linux-kernel - Re: [PATCH v2 03/10] arm64: mm: Permit contiguous descriptors to be rewritten

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Message-Id: <eff29cc6-29e6-4eda-8b44-aef30095252c@app.fastmail.com>
Date: Tue, 27 Jan 2026 18:02:55 +0100
From: "Ard Biesheuvel" <ardb@...nel.org>
To: "Ryan Roberts" <ryan.roberts@....com>
Cc: linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 will@...nel.org, catalin.marinas@....com, mark.rutland@....com,
 "Anshuman Khandual" <anshuman.khandual@....com>,
 "Liz Prucka" <lizprucka@...gle.com>, "Seth Jenkins" <sethjenkins@...gle.com>,
 "Kees Cook" <kees@...nel.org>, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2 03/10] arm64: mm: Permit contiguous descriptors to be rewritten



On Tue, 27 Jan 2026, at 17:59, Ryan Roberts wrote:
> On 27/01/2026 15:03, Ard Biesheuvel wrote:
>> On Tue, 27 Jan 2026 at 10:45, Ryan Roberts <ryan.roberts@....com> wrote:
>>>
>>> On 26/01/2026 09:26, Ard Biesheuvel wrote:
>>>> From: Ard Biesheuvel <ardb@...nel.org>
>>>>
>>>> Currently, pgattr_change_is_safe() is overly pedantic when it comes to
>>>> descriptors with the contiguous hint attribute set, as it rejects
>>>> assignments even if the old and the new value are the same.
>>>>
>>>> So relax the check to allow that.
>>>
>>> But why do we require the relaxation? Why are we re-writing a PTE in the first
>>> place? Either the caller already knows it's the same in which case it can be
>>> avoided, or it doesn't know in which case it is accidentally the same and couple
>>> probably just as easily been accidentally different? So it's better to warn
>>> regardless I would think?
>>>
>> 
>> Based on rule RJQQTC in your reply to another patch in this series, my
>> conclusion here is that we can drop this check entirely.
>
> Hmm, I don't think that would be quite right; The rule permits _some_ bits of
> the PTE to change in a live mapping as long as the CONT bit remains unchanged.
> If you change the CONT bit on a live mapping, you could end up with overlapping
> TLB entries which would not go well on a system without bbml2.

I'm not suggesting we add it to 'mask', just to remove the check that forbids any manipulation of an entry that has PTE_CONT set. So toggling PTE_CONT itself would still be caught by the check.