Re: [RFC PATCH v5 00/18] pkeys-based page table hardening

Yang Shi
2025-08-29T22:32:04Z

On 8/27/25 9:09 AM, Kevin Brodsky wrote:
> On 26/08/2025 21:18, Yang Shi wrote:
>>>>> from a contiguous cache of pages could help minimise the overhead, as
>>>>> proposed for x86 in [1].
>>>> I'm a little bit confused about how this can work. The contiguous
>>>> cache of pages should be some large page, for example, 2M. But the
>>>> page table pages allocated from the cache may have different
>>>> permissions if I understand correctly. The default permission is RO,
>>>> but some of them may become R/W at sometime, for example, when calling
>>>> set_pte_at(). You still need to split the linear mapping, right?
>>> When such a helper is called, *all* PTPs become writeable - there is no
>>> per-PTP permission switching.
>> OK, so all PTPs in the same contiguous cache will become writeable
>> even though the helper (i.e. set_pte_at()) is just called on one of
>> the PTPs.  But doesn't it compromise the page table hardening somehow?
>> The PTPs from the same cache may belong to different processes.
> First just a note that this is true regardless of how the PTPs are
> allocated (i.e. this is already the case in this version of the series).
>
> Either way, yes you are right, this approach does not introduce any
> isolation *between* page tables - pgtable helpers are able to write to
> all page tables. In principle it should be possible to use a different
> pkey for kernel and user page tables, but that would make the kpkeys
> level switching in helpers quite a bit more complicated. Isolating
> further is impractical as we have so few pkeys (just 8 on arm64).
>
> That said, what kpkeys really tries to protect against is the direct
> corruption of critical data by arbitrary (unprivileged) code. If the
> attacker is able to manipulate calls to set_pte() and the likes, kpkeys
> cannot provide much protection - even if we restricted the writes to a
> specific set of page tables, the attacker would still be able to insert
> a translation to any arbitrary physical page.

I see. Thanks for elaborating this.

Yang

>
> - Kevin