Re: [PATCH 3/4] rseq: Make rseq work with protection keys

[Mathieu Desnoyers]

On 2025-02-21 14:48, Dave Hansen wrote:
> On 2/21/25 11:38, Mathieu Desnoyers wrote:
> > I agree that switching to permissive key in the fast path would be
> > simpler. AFAIU, the switch_to_permissive_pkey_reg() is only a pkey
> > read when the key is already permissive.
>
> Unfortunately, on x86, PKRU is almost never in its permissive state. We
> chose a policy (stored in the global init_pkru_value variable) that
> allows R/W access to pkey 0, but disables access to everything else.
> It's 0xfffffff5, IIRC.
>
> This ensures deny-by-default behavior and ensures that threads cloned
> off long ago don't have a dangerous PKRU value for newly-allocated and
> pkey-protected memory.
>
> If I had a time machine, it'd be interesting to go back and try to make
> PKRU's default value be all 0's and also represent the logically most
> restrictive value.

Can we assume (or require) that struct rseq and struct rseq_cs reside in
pkey-0 memory ?

In that case, we could add something to the pkey API that switches to a
permissive state only if pkey 0 cannot be accessed.

Therefore it would only trigger a pkey read in the common case, and
issue a pkey write only if pkey 0 is not accessible.

Thoughts ?

Thanks,

Mathieu



--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com