Re: [PATCH v5 06/30] arm64: context switch POR_EL0 register

[Dave Hansen <dave.hansen@intel.com>]

On 9/11/24 08:01, Kevin Brodsky wrote:
> On 22/08/2024 17:10, Joey Gouly wrote:
>> @@ -371,6 +382,9 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
>>  		if (system_supports_tpidr2())
>>  			p->thread.tpidr2_el0 = read_sysreg_s(SYS_TPIDR2_EL0);
>>  
>> +		if (system_supports_poe())
>> +			p->thread.por_el0 = read_sysreg_s(SYS_POR_EL0);
> Here we are only reloading POR_EL0's value if the target is a user
> thread. However, as this series stands, POR_EL0 is also relevant to
> kthreads, because any uaccess or GUP done from a kthread will also be
> checked against POR_EL0. This is especially important in cases like the
> io_uring kthread, which accesses the memory of the user process that
> spawned it. To prevent such a kthread from inheriting a stale value of
> POR_EL0, it seems that we should reload POR_EL0's value in all cases
> (user and kernel thread).

The problem with this is trying to figure out which POR_EL0 to use.  The
kthread could have been spawned ages ago and might not have a POR_EL0
which is very different from the current value of any of the threads in
the process right now.

There's also no great way for a kthread to reach out and grab an updated
value.  It's all completely inherently racy.

> Other approaches could also be considered (e.g. resetting POR_EL0 to
> unrestricted when creating a kthread), see my reply on v4 [1].

I kinda think this is the only way to go.  It's the only sensible,
predictable way.  I _think_ it's what x86 will end up doing with PKRU,
but there's been enough churn there that I'd need to go double check
what happens in practice.

Either way, it would be nice to get an io_uring test in here that
actually spawns kthreads:

	tools/testing/selftests/mm/protection_keys.c