[PATCH 1/2] mm/uffd: Fix pte marker when fork() without fork event

[Peter Xu <peterx@redhat.com>]

When fork(), dst_vma is not guaranteed to have VM_UFFD_WP even if src may
have it and has pte marker installed.  The warning is improper along with
the comment.  The right thing is to inherit the pte marker when needed, or
keep the dst pte empty.

A vague guess is this happened by an accident when there's the prior patch
to introduce src/dst vma into this helper during the uffd-wp feature got
developed and I probably messed up in the rebase, since if we replace
dst_vma with src_vma the warning & comment it all makes sense too.

Hugetlb did exactly the right here (copy_hugetlb_page_range()).  Fix the
general path.

Reproducer:

https://github.com/xupengfe/syzkaller_logs/blob/main/221208_115556_copy_page_range/repro.c

Cc: <stable@vger.kernel.org> # 5.19+
Fixes: c56d1b62cce8 ("mm/shmem: handle uffd-wp during fork()")
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=216808
Reported-by: Pengfei Xu <pengfei.xu@intel.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
---
 mm/memory.c | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index aad226daf41b..032ef700c3e8 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -828,12 +828,8 @@ copy_nonpresent_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
 			return -EBUSY;
 		return -ENOENT;
 	} else if (is_pte_marker_entry(entry)) {
-		/*
-		 * We're copying the pgtable should only because dst_vma has
-		 * uffd-wp enabled, do sanity check.
-		 */
-		WARN_ON_ONCE(!userfaultfd_wp(dst_vma));
-		set_pte_at(dst_mm, addr, dst_pte, pte);
+		if (userfaultfd_wp(dst_vma))
+			set_pte_at(dst_mm, addr, dst_pte, pte);
 		return 0;
 	}
 	if (!userfaultfd_wp(dst_vma))
-- 
2.37.3