Dave Hansen [off-list ref] writes:

On 9/11/24 08:01, Kevin Brodsky wrote:

quoted

On 22/08/2024 17:10, Joey Gouly wrote:

quoted

@@ -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.

quoted

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.

that is also what powerpc does. 

/* usage of kthread_use_mm() should inherit the
 * AMR value of the operating address space. But, the AMR value is
 * thread-specific and we inherit the address space and not thread
 * access restrictions. Because of this ignore AMR value when accessing
 * userspace via kernel thread.
 */
static __always_inline u64 current_thread_amr(void)
{
	if (current->thread.regs)
		return current->thread.regs->amr;
	return default_amr;
}


-aneesh