Re: [PATCH 2/7] mm/hugetlb: Fix hugetlb_follow_page_mask() on permission checks

[David Hildenbrand <david@redhat.com>]

On 13.06.23 23:53, Peter Xu wrote:
> It seems hugetlb_follow_page_mask() was missing permission checks.  For
> example, one follow_page() can get the hugetlb page with FOLL_WRITE even if
> the page is read-only.

I'm curious if there even is a follow_page() user that operates on 
hugetlb ...

s390x secure storage does not apply to hugetlb IIRC.

ksm.c? no.

huge_memory.c ? no

So what remains is most probably mm/migrate.c, which never sets FOLL_WRITE.


Or am I missing something a user?

>  > And it wasn't there even in the old follow_page_mask(), where we can
> reference from before commit 57a196a58421 ("hugetlb: simplify hugetlb
> handling in follow_page_mask").
> 
> Let's add them, namely, either the need to CoW due to missing write bit, or
> proper CoR on !AnonExclusive pages over R/O pins to reject the follow page.
> That brings this function closer to follow_hugetlb_page().
> 
> I just doubt how many of us care for that, for FOLL_PIN follow_page doesn't
> really happen at all.  But we'll care, and care more if we switch over
> slow-gup to use hugetlb_follow_page_mask().  We'll also care when to return
> -EMLINK then, as that's the gup internal api to mean "we should do CoR".
> 
> When at it, switching the try_grab_page() to use WARN_ON_ONCE(), to be
> clear that it just should never fail.
> 
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>   mm/hugetlb.c | 22 ++++++++++++++++------
>   1 file changed, 16 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 82dfdd96db4c..9c261921b2cf 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -6481,8 +6481,21 @@ struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
>   	ptl = huge_pte_lock(h, mm, pte);
>   	entry = huge_ptep_get(pte);
>   	if (pte_present(entry)) {
> -		page = pte_page(entry) +
> -				((address & ~huge_page_mask(h)) >> PAGE_SHIFT);
> +		page = pte_page(entry);
> +
> +		if (gup_must_unshare(vma, flags, page)) {
> +			/* Tell the caller to do Copy-On-Read */
> +			page = ERR_PTR(-EMLINK);
> +			goto out;
> +		}
> +
> +		if ((flags & FOLL_WRITE) && !pte_write(entry)) {
> +			page = NULL;
> +			goto out;
> +		}
> +
> +		page += ((address & ~huge_page_mask(h)) >> PAGE_SHIFT);
> +
>   		/*
>   		 * Note that page may be a sub-page, and with vmemmap
>   		 * optimizations the page struct may be read only.
> @@ -6492,10 +6505,7 @@ struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
>   		 * try_grab_page() should always be able to get the page here,
>   		 * because we hold the ptl lock and have verified pte_present().
>   		 */
> -		if (try_grab_page(page, flags)) {
> -			page = NULL;
> -			goto out;
> -		}
> +		WARN_ON_ONCE(try_grab_page(page, flags));
>   	}
>   out:
>   	spin_unlock(ptl);

-- 
Cheers,

David / dhildenb