linux-kernel - Re: [PATCH 6/7] i915: Convert to verify_page

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Date:   Mon, 12 Apr 2021 13:08:38 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        boris.ostrovsky@...cle.com, jgross@...e.com,
        sstabellini@...nel.org, x86@...nel.org,
        jani.nikula@...ux.intel.com, joonas.lahtinen@...ux.intel.com,
        rodrigo.vivi@...el.com, chris@...is-wilson.co.uk,
        intel-gfx@...ts.freedesktop.org, linux-mm@...ck.org, hch@....de
Subject: Re: [PATCH 6/7] i915: Convert to verify_page_range()

On Mon, Apr 12, 2021 at 10:00:18AM +0200, Peter Zijlstra wrote:
> check_{present,absent}() only need R/O access, use verify_page_range()
> instead to remove modular use of apply_to_page_range().
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  drivers/gpu/drm/i915/gem/selftests/i915_gem_mman.c |   16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 deletions(-)
> 
> --- a/drivers/gpu/drm/i915/gem/selftests/i915_gem_mman.c
> +++ b/drivers/gpu/drm/i915/gem/selftests/i915_gem_mman.c
> @@ -1225,9 +1225,9 @@ static int igt_mmap_gpu(void *arg)
>  	return 0;
>  }
>  
> -static int check_present_pte(pte_t *pte, unsigned long addr, void *data)
> +static int check_present_pte(pte_t pte, unsigned long addr, void *data)
>  {
> -	if (!pte_present(*pte) || pte_none(*pte)) {
> +	if (!pte_present(pte) || pte_none(pte)) {
>  		pr_err("missing PTE:%lx\n",
>  		       (addr - (unsigned long)data) >> PAGE_SHIFT);
>  		return -EINVAL;
> @@ -1236,9 +1236,9 @@ static int check_present_pte(pte_t *pte,
>  	return 0;
>  }
>  
> -static int check_absent_pte(pte_t *pte, unsigned long addr, void *data)
> +static int check_absent_pte(pte_t pte, unsigned long addr, void *data)
>  {
> -	if (pte_present(*pte) && !pte_none(*pte)) {
> +	if (pte_present(pte) && !pte_none(pte)) {
>  		pr_err("present PTE:%lx; expected to be revoked\n",
>  		       (addr - (unsigned long)data) >> PAGE_SHIFT);
>  		return -EINVAL;
> @@ -1249,14 +1249,14 @@ static int check_absent_pte(pte_t *pte,
>  
>  static int check_present(unsigned long addr, unsigned long len)
>  {
> -	return apply_to_page_range(current->mm, addr, len,
> -				   check_present_pte, (void *)addr);
> +	return verify_page_range(current->mm, addr, len,
> +				 check_present_pte, (void *)addr);

For example, switch to returning bad addr through verify_page_range(),
or have a by-reference value, etc:

	unsigned long failed;

	failed = verify_page_range(current->mm< addr, len, check_present_pte);
	if (failed) {
  		pr_err("missing PTE:%lx\n",
  		       (addr - failed) >> PAGE_SHIFT);


-- 
Kees Cook