Re: [PATCH v3 08/14] arm64: exec: Adjust affinity for compat tasks with mismatched 32-bit EL0

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Nov 13, 2020 at 09:37:13AM +0000, Will Deacon wrote:
> When exec'ing a 32-bit task on a system with mismatched support for
> 32-bit EL0, try to ensure that it starts life on a CPU that can actually
> run it.
> 
> Signed-off-by: Will Deacon <will@kernel.org>
> ---
>  arch/arm64/kernel/process.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> index 1540ab0fbf23..17b94007fed4 100644
> --- a/arch/arm64/kernel/process.c
> +++ b/arch/arm64/kernel/process.c
> @@ -625,6 +625,16 @@ unsigned long arch_align_stack(unsigned long sp)
>  	return sp & ~0xf;
>  }
>  
> +static void adjust_compat_task_affinity(struct task_struct *p)
> +{
> +	const struct cpumask *mask = system_32bit_el0_cpumask();
> +
> +	if (restrict_cpus_allowed_ptr(p, mask))
> +		set_cpus_allowed_ptr(p, mask);

This silently destroys user state, at the very least that ought to go
with a WARN or something. Ideally SIGKILL though. What's to stop someone
from doing a sched_setaffinity() right after the execve, same problem.
So why bother..

> +
> +	set_tsk_thread_flag(current, TIF_NOTIFY_RESUME);
> +}
> +
>  /*
>   * Called from setup_new_exec() after (COMPAT_)SET_PERSONALITY.
>   */
> @@ -635,7 +645,7 @@ void arch_setup_new_exec(void)
>  	if (is_compat_task()) {
>  		mmflags = MMCF_AARCH32;
>  		if (static_branch_unlikely(&arm64_mismatched_32bit_el0))
> -			set_tsk_thread_flag(current, TIF_NOTIFY_RESUME);
> +			adjust_compat_task_affinity(current);
>  	}
>  
>  	current->mm->context.flags = mmflags;
> -- 
> 2.29.2.299.gdc1121823c-goog
>