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mm/userfaultfd: rely on vma->vm_page_prot in uffd_wp_range()

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Patch series "mm: uffd-wp + change_protection() cleanups".

Cleanup page protection handling in uffd-wp when calling
change_protection() and improve unprotecting uffd=wp in private mappings,
trying to set PTEs writable again if possible just like we do during
mprotect() when upgrading write permissions.  Make the change_protection()
interface harder to get wrong :)

I consider both pages primarily cleanups, although patch #1 fixes a corner
case with uffd-wp and softdirty tracking for shmem.  @Peter, please let me
know if we should flag patch #1 as pure cleanup -- I have no idea how
important softdirty tracking on shmem is.


This patch (of 2):

uffd_wp_range() currently calculates page protection manually using
vm_get_page_prot().  This will ignore any other reason for active
writenotify: one mechanism applicable to shmem is softdirty tracking.

For example, the following sequence

1) Write to mapped shmem page
2) Clear softdirty
3) Register uffd-wp covering the mapped page
4) Unregister uffd-wp covering the mapped page
5) Write to page again

will not set the modified page softdirty, because uffd_wp_range() will
ignore that writenotify is required for softdirty tracking and simply map
the page writable again using change_protection().  Similarly, instead of
unregistering, protecting followed by un-protecting the page using uffd-wp
would result in the same situation.

Now that we enable writenotify whenever enabling uffd-wp on a VMA,
vma->vm_page_prot will already properly reflect our requirements: the
default is to write-protect all PTEs.  However, for shared mappings we
would now not remap the PTEs writable if possible when unprotecting, just
like for private mappings (COW).  To compensate, set
MM_CP_TRY_CHANGE_WRITABLE just like mprotect() does to try mapping
individual PTEs writable.

For private mappings, this change implies that we will now always try
setting PTEs writable when un-protecting, just like when upgrading write
permissions using mprotect(), which is an improvement.

For shared mappings, we will only set PTEs writable if
can_change_pte_writable()/can_change_pmd_writable() indicates that it's
ok.  For ordinary shmem, this will be the case when PTEs are dirty, which
should usually be the case -- otherwise we could special-case shmem in
can_change_pte_writable()/can_change_pmd_writable() easily, because shmem
itself doesn't require writenotify.

Note that hugetlb does not yet implement MM_CP_TRY_CHANGE_WRITABLE, so we
won't try setting PTEs writable when unprotecting or when unregistering
uffd-wp.  This can be added later on top by implementing
MM_CP_TRY_CHANGE_WRITABLE.

While commit ffd0579396 ("userfaultfd: wp: support write protection for
userfault vma range") introduced that code, it should only be applicable
to uffd-wp on shared mappings -- shmem (hugetlb does not support softdirty
tracking).  I don't think this corner cases justifies to cc stable.  Let's
just handle it correctly and prepare for change_protection() cleanups.

[david@redhat.com: o need for additional harmless checks if we're wr-protecting either way]
  Link: https://lkml.kernel.org/r/71412742-a71f-9c74-865f-773ad83db7a5@redhat.com
Link: https://lkml.kernel.org/r/20221223155616.297723-1-david@redhat.com
Link: https://lkml.kernel.org/r/20221223155616.297723-2-david@redhat.com
Fixes: b1f9e87686 ("mm/uffd: enable write protection for shmem & hugetlbfs")
Signed-off-by: David Hildenbrand <david@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Nadav Amit <nadav.amit@gmail.com>
Cc: Peter Xu <peterx@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
David Hildenbrand 2022-12-23 16:56:15 +01:00 committed by Andrew Morton
parent a9af8e6bb3
commit 931298e103
1 changed files with 13 additions and 5 deletions
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18
mm/userfaultfd.c
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@ -713,17 +713,25 @@ ssize_t mcopy_continue(struct mm_struct *dst_mm, unsigned long start,
void uffd_wp_range(struct mm_struct *dst_mm, struct vm_area_struct *dst_vma,
unsigned long start, unsigned long len, bool enable_wp)
{
unsigned int mm_cp_flags;
struct mmu_gather tlb;
pgprot_t newprot;
if (enable_wp)
newprot = vm_get_page_prot(dst_vma->vm_flags & ~(VM_WRITE));
mm_cp_flags = MM_CP_UFFD_WP;
else
newprot = vm_get_page_prot(dst_vma->vm_flags);
mm_cp_flags = MM_CP_UFFD_WP_RESOLVE;
/*
* vma->vm_page_prot already reflects that uffd-wp is enabled for this
* VMA (see userfaultfd_set_vm_flags()) and that all PTEs are supposed
* to be write-protected as default whenever protection changes.
* Try upgrading write permissions manually.
*/
if (!enable_wp && vma_wants_manual_pte_write_upgrade(dst_vma))
mm_cp_flags |= MM_CP_TRY_CHANGE_WRITABLE;
tlb_gather_mmu(&tlb, dst_mm);
change_protection(&tlb, dst_vma, start, start + len, newprot,
enable_wp ? MM_CP_UFFD_WP : MM_CP_UFFD_WP_RESOLVE);
change_protection(&tlb, dst_vma, start, start + len, vma->vm_page_prot,
mm_cp_flags);
tlb_finish_mmu(&tlb);
}