mm/uffd: fix pte marker when fork() without fork event

Summary

In the Linux kernel, the following vulnerability has been resolved: mm/uffd: fix pte marker when fork() without fork event Patch series "mm: Fixes on pte markers". Patch 1 resolves the syzkiller report from Pengfei. Patch 2 further harden pte markers when used with the recent swapin error markers. The major case is we should persist a swapin error marker after fork(), so child shouldn't read a corrupted page. This patch (of 2): 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 Bugzilla report: https://bugzilla.kernel.org/show_bug.cgi?id=216808