Re: [PATCH v6 01/30] mm: Introduce kpkeys

[Kevin Brodsky <kevin.brodsky@arm.com>]

On 15/04/2026 15:00, David Hildenbrand (Arm) wrote:
> On 2/27/26 18:54, Kevin Brodsky wrote:
>> kpkeys is a simple framework to enable the use of protection keys
>> (pkeys) to harden the kernel itself. This patch introduces the basic
>> API in <linux/kpkeys.h>: a couple of functions to set and restore
>> the pkey register and macros to define guard objects.
>>
>> kpkeys introduces a new concept on top of pkeys: the kpkeys level.
>> Each level is associated to a set of permissions for the pkeys
>> managed by the kpkeys framework. kpkeys_set_level(lvl) sets those
>> permissions according to lvl, and returns the original pkey
>> register, to be later restored by kpkeys_restore_pkey_reg(). To
>> start with, only KPKEYS_LVL_DEFAULT is available, which is meant
>> to grant RW access to KPKEYS_PKEY_DEFAULT (i.e. all memory since
>> this is the only available pkey for now).
>>
>> Because each architecture implementing pkeys uses a different
>> representation for the pkey register, and may reserve certain pkeys
>> for specific uses, support for kpkeys must be explicitly indicated
>> by selecting ARCH_HAS_KPKEYS and defining the following functions in
>> <asm/kpkeys.h>, in addition to the macros provided in
>> <asm-generic/kpkeys.h>:
> I don't quite understand the reason for using levels. Levels sounds like
> it would all be in some ordered fashion, where higher levels have access
> to lower levels.

That was originally the idea indeed, but in practice I don't expect
levels to have a strict ordering, as it's not practical for composing
features.

> Think of that as a key that can unlock all "lower" locks, not just a
> single lock.
>
> Then, the question is about the ordering once we introduce new
> keys/locks. With two, it obviously doesn't matter :)
>
> So naturally I wonder whether levels is really the right abstraction
> here, and why we are not simply using "distinct" keys, like
>
> KPKEY_DEFAULT
> KPKEY_PGTABLE
> KPKEY_SUPER_SECRET1
> KPKEY_SUPER_SECRET2
>
> Is it because you want KPKEY_PGTABLE also be able to write to KPKEY_DEFAULT?

Right, and in general a given level may be able to write to any number
of pkeys. That's why I don't want to conflate pkeys and levels. Agreed
that "level" might not be the clearest term though, since there's no
strict ordering.

> But how would you handle KPKEY_SUPER_SECRET1 and KPKEY_SUPER_SECRET2 then?

Presumably those would also have access to KPKEY_DEFAULT. However, if
you consider the reverse situation where the level is less privileged
than the default (say an eBPF program), then write access to
KPKEY_DEFAULT would not be granted.

Also worth noting on the notion of level that POE2 will bring further
per-level restrictions, besides which pkeys can be accessed. For
instance, we could prevent an unprivileged level from executing certain
instructions. This isn't in scope for this series, but this is a
consideration in the design of the kpkeys abstractions.

- Kevin