The patch titled Subject: mm/userfaultfd: enable writenotify while userfaultfd-wp is enabled for a VMA has been added to the -mm mm-hotfixes-unstable branch. Its filename is mm-userfaultfd-enable-writenotify-while-userfaultfd-wp-is-enabled-for-a-vma.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: David Hildenbrand david@redhat.com Subject: mm/userfaultfd: enable writenotify while userfaultfd-wp is enabled for a VMA Date: Fri, 9 Dec 2022 09:09:12 +0100

Currently, we don't enable writenotify when enabling userfaultfd-wp on a shared writable mapping (for now only shmem and hugetlb). The consequence is that vma->vm_page_prot will still include write permissions, to be set as default for all PTEs that get remapped (e.g., mprotect(), NUMA hinting, page migration, ...).

So far, vma->vm_page_prot is assumed to be a safe default, meaning that we only add permissions (e.g., mkwrite) but not remove permissions (e.g., wrprotect). For example, when enabling softdirty tracking, we enable writenotify. With uffd-wp on shared mappings, that changed. More details on vma->vm_page_prot semantics were summarized in [1].

This is problematic for uffd-wp: we'd have to manually check for a uffd-wp PTEs/PMDs and manually write-protect PTEs/PMDs, which is error prone. Prone to such issues is any code that uses vma->vm_page_prot to set PTE permissions: primarily pte_modify() and mk_pte().

Instead, let's enable writenotify such that PTEs/PMDs/... will be mapped write-protected as default and we will only allow selected PTEs that are definitely safe to be mapped without write-protection (see can_change_pte_writable()) to be writable. In the future, we might want to enable write-bit recovery -- e.g., can_change_pte_writable() -- at more locations, for example, also when removing uffd-wp protection.

This fixes two known cases:

(a) remove_migration_pte() mapping uffd-wp'ed PTEs writable, resulting in uffd-wp not triggering on write access. (b) do_numa_page() / do_huge_pmd_numa_page() mapping uffd-wp'ed PTEs/PMDs writable, resulting in uffd-wp not triggering on write access.

Note that do_numa_page() / do_huge_pmd_numa_page() can be reached even without NUMA hinting (which currently doesn't seem to be applicable to shmem), for example, by using uffd-wp with a PROT_WRITE shmem VMA. On such a VMA, userfaultfd-wp is currently non-functional.

Note that when enabling userfaultfd-wp, there is no need to walk page tables to enforce the new default protection for the PTEs: we know that they cannot be uffd-wp'ed yet, because that can only happen after enabling uffd-wp for the VMA in general.

Also note that this makes mprotect() on ranges with uffd-wp'ed PTEs not accidentally set the write bit -- which would result in uffd-wp not triggering on later write access. This commit makes uffd-wp on shmem behave just like uffd-wp on anonymous memory in that regard, even though, mixing mprotect with uffd-wp is controversial.

[1] https://lkml.kernel.org/r/92173bad-caa3-6b43-9d1e-9a471fdbc184@redhat.com

Link: https://lkml.kernel.org/r/20221209080912.7968-1-david@redhat.com Fixes: b1f9e876862d ("mm/uffd: enable write protection for shmem & hugetlbfs") Signed-off-by: David Hildenbrand david@redhat.com Reported-by: Ives van Hoorne ives@codesandbox.io Debugged-by: Peter Xu peterx@redhat.com Acked-by: Peter Xu peterx@redhat.com Cc: Hugh Dickins hughd@google.com Cc: Alistair Popple apopple@nvidia.com Cc: Mike Rapoport rppt@linux.vnet.ibm.com Cc: Nadav Amit nadav.amit@gmail.com Cc: Andrea Arcangeli aarcange@redhat.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

fs/userfaultfd.c | 28 ++++++++++++++++++++++------ mm/mmap.c | 4 ++++ 2 files changed, 26 insertions(+), 6 deletions(-)

--- a/fs/userfaultfd.c~mm-userfaultfd-enable-writenotify-while-userfaultfd-wp-is-enabled-for-a-vma +++ a/fs/userfaultfd.c @@ -108,6 +108,21 @@ static bool userfaultfd_is_initialized(s return ctx->features & UFFD_FEATURE_INITIALIZED; }

+static void userfaultfd_set_vm_flags(struct vm_area_struct *vma, + vm_flags_t flags) +{ + const bool uffd_wp_changed = (vma->vm_flags ^ flags) & VM_UFFD_WP; + + vma->vm_flags = flags; + /* + * For shared mappings, we want to enable writenotify while + * userfaultfd-wp is enabled (see vma_wants_writenotify()). We'll simply + * recalculate vma->vm_page_prot whenever userfaultfd-wp changes. + */ + if ((vma->vm_flags & VM_SHARED) && uffd_wp_changed) + vma_set_page_prot(vma); +} + static int userfaultfd_wake_function(wait_queue_entry_t *wq, unsigned mode, int wake_flags, void *key) { @@ -618,7 +633,8 @@ static void userfaultfd_event_wait_compl for_each_vma(vmi, vma) { if (vma->vm_userfaultfd_ctx.ctx == release_new_ctx) { vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX; - vma->vm_flags &= ~__VM_UFFD_FLAGS; + userfaultfd_set_vm_flags(vma, + vma->vm_flags & ~__VM_UFFD_FLAGS); } } mmap_write_unlock(mm); @@ -652,7 +668,7 @@ int dup_userfaultfd(struct vm_area_struc octx = vma->vm_userfaultfd_ctx.ctx; if (!octx || !(octx->features & UFFD_FEATURE_EVENT_FORK)) { vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX; - vma->vm_flags &= ~__VM_UFFD_FLAGS; + userfaultfd_set_vm_flags(vma, vma->vm_flags & ~__VM_UFFD_FLAGS); return 0; }

@@ -733,7 +749,7 @@ void mremap_userfaultfd_prep(struct vm_a } else { /* Drop uffd context if remap feature not enabled */ vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX; - vma->vm_flags &= ~__VM_UFFD_FLAGS; + userfaultfd_set_vm_flags(vma, vma->vm_flags & ~__VM_UFFD_FLAGS); } }

@@ -895,7 +911,7 @@ static int userfaultfd_release(struct in prev = vma; }

- vma->vm_flags = new_flags; + userfaultfd_set_vm_flags(vma, new_flags); vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX; } mmap_write_unlock(mm); @@ -1463,7 +1479,7 @@ static int userfaultfd_register(struct u * the next vma was merged into the current one and * the current one has not been updated yet. */ - vma->vm_flags = new_flags; + userfaultfd_set_vm_flags(vma, new_flags); vma->vm_userfaultfd_ctx.ctx = ctx;

if (is_vm_hugetlb_page(vma) && uffd_disable_huge_pmd_share(vma)) @@ -1648,7 +1664,7 @@ static int userfaultfd_unregister(struct * the next vma was merged into the current one and * the current one has not been updated yet. */ - vma->vm_flags = new_flags; + userfaultfd_set_vm_flags(vma, new_flags); vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;

skip: --- a/mm/mmap.c~mm-userfaultfd-enable-writenotify-while-userfaultfd-wp-is-enabled-for-a-vma +++ a/mm/mmap.c @@ -1524,6 +1524,10 @@ int vma_wants_writenotify(struct vm_area if (vma_soft_dirty_enabled(vma) && !is_vm_hugetlb_page(vma)) return 1;

+ /* Do we need write faults for uffd-wp tracking? */ + if (userfaultfd_wp(vma)) + return 1; + /* Specialty mapping? */ if (vm_flags & VM_PFNMAP) return 0; _

Patches currently in -mm which might be from david@redhat.com are

mm-swap-fix-swp_pfn_bits-with-config_phys_addr_t_64bit-on-32bit.patch mm-swap-fix-swp_pfn_bits-with-config_phys_addr_t_64bit-on-32bit-v2.patch mm-swap-fix-swp_pfn_bits-with-config_phys_addr_t_64bit-on-32bit-fix.patch mm-userfaultfd-enable-writenotify-while-userfaultfd-wp-is-enabled-for-a-vma.patch selftests-vm-add-ksm-unmerge-tests.patch mm-pagewalk-dont-trigger-test_walk-in-walk_page_vma.patch selftests-vm-add-test-to-measure-madv_unmergeable-performance.patch mm-ksm-simplify-break_ksm-to-not-rely-on-vm_fault_write.patch mm-remove-vm_fault_write.patch mm-ksm-fix-ksm-cow-breaking-with-userfaultfd-wp-via-fault_flag_unshare.patch mm-pagewalk-add-walk_page_range_vma.patch mm-ksm-convert-break_ksm-to-use-walk_page_range_vma.patch mm-gup-remove-foll_migration.patch mm-gup_test-fix-pin_longterm_test_read-with-highmem.patch selftests-vm-madv_populate-fix-missing-madv_populate_readwrite-definitions.patch selftests-vm-cow-fix-compile-warning-on-32bit.patch selftests-vm-ksm_functional_tests-fixes-for-32bit.patch