The patch titled Subject: userfaultfd: mark uffd_wp regardless of VM_WRITE flag has been added to the -mm tree. Its filename is userfaultfd-mark-uffd_wp-regardless-of-vm_write-flag.patch
This patch should soon appear at https://ozlabs.org/~akpm/mmots/broken-out/userfaultfd-mark-uffd_wp-regardles... and later at https://ozlabs.org/~akpm/mmotm/broken-out/userfaultfd-mark-uffd_wp-regardles...
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------------------------------------------------------ From: Nadav Amit namit@vmware.com Subject: userfaultfd: mark uffd_wp regardless of VM_WRITE flag
When a PTE is set by UFFD operations such as UFFDIO_COPY, the PTE is currently only marked as write-protected if the VMA has VM_WRITE flag set. This seems incorrect or at least would be unexpected by the users.
Consider the following sequence of operations that are being performed on a certain page:
mprotect(PROT_READ) UFFDIO_COPY(UFFDIO_COPY_MODE_WP) mprotect(PROT_READ|PROT_WRITE)
At this point the user would expect to still get UFFD notification when the page is accessed for write, but the user would not get one, since the PTE was not marked as UFFD_WP during UFFDIO_COPY.
Fix it by always marking PTEs as UFFD_WP regardless on the write-permission in the VMA flags.
Link: https://lkml.kernel.org/r/20220217211602.2769-1-namit@vmware.com Fixes: 292924b26024 ("userfaultfd: wp: apply _PAGE_UFFD_WP bit") Signed-off-by: Nadav Amit namit@vmware.com Cc: Axel Rasmussen axelrasmussen@google.com Cc: Mike Rapoport rppt@linux.vnet.ibm.com Cc: Andrea Arcangeli aarcange@redhat.com Cc: Peter Xu peterx@redhat.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---
mm/userfaultfd.c | 15 +++++++++------ 1 file changed, 9 insertions(+), 6 deletions(-)
--- a/mm/userfaultfd.c~userfaultfd-mark-uffd_wp-regardless-of-vm_write-flag +++ a/mm/userfaultfd.c @@ -72,12 +72,15 @@ int mfill_atomic_install_pte(struct mm_s _dst_pte = pte_mkdirty(_dst_pte); if (page_in_cache && !vm_shared) writable = false; - if (writable) { - if (wp_copy) - _dst_pte = pte_mkuffd_wp(_dst_pte); - else - _dst_pte = pte_mkwrite(_dst_pte); - } + + /* + * Always mark a PTE as write-protected when needed, regardless of + * VM_WRITE, which the user might change. + */ + if (wp_copy) + _dst_pte = pte_mkuffd_wp(_dst_pte); + else if (writable) + _dst_pte = pte_mkwrite(_dst_pte);
dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);
_
Patches currently in -mm which might be from namit@vmware.com are
userfaultfd-mark-uffd_wp-regardless-of-vm_write-flag.patch