Re: [PATCH RFC v9 00/25] pkeys-based page table hardening

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

On 08/09/2026 18:04, Linu Cherian wrote:
>> [...]
>>
>>>>>> Open questions
>>>>>> ==============
>>>>>>
>>>>>> A few aspects in this RFC that are debatable and/or worth discussing:
>>>>>>
>>>>>> - There is currently no restriction on how kpkeys contexts map to pkeys
>>>>>>   permissions. A typical approach is to allocate one pkey per context and
>>>>>>   make it writable in that context only. As the number of contexts
>>>>> Probably to avoid the assumption, may be we can we have something like
>>>>> below 
>>>>>
>>>>> For a pkey P, we could define
>>>>> PKEY_P_PERM_CTXT_OTHERS	 //permission for pkey p in other contexts
>>>>> PKEY_P_PERM_CTXT_SELF	 //permission for pkey p in self context
>>>>>
>>>>> With the assumption of one pkey mapped for every context,
>>>>> the permission for the default context would look something like,
>>>>>
>>>>> PKEY_DEF_PERM_CTXT_SELF << PKEY_DEF_PKEY_SHIFT |
>>>>> PKEY_CT0_PERM_CTXT_OTHERS << PKEY_CT0_PKEY_SHIFT | 
>>>>> PKEY_CT1_PERM_CTXT_OTHERS << PKEY_CT1_PKEY_SHIFT |
>>>>> ...(for all valid contexts)
>>>>>
>>>>> where,
>>>>> Permission key, PKEY_DEF is associated with context DEFAULT,
>>>>> Permission key, PKEY_CT0 is associated with context CT0,
>>>>> Permission key, PKEY_CT1 is associated with context CT1
>>>> This adds assumptions rather than avoiding them. *Typically* when adding
>>>> a context you'd allocate a pkey that's only writable by this context,
>>>> but it doesn't have to be this way.
>>> Okay agree. Then may be something like
>>>
>>> Define permissions:
>>>
>>> For default context,
>>> KPKEYS_CTX_DEFAULT_PERM_PKEY_DEF
>>> KPKEYS_CTX_DEFAULT_PERM_PKEY_CT0
>>>
>>> For CT0 context,
>>> KPKEYS_CTX_CT0_PERM_PKEY_DEF
>>> KPKEYS_CTX_CT0_PERM_PKEY_CT0
>>>
>>> Define POR_EL1:
>>>
>>> For default context,
>>> KPKEYS_POR_EL1_DEFAULT
>>>
>>> For CT0 context,
>>> KPKEYS_POR_EL1_CT0
>>>
>>> Finally,
>>> #define POR_EL1_INIT KPKEYS_POR_EL1_DEFAULT
>> We cannot do this because POR_EL1 is arm64-specific and its format is
>> not at all the same as x86's PKRS for instance.
>>
>> I think what you're getting at is that the permissions for each pkeys in
>> a given context could be defined at the generic level. This could be
>> done, but I'm not sure this is essential, and we may not need all archs
>> to use exactly the same permissions. There's also the issue that x86
>> only encodes RW permissions directly, not X.
> Really didnt mean to keep these macros generic. Sorry for the confusion. I could
> have replied this on a arm64 specific patch.
>
> The original intention was to suggest the use of macros similar to above in the arm64
> world.

Ah I see :) The problem is that again, kpkeys is not supposed to take
over the entire POR_EL1, just the pkeys we've allocated for it (0 and 1
for now). What could be done is something like

#define CTX_DEFAULT_PKEY_0_PERMS RW
#define CTX_DEFAULT_PKEY_PGTABLES_PERMS R

#define CTX_PGTABLES_PKEY_0_PERMS RW
#define CTX_PGTABLES_PKEY_PGTABLES_PERMS RW

That seems very heavy to me though. Is there really something wrong with
what por_set_kpkeys_context() does? I'd argue it's at least as readable
as #defining the configuration for each context, especially when the
number of pkeys increases.


>
>>> Probably using something similar would make the idea of kpkeys context 
>>> more evident in the code as well ?
>>>
>>>> The configuration space is more easily understood by considering the
>>>> other use-cases we've investigated (struct cred protection and eBPF
>>>> isolation, linked further down). For instance, for cred protection, we
>>>> had KPKEYS_LVL_UNRESTRICTED with write access to all pkeys, and for eBPF
>>>> isolation, we need a level that is less privileged and therefore does
>>>> *not* have write access to pkey 0.
>>>>
>>>>>>   increases, we may however run out of pkeys, especially on arm64 (just
>>>>>>   8 pkeys with POE). Depending on the use-cases, it may be acceptable to
>>>>>>   use the same pkey for the data associated to multiple contexts.
>>>>> Lets say two contexts A and B, use the same pkey P as their permission matches.
>>>>> But then, when we enter context A, permission for pkey P gets
>>>>> relaxed, then that would relax permission for pages associated with
>>>>> context B as well which is unintended ?
>>>> That may be exactly what is intended, it all depends on the use-case. C1
>>>> may have a private pkey P1, and C2 P2, and then P3 that is shared by C1
>>>> and C2 (writable by both)
>>> Got it. With each context defining permissions for each pkey owned by
>>> kpkeys makes sense. 
>>>
>>> Also do we need to assume that nesting of different contexts is not valid ?
>>> For example,
>>> Default context:
>>> 	enter CTX 0
>>> 		enter CTX 1
>>> 		leave CTX 1
>>> 	leave CTX 0
>>> Default context:
>> That is a good question. The enter/leave logic does support nesting,
>> since leave() restores the pkeys register as it was on enter(), but
>> whether the inner context has sufficient permissions will depend on the
>> situation. Certainly if nesting is expected then it has to be taken into
>> account when defining the permissions for each context.
>>
> Exactly. Also if nesting need to be disallowed for certain contexts,
> then it needs to be taken care as well ?  Feels like would be worth
> covering the aspect of nesting in documentation and cover letter.

I think having logic allowing/denying nesting would be overkill at this
stage, but agreed this should be documented.

- Kevin