[PATCH v1 0/8] Fix set_huge_pte_at() panic on arm64

Ryan Roberts ryan.roberts at arm.com
Fri Sep 22 02:19:59 AEST 2023

Hi All,

This series fixes a bug in arm64's implementation of set_huge_pte_at(), which
can result in an unprivileged user causing a kernel panic. The problem was
triggered when running the new uffd poison mm selftest for HUGETLB memory. This
test (and the uffd poison feature) was merged for v6.6-rc1. However, upon
inspection there are multiple other pre-existing paths that can trigger this

Ideally, I'd like to get this fix in for v6.6 if possible? And I guess it should
be backported too, given there are call sites where this can theoretically
happen that pre-date v6.6-rc1 (I've cc'ed stable at vger.kernel.org).

Description of Bug

arm64's huge pte implementation supports multiple huge page sizes, some of which
are implemented in the page table with contiguous mappings. So set_huge_pte_at()
needs to work out how big the logical pte is, so that it can also work out how
many physical ptes (or pmds) need to be written. It does this by grabbing the
folio out of the pte and querying its size.

However, there are cases when the pte being set is actually a swap entry. But
this also used to work fine, because for huge ptes, we only ever saw migration
entries and hwpoison entries. And both of these types of swap entries have a PFN
embedded, so the code would grab that and everything still worked out.

But over time, more calls to set_huge_pte_at() have been added that set swap
entry types that do not embed a PFN. And this causes the code to go bang. The
triggering case is for the uffd poison test, commit 99aa77215ad0 ("selftests/mm:
add uffd unit test for UFFDIO_POISON"), which sets a PTE_MARKER_POISONED swap
entry. But review shows there are other places too (PTE_MARKER_UFFD_WP).

If CONFIG_DEBUG_VM is enabled, we do at least get a BUG(), but otherwise, it
will dereference a bad pointer in page_folio():

    static inline struct folio *hugetlb_swap_entry_to_folio(swp_entry_t entry)
        VM_BUG_ON(!is_migration_entry(entry) && !is_hwpoison_entry(entry));

        return page_folio(pfn_to_page(swp_offset_pfn(entry)));

So the root cause is due to commit 18f3962953e4 ("mm: hugetlb: kill
set_huge_swap_pte_at()"), which aimed to simplify the interface to the core code
by removing set_huge_swap_pte_at() (which took a page size parameter) and
replacing it with calls to set_huge_swap_pte_at() where the size was inferred
from the folio, as descibed above. While that commit didn't break anything at
the time, it did break the interface because it couldn't handle swap entries
without PFNs. And since then new callers have come along which rely on this


The simplest fix would have been to revert the dodgy cleanup commit, but since
things have moved on, this would have required an audit of all the new
set_huge_pte_at() call sites to see if they should be converted to
set_huge_swap_pte_at(). As per the original intent of the change, it would also
leave us open to future bugs when people invariably get it wrong and call the
wrong helper.

So instead, I've converted the first parameter of set_huge_pte_at() to be a vma
rather than an mm. This means that the arm64 code can easily recover the huge
page size in all cases. It's a bigger change, due to needing to touch the arches
that implement the function, but it is entirely mechanical, so in my view, low

I've compile-tested all touched arches; arm64, parisc, powerpc, riscv, s390 (and
additionally x86_64). I've additionally booted and run mm selftests against
arm64, where I observe the uffd poison test is fixed, and there are no other


patches 1-7: Convert core mm and arches to pass vma instead of mm
patch: 8: Fixes the arm64 bug

Patches based on v6.6-rc2.


Ryan Roberts (8):
  parisc: hugetlb: Convert set_huge_pte_at() to take vma
  powerpc: hugetlb: Convert set_huge_pte_at() to take vma
  riscv: hugetlb: Convert set_huge_pte_at() to take vma
  s390: hugetlb: Convert set_huge_pte_at() to take vma
  sparc: hugetlb: Convert set_huge_pte_at() to take vma
  mm: hugetlb: Convert set_huge_pte_at() to take vma
  arm64: hugetlb: Convert set_huge_pte_at() to take vma
  arm64: hugetlb: Fix set_huge_pte_at() to work with all swap entries

 arch/arm64/include/asm/hugetlb.h              |  2 +-
 arch/arm64/mm/hugetlbpage.c                   | 22 ++++----------
 arch/parisc/include/asm/hugetlb.h             |  2 +-
 arch/parisc/mm/hugetlbpage.c                  |  4 +--
 .../include/asm/nohash/32/hugetlb-8xx.h       |  3 +-
 arch/powerpc/mm/book3s64/hugetlbpage.c        |  2 +-
 arch/powerpc/mm/book3s64/radix_hugetlbpage.c  |  2 +-
 arch/powerpc/mm/nohash/8xx.c                  |  2 +-
 arch/powerpc/mm/pgtable.c                     |  7 ++++-
 arch/riscv/include/asm/hugetlb.h              |  2 +-
 arch/riscv/mm/hugetlbpage.c                   |  3 +-
 arch/s390/include/asm/hugetlb.h               |  8 +++--
 arch/s390/mm/hugetlbpage.c                    |  8 ++++-
 arch/sparc/include/asm/hugetlb.h              |  8 +++--
 arch/sparc/mm/hugetlbpage.c                   |  8 ++++-
 include/asm-generic/hugetlb.h                 |  6 ++--
 include/linux/hugetlb.h                       |  6 ++--
 mm/damon/vaddr.c                              |  2 +-
 mm/hugetlb.c                                  | 30 +++++++++----------
 mm/migrate.c                                  |  2 +-
 mm/rmap.c                                     | 10 +++----
 mm/vmalloc.c                                  |  5 +++-
 22 files changed, 80 insertions(+), 64 deletions(-)


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