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Subject[PATCH RFC 08/24] userfaultfd: wp: hook userfault handler to write protection fault
Date
From: Andrea Arcangeli <aarcange@redhat.com>

There are several cases write protection fault happens. It could be a
write to zero page, swaped page or userfault write protected
page. When the fault happens, there is no way to know if userfault
write protect the page before. Here we just blindly issue a userfault
notification for vma with VM_UFFD_WP regardless if app write protects
it yet. Application should be ready to handle such wp fault.

v1: From: Shaohua Li <shli@fb.com>

v2: Handle the userfault in the common do_wp_page. If we get there a
pagetable is present and readonly so no need to do further processing
until we solve the userfault.

In the swapin case, always swapin as readonly. This will cause false
positive userfaults. We need to decide later if to eliminate them with
a flag like soft-dirty in the swap entry (see _PAGE_SWP_SOFT_DIRTY).

hugetlbfs wouldn't need to worry about swapouts but and tmpfs would
be handled by a swap entry bit like anonymous memory.

The main problem with no easy solution to eliminate the false
positives, will be if/when userfaultfd is extended to real filesystem
pagecache. When the pagecache is freed by reclaim we can't leave the
radix tree pinned if the inode and in turn the radix tree is reclaimed
as well.

The estimation is that full accuracy and lack of false positives could
be easily provided only to anonymous memory (as long as there's no
fork or as long as MADV_DONTFORK is used on the userfaultfd anonymous
range) tmpfs and hugetlbfs, it's most certainly worth to achieve it
but in a later incremental patch.

v3: Add hooking point for THP wrprotect faults.

CC: Shaohua Li <shli@fb.com>
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
---
mm/memory.c | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/mm/memory.c b/mm/memory.c
index 4ad2d293ddc2..89d51d1650e4 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2482,6 +2482,11 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
{
struct vm_area_struct *vma = vmf->vma;

+ if (userfaultfd_wp(vma)) {
+ pte_unmap_unlock(vmf->pte, vmf->ptl);
+ return handle_userfault(vmf, VM_UFFD_WP);
+ }
+
vmf->page = vm_normal_page(vma, vmf->address, vmf->orig_pte);
if (!vmf->page) {
/*
@@ -2799,6 +2804,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
inc_mm_counter_fast(vma->vm_mm, MM_ANONPAGES);
dec_mm_counter_fast(vma->vm_mm, MM_SWAPENTS);
pte = mk_pte(page, vma->vm_page_prot);
+ if (userfaultfd_wp(vma))
+ vmf->flags &= ~FAULT_FLAG_WRITE;
if ((vmf->flags & FAULT_FLAG_WRITE) && reuse_swap_page(page, NULL)) {
pte = maybe_mkwrite(pte_mkdirty(pte), vma);
vmf->flags &= ~FAULT_FLAG_WRITE;
@@ -3662,8 +3669,11 @@ static inline vm_fault_t create_huge_pmd(struct vm_fault *vmf)
/* `inline' is required to avoid gcc 4.1.2 build error */
static inline vm_fault_t wp_huge_pmd(struct vm_fault *vmf, pmd_t orig_pmd)
{
- if (vma_is_anonymous(vmf->vma))
+ if (vma_is_anonymous(vmf->vma)) {
+ if (userfaultfd_wp(vmf->vma))
+ return handle_userfault(vmf, VM_UFFD_WP);
return do_huge_pmd_wp_page(vmf, orig_pmd);
+ }
if (vmf->vma->vm_ops->huge_fault)
return vmf->vma->vm_ops->huge_fault(vmf, PE_SIZE_PMD);

--
2.17.1
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