From: Jann Horn jannh@google.com
commit 8d3c106e19e8d251da31ff4cc7462e4565d65084 upstream.
pagetable walks on address ranges mapped by VMAs can be done under the mmap lock, the lock of an anon_vma attached to the VMA, or the lock of the VMA's address_space. Only one of these needs to be held, and it does not need to be held in exclusive mode.
Under those circumstances, the rules for concurrent access to page table entries are:
- Terminal page table entries (entries that don't point to another page table) can be arbitrarily changed under the page table lock, with the exception that they always need to be consistent for hardware page table walks and lockless_pages_from_mm(). This includes that they can be changed into non-terminal entries. - Non-terminal page table entries (which point to another page table) can not be modified; readers are allowed to READ_ONCE() an entry, verify that it is non-terminal, and then assume that its value will stay as-is.
Retracting a page table involves modifying a non-terminal entry, so page-table-level locks are insufficient to protect against concurrent page table traversal; it requires taking all the higher-level locks under which it is possible to start a page walk in the relevant range in exclusive mode.
The collapse_huge_page() path for anonymous THP already follows this rule, but the shmem/file THP path was getting it wrong, making it possible for concurrent rmap-based operations to cause corruption.
Link: https://lkml.kernel.org/r/20221129154730.2274278-1-jannh@google.com Link: https://lkml.kernel.org/r/20221128180252.1684965-1-jannh@google.com Link: https://lkml.kernel.org/r/20221125213714.4115729-1-jannh@google.com Fixes: 27e1f8273113 ("khugepaged: enable collapse pmd for pte-mapped THP") Signed-off-by: Jann Horn jannh@google.com Reviewed-by: Yang Shi shy828301@gmail.com Acked-by: David Hildenbrand david@redhat.com Cc: John Hubbard jhubbard@nvidia.com Cc: Peter Xu peterx@redhat.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org [backport fixed up manually: collapse_pte_mapped_thp returns different type] Signed-off-by: Jann Horn jannh@google.com Signed-off-by: Sasha Levin sashal@kernel.org --- mm/khugepaged.c | 56 +++++++++++++++++++++++++++++++++++++++++++++---- 1 file changed, 52 insertions(+), 4 deletions(-)
diff --git a/mm/khugepaged.c b/mm/khugepaged.c index 70b7ac66411c..28d8459d7aae 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -1360,16 +1360,37 @@ static void khugepaged_add_pte_mapped_thp(struct mm_struct *mm, spin_unlock(&khugepaged_mm_lock); }
+/* + * A note about locking: + * Trying to take the page table spinlocks would be useless here because those + * are only used to synchronize: + * + * - modifying terminal entries (ones that point to a data page, not to another + * page table) + * - installing *new* non-terminal entries + * + * Instead, we need roughly the same kind of protection as free_pgtables() or + * mm_take_all_locks() (but only for a single VMA): + * The mmap lock together with this VMA's rmap locks covers all paths towards + * the page table entries we're messing with here, except for hardware page + * table walks and lockless_pages_from_mm(). + */ static void collapse_and_free_pmd(struct mm_struct *mm, struct vm_area_struct *vma, unsigned long addr, pmd_t *pmdp) { - spinlock_t *ptl; pmd_t pmd;
mmap_assert_write_locked(mm); - ptl = pmd_lock(vma->vm_mm, pmdp); + if (vma->vm_file) + lockdep_assert_held_write(&vma->vm_file->f_mapping->i_mmap_rwsem); + /* + * All anon_vmas attached to the VMA have the same root and are + * therefore locked by the same lock. + */ + if (vma->anon_vma) + lockdep_assert_held_write(&vma->anon_vma->root->rwsem); + pmd = pmdp_collapse_flush(vma, addr, pmdp); - spin_unlock(ptl); mm_dec_nr_ptes(mm); page_table_check_pte_clear_range(mm, addr, pmd); pte_free(mm, pmd_pgtable(pmd)); @@ -1410,6 +1431,14 @@ void collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr) if (!hugepage_vma_check(vma, vma->vm_flags | VM_HUGEPAGE, false, false)) return;
+ /* + * Symmetry with retract_page_tables(): Exclude MAP_PRIVATE mappings + * that got written to. Without this, we'd have to also lock the + * anon_vma if one exists. + */ + if (vma->anon_vma) + return; + /* Keep pmd pgtable for uffd-wp; see comment in retract_page_tables() */ if (userfaultfd_wp(vma)) return; @@ -1426,6 +1455,20 @@ void collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr) if (!pmd) goto drop_hpage;
+ /* + * We need to lock the mapping so that from here on, only GUP-fast and + * hardware page walks can access the parts of the page tables that + * we're operating on. + * See collapse_and_free_pmd(). + */ + i_mmap_lock_write(vma->vm_file->f_mapping); + + /* + * This spinlock should be unnecessary: Nobody else should be accessing + * the page tables under spinlock protection here, only + * lockless_pages_from_mm() and the hardware page walker can access page + * tables while all the high-level locks are held in write mode. + */ start_pte = pte_offset_map_lock(mm, pmd, haddr, &ptl);
/* step 1: check all mapped PTEs are to the right huge page */ @@ -1476,6 +1519,9 @@ void collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr)
/* step 4: collapse pmd */ collapse_and_free_pmd(mm, vma, haddr, pmd); + + i_mmap_unlock_write(vma->vm_file->f_mapping); + drop_hpage: unlock_page(hpage); put_page(hpage); @@ -1483,6 +1529,7 @@ void collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr)
abort: pte_unmap_unlock(start_pte, ptl); + i_mmap_unlock_write(vma->vm_file->f_mapping); goto drop_hpage; }
@@ -1531,7 +1578,8 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff) * An alternative would be drop the check, but check that page * table is clear before calling pmdp_collapse_flush() under * ptl. It has higher chance to recover THP for the VMA, but - * has higher cost too. + * has higher cost too. It would also probably require locking + * the anon_vma. */ if (vma->anon_vma) continue;