mm/userfaultfd: don't consider uffd-wp bit of writable migration entries

If we end up with a writable migration entry that has the uffd-wp bit set,
we already messed up: the source PTE/PMD was writable, which means we
could have modified the page without notifying uffd first.  Setting the
uffd-wp bit always implies converting migration entries to !writable
migration entries.

Commit 8f34f1eac382 ("mm/userfaultfd: fix uffd-wp special cases for
fork()") documents that "3.  Forget to carry over uffd-wp bit for a write
migration huge pmd entry", but it doesn't really say why that should be
relevant.

So let's remove that code to avoid hiding an eventual underlying issue (in
the future, we might want to warn when creating writable migration entries
that have the uffd-wp bit set -- or even better when turning a PTE
writable that still has the uffd-wp bit set).

This now matches the handling for hugetlb migration entries in
hugetlb_change_protection().

In copy_huge_pmd()/copy_nonpresent_pte()/copy_hugetlb_page_range(), we
still transfer the uffd-bit also for writable migration entries, but
simply because we have unified handling for "writable" and
"readable-exclusive" migration entries, and we care about transferring the
uffd-wp bit for the latter.

Link: https://lkml.kernel.org/r/20230405160236.587705-3-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Cc: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 03d78901a7a799483585b28094e24f18d061702e..0dbce09916c444e6f450531521acc9b5b975493f 100644 (file)
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1845,8 +1845,6 @@ int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
                        newpmd = swp_entry_to_pmd(entry);
                        if (pmd_swp_soft_dirty(*pmd))
                                newpmd = pmd_swp_mksoft_dirty(newpmd);
-                       if (pmd_swp_uffd_wp(*pmd))
-                               newpmd = pmd_swp_mkuffd_wp(newpmd);
                } else {
                        newpmd = *pmd;
                }

diff --git a/mm/mprotect.c b/mm/mprotect.c
index 2041941558634584d3035137c8c7ca1b8713ba7b..92d3d3ca390a09de5f883d0b620608c3b4593aed 100644 (file)
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -223,8 +223,6 @@ static long change_pte_range(struct mmu_gather *tlb,
                                newpte = swp_entry_to_pte(entry);
                                if (pte_swp_soft_dirty(oldpte))
                                        newpte = pte_swp_mksoft_dirty(newpte);
-                               if (pte_swp_uffd_wp(oldpte))
-                                       newpte = pte_swp_mkuffd_wp(newpte);
                        } else if (is_writable_device_private_entry(entry)) {
                                /*
                                 * We do not preserve soft-dirtiness. See