone_pgtable() doesn't
seem to be 32-bit specific.

Guenter