Re: [PATCH v11 19/39] arm64/mm: Handle GCS data aborts

[Catalin Marinas <catalin.marinas@arm.com>]

On Thu, Aug 22, 2024 at 02:15:22AM +0100, Mark Brown wrote:
> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> index 451ba7cbd5ad..3ada31c2ac12 100644
> --- a/arch/arm64/mm/fault.c
> +++ b/arch/arm64/mm/fault.c
> @@ -486,6 +486,14 @@ static void do_bad_area(unsigned long far, unsigned long esr,
>  	}
>  }
>  
> +static bool is_gcs_fault(unsigned long esr)
> +{
> +	if (!esr_is_data_abort(esr))
> +		return false;
> +
> +	return ESR_ELx_ISS2(esr) & ESR_ELx_GCS;
> +}
> +
>  static bool is_el0_instruction_abort(unsigned long esr)
>  {
>  	return ESR_ELx_EC(esr) == ESR_ELx_EC_IABT_LOW;
> @@ -500,6 +508,23 @@ static bool is_write_abort(unsigned long esr)
>  	return (esr & ESR_ELx_WNR) && !(esr & ESR_ELx_CM);
>  }
>  
> +static bool is_invalid_gcs_access(struct vm_area_struct *vma, u64 esr)
> +{
> +	if (!system_supports_gcs())
> +		return false;
> +
> +	if (unlikely(is_gcs_fault(esr))) {
> +		/* GCS accesses must be performed on a GCS page */
> +		if (!(vma->vm_flags & VM_SHADOW_STACK))
> +			return true;

This first check covers the GCSPOPM/RET etc. permission faults on
non-GCS vmas. It looks correct.

> +	} else if (unlikely(vma->vm_flags & VM_SHADOW_STACK)) {
> +		/* Only GCS operations can write to a GCS page */
> +		return is_write_abort(esr);
> +	}

I don't think that's right. The ESR on this path may not even indicate a
data abort and ESR.WnR bit check wouldn't make sense.

I presume we want to avoid an infinite loop on a (writeable) GCS page
when the user does a normal STR but the CPU raises a permission fault. I
think this function needs to just return false if !esr_is_data_abort().

> +
> +	return false;
> +}
> +
>  static int __kprobes do_page_fault(unsigned long far, unsigned long esr,
>  				   struct pt_regs *regs)
>  {
> @@ -535,6 +560,14 @@ static int __kprobes do_page_fault(unsigned long far, unsigned long esr,
>  		/* It was exec fault */
>  		vm_flags = VM_EXEC;
>  		mm_flags |= FAULT_FLAG_INSTRUCTION;
> +	} else if (is_gcs_fault(esr)) {
> +		/*
> +		 * The GCS permission on a page implies both read and
> +		 * write so always handle any GCS fault as a write fault,
> +		 * we need to trigger CoW even for GCS reads.
> +		 */
> +		vm_flags = VM_WRITE;
> +		mm_flags |= FAULT_FLAG_WRITE;
>  	} else if (is_write_abort(esr)) {
>  		/* It was write fault */
>  		vm_flags = VM_WRITE;
> @@ -568,6 +601,13 @@ static int __kprobes do_page_fault(unsigned long far, unsigned long esr,
>  	if (!vma)
>  		goto lock_mmap;
>  
> +	if (is_invalid_gcs_access(vma, esr)) {
> +		vma_end_read(vma);
> +		fault = 0;
> +		si_code = SEGV_ACCERR;
> +		goto bad_area;
> +	}

Here there's a risk that the above function returns true for some
unrelated fault that happens to have bit 6 in ESR set.

-- 
Catalin