Re: [PATCH v5 6/7] mm: userfaultfd: fix missing cache flush in mcopy_atomic_pte() and __mcopy_atomic()

[Mike Kravetz <mike.kravetz@oracle.com>]

On 2/10/22 04:30, Muchun Song wrote:
> The userfaultfd calls mcopy_atomic_pte() and __mcopy_atomic() which do not
> do any cache flushing for the target page.  Then the target page will be
> mapped to the user space with a different address (user address), which might
> have an alias issue with the kernel address used to copy the data from the
> user to.  Fix this by insert flush_dcache_page() after copy_from_user()
> succeeds.
> 
> Fixes: b6ebaedb4cb1 ("userfaultfd: avoid mmap_sem read recursion in mcopy_atomic")
> Fixes: c1a4de99fada ("userfaultfd: mcopy_atomic|mfill_zeropage: UFFDIO_COPY|UFFDIO_ZEROPAGE preparation")
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> ---
>  mm/userfaultfd.c | 3 +++
>  1 file changed, 3 insertions(+)

Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>

-- 
Mike Kravetz

> 
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index 0780c2a57ff1..6ccc534d1c1c 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -150,6 +150,8 @@ static int mcopy_atomic_pte(struct mm_struct *dst_mm,
>  			/* don't free the page */
>  			goto out;
>  		}
> +
> +		flush_dcache_page(page);
>  	} else {
>  		page = *pagep;
>  		*pagep = NULL;
> @@ -625,6 +627,7 @@ static __always_inline ssize_t __mcopy_atomic(struct mm_struct *dst_mm,
>  				err = -EFAULT;
>  				goto out;
>  			}
> +			flush_dcache_page(page);
>  			goto retry;
>  		} else
>  			BUG_ON(page);