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

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Message-ID: <da0b8783-590e-42a0-8937-f786a2ebcafa@arm.com>
Date: Tue, 27 Jan 2026 16:59:25 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Ard Biesheuvel <ardb@...nel.org>
Cc: Ard Biesheuvel <ardb+git@...gle.com>, 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 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.