Hi Dave,
On Wed, Sep 11, 2024 at 08:33:54AM -0700, Dave Hansen wrote:
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.
I agree.
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
It would be good to update Documentation/core-api/protection-keys.rst as well, since the example with read() raises more questions than it answers!
Kevin, Joey -- I've got this series queued in arm64 as-is, so perhaps you could send some patches on top so we can iron this out in time for 6.12? I'll also be at LPC next week if you're about.
Cheers,
Will