mm: userfaultfd: fix missing cache flush in mcopy_atomic_pte() and __mcopy_atomic()

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.

Link: https://lkml.kernel.org/r/20220210123058.79206-7-songmuchun@bytedance.com
Fixes: b6ebaedb ("userfaultfd: avoid mmap_sem read recursion in mcopy_atomic")
Fixes: c1a4de99 ("userfaultfd: mcopy_atomic|mfill_zeropage: UFFDIO_COPY|UFFDIO_ZEROPAGE preparation")
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Fam Zheng <fam.zheng@bytedance.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Lars Persson <lars.persson@axis.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Xiongchun Duan <duanxiongchun@bytedance.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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);