On 3/24/23 7:26 PM, Peter Xu wrote:
> This patch fixes an issue that a hugetlb uffd-wr-protected mapping can be
> writable even with uffd-wp bit set. It only happens with hugetlb private
> mappings, when someone firstly wr-protects a missing pte (which will
> install a pte marker), then a write to the same page without any prior
> access to the page.
>
> Userfaultfd-wp trap for hugetlb was implemented in hugetlb_fault() before
> reaching hugetlb_wp() to avoid taking more locks that userfault won't need.
> However there's one CoW optimization path that can trigger hugetlb_wp()
> inside hugetlb_no_page(), which will bypass the trap.
>
> This patch skips hugetlb_wp() for CoW and retries the fault if uffd-wp bit
> is detected. The new path will only trigger in the CoW optimization path
> because generic hugetlb_fault() (e.g. when a present pte was wr-protected)
> will resolve the uffd-wp bit already. Also make sure anonymous UNSHARE
> won't be affected and can still be resolved, IOW only skip CoW not CoR.
>
> This patch will be needed for v5.19+ hence copy stable.
>
> Reported-by: Muhammad Usama Anjum <usama.anjum@xxxxxxxxxxxxx>
> Cc: linux-stable <stable@xxxxxxxxxxxxxxx>
> Fixes: 166f3ecc0daf ("mm/hugetlb: hook page faults for uffd write protection")
> Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
Tested-by: Muhammad Usama Anjum <usama.anjum@xxxxxxxxxxxxx>
> ---
>
> Notes:
>
> v2 is not on the list but in an attachment in the reply; this v3 is mostly
> to make sure it's not the same as the patch used to be attached. Sorry
> Andrew, we need to drop the queued one as I rewrote the commit message.
>
> Muhammad, I didn't attach your T-b because of the slight functional change.
> Please feel free to re-attach if it still works for you (which I believe
> should).
Thank you for the fix.
>
> thanks,
> ---
> mm/hugetlb.c | 14 ++++++++++++--
> 1 file changed, 12 insertions(+), 2 deletions(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 8bfd07f4c143..a58b3739ed4b 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -5478,7 +5478,7 @@ static vm_fault_t hugetlb_wp(struct mm_struct *mm, struct vm_area_struct *vma,
> struct folio *pagecache_folio, spinlock_t *ptl)
> {
> const bool unshare = flags & FAULT_FLAG_UNSHARE;
> - pte_t pte;
> + pte_t pte = huge_ptep_get(ptep);
> struct hstate *h = hstate_vma(vma);
> struct page *old_page;
> struct folio *new_folio;
> @@ -5487,6 +5487,17 @@ static vm_fault_t hugetlb_wp(struct mm_struct *mm, struct vm_area_struct *vma,
> unsigned long haddr = address & huge_page_mask(h);
> struct mmu_notifier_range range;
>
> + /*
> + * Never handle CoW for uffd-wp protected pages. It should be only
> + * handled when the uffd-wp protection is removed.
> + *
> + * Note that only the CoW optimization path (in hugetlb_no_page())
> + * can trigger this, because hugetlb_fault() will always resolve
> + * uffd-wp bit first.
> + */
> + if (!unshare && huge_pte_uffd_wp(pte))
> + return 0;
> +
> /*
> * hugetlb does not support FOLL_FORCE-style write faults that keep the
> * PTE mapped R/O such as maybe_mkwrite() would do.
> @@ -5500,7 +5511,6 @@ static vm_fault_t hugetlb_wp(struct mm_struct *mm, struct vm_area_struct *vma,
> return 0;
> }
>
> - pte = huge_ptep_get(ptep);
> old_page = pte_page(pte);
>
> delayacct_wpcopy_start();
--
BR,
Muhammad Usama Anjum
