[PATCH v5 3/7] arm64: uaccess: Add additional userspace GCS accessors

[Jeremy Linton]

On 8/11/25 9:10 AM, Jeremy Linton wrote:
> Uprobes need more advanced read, push, and pop userspace GCS
> functionality. Implement those features using the existing gcsstr()
> and copy_from_user().
> 
> Its important to note that GCS pages can be read by normal
> instructions, but the hardware validates that pages used by GCS
> specific operations, have a GCS privilege set. We aren't validating this
> in load_user_gcs because it requires stabilizing the VMA over the read
> which may fault.
> 
> Signed-off-by: Jeremy Linton <jeremy.linton at arm.com>
> Reviewed-by: Catalin Marinas <catalin.marinas at arm.com>
> ---
>   arch/arm64/include/asm/gcs.h | 54 ++++++++++++++++++++++++++++++++++++
>   1 file changed, 54 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/gcs.h b/arch/arm64/include/asm/gcs.h
> index e3b360c9dba4..bd9ff1dedcd9 100644
> --- a/arch/arm64/include/asm/gcs.h
> +++ b/arch/arm64/include/asm/gcs.h
> @@ -116,6 +116,47 @@ static inline void put_user_gcs(unsigned long val, unsigned long __user *addr,
>   	uaccess_ttbr0_disable();
>   }
>   
> +static inline void push_user_gcs(unsigned long val, int *err)
> +{
> +	u64 gcspr = read_sysreg_s(SYS_GCSPR_EL0);
> +
> +	gcspr -= sizeof(u64);
> +	put_user_gcs(val, (unsigned long __user *)gcspr, err);
> +	if (!*err)
> +		write_sysreg_s(gcspr, SYS_GCSPR_EL0);
> +}
> +
> +/*
> + * Unlike put/push_user_gcs() above, get/pop_user_gsc() doesn't
> + * validate the GCS permission is set on the page being read.  This
> + * differs from how the hardware works when it consumes data stored at
> + * GCSPR. Callers should assure this is acceptable.
> + */
> +static inline u64 get_user_gcs(unsigned long __user *addr, int *err)
> +{
> +	unsigned long ret;
> +	u64 load = 0;
> +
> +	/* Ensure previous GCS operation are visible before we read the page */
> +	gcsb_dsync();
> +	ret = copy_from_user(&load, addr, sizeof(load));
> +	if (ret != 0)
> +		*err = ret;
> +	return load;
> +}
> +
> +static inline u64 pop_user_gcs(int *err)
> +{
> +	u64 gcspr = read_sysreg_s(SYS_GCSPR_EL0);
> +	u64 read_val;
> +
> +	read_val = get_user_gcs((unsigned long __user *)gcspr, err);
> +	if (!*err)
> +		write_sysreg_s(gcspr + sizeof(u64), SYS_GCSPR_EL0);
> +
> +	return read_val;
> +}
> +
>   #else
>   
>   static inline bool task_gcs_el0_enabled(struct task_struct *task)
> @@ -126,6 +167,10 @@ static inline bool task_gcs_el0_enabled(struct task_struct *task)
>   static inline void gcs_set_el0_mode(struct task_struct *task) { }
>   static inline void gcs_free(struct task_struct *task) { }
>   static inline void gcs_preserve_current_state(void) { }
> +static inline void put_user_gcs(unsigned long val, unsigned long __user *addr,
> +				int *err) { }
> +static inline void push_user_gcs(unsigned long val, int *err) { }
> +
>   static inline unsigned long gcs_alloc_thread_stack(struct task_struct *tsk,
>   						   const struct kernel_clone_args *args)
>   {
> @@ -136,6 +181,15 @@ static inline int gcs_check_locked(struct task_struct *task,
>   {
>   	return 0;
>   }
> +static inline u64 load_user_gcs(unsigned long __user *addr, int *err)
> +{
> +	*err = -EFAULT;
> +	return 0;
> +}

I sent an old version sine this was sitting around during the merge 
window. This should have been renamed along with the version above to
'get_user_gcs'.


> +static inline u64 pop_user_gcs(int *err)
> +{
> +	return 0;
> +}
>   
>   #endif
>