Re: [PATCH v12 21/39] arm64/gcs: Ensure that new threads have a GCS

[Szabolcs Nagy <Szabolcs.Nagy@arm.com>]

The 08/29/2024 00:27, Mark Brown wrote:
> Unfortunately plain clone() is not extensible and existing clone3()
> users will not specify a stack so all existing code would be broken if
> we mandated specifying the stack explicitly.  For compatibility with
> these cases and also x86 (which did not initially implement clone3()
> support for shadow stacks) if no GCS is specified we will allocate one
> so when a thread is created which has GCS enabled allocate one for it.
> We follow the extensively discussed x86 implementation and allocate
> min(RLIMIT_STACK, 2G).  Since the GCS only stores the call stack and not
> any variables this should be more than sufficient for most applications.

the code has RLIMIT_STACK/2

(which is what i expect on arm64, since gcs entry size
is min stack frame / 2 if the stack is correctly aligned)

> 
> GCSs allocated via this mechanism will be freed when the thread exits.

i see gcs still mapped after thread exit when testing.

> +static unsigned long gcs_size(unsigned long size)
> +{
> +	if (size)
> +		return PAGE_ALIGN(size);

no /2

> +
> +	/* Allocate RLIMIT_STACK/2 with limits of PAGE_SIZE..2G */
> +	size = PAGE_ALIGN(min_t(unsigned long long,
> +				rlimit(RLIMIT_STACK) / 2, SZ_2G));

has /2

> +	return max(PAGE_SIZE, size);
> +}
> +
> +unsigned long gcs_alloc_thread_stack(struct task_struct *tsk,
> +				     const struct kernel_clone_args *args)
> +{
> +	unsigned long addr, size;
> +
> +	if (!system_supports_gcs())
> +		return 0;
> +
> +	if (!task_gcs_el0_enabled(tsk))
> +		return 0;
> +
> +	if ((args->flags & (CLONE_VFORK | CLONE_VM)) != CLONE_VM) {
> +		tsk->thread.gcspr_el0 = read_sysreg_s(SYS_GCSPR_EL0);
> +		return 0;
> +	}
> +
> +	size = args->stack_size;

no /2 (i think this should be divided)

> +
> +	size = gcs_size(size);
> +	addr = alloc_gcs(0, size);
> +	if (IS_ERR_VALUE(addr))
> +		return addr;
> +
> +	tsk->thread.gcs_base = addr;
> +	tsk->thread.gcs_size = size;
> +	tsk->thread.gcspr_el0 = addr + size - sizeof(u64);
> +
> +	return addr;
> +}
...
>  void gcs_free(struct task_struct *task)
>  {
> +
> +	/*
> +	 * When fork() with CLONE_VM fails, the child (tsk) already
> +	 * has a GCS allocated, and exit_thread() calls this function
> +	 * to free it.  In this case the parent (current) and the
> +	 * child share the same mm struct.
> +	 */
> +	if (!task->mm || task->mm != current->mm)
> +		return;
> +
>  	if (task->thread.gcs_base)
>  		vm_munmap(task->thread.gcs_base, task->thread.gcs_size);

not sure why this logic fails to free thread gcs
(created with clone3 in glibc)

other the gcs leak, my tests pass.