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

[Dave Hansen]

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.