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

[Kevin Brodsky]

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).

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

Kevin

[1]
https://lore.kernel.org/linux-arm-kernel/b4f8b351-4c83-43b4-bfbe-8f67f3f56fb9@arm.com/