[ Upstream commit 8782fb61cc848364e1e1599d76d3c9dd58a1cc06 ]

The mmap lock protects the page walker from changes to the page tables during the walk. However a read lock is insufficient to protect those areas which don't have a VMA as munmap() detaches the VMAs before downgrading to a read lock and actually tearing down PTEs/page tables.

For users of walk_page_range() the solution is to simply call pte_hole() immediately without checking the actual page tables when a VMA is not present. We now never call __walk_page_range() without a valid vma.

For walk_page_range_novma() the locking requirements are tightened to require the mmap write lock to be taken, and then walking the pgd directly with 'no_vma' set.

This in turn means that all page walkers either have a valid vma, or it's that special 'novma' case for page table debugging. As a result, all the odd '(!walk->vma && !walk->no_vma)' tests can be removed.

Fixes: dd2283f2605e ("mm: mmap: zap pages with read mmap_sem in munmap") Reported-by: Jann Horn jannh@google.com Signed-off-by: Steven Price steven.price@arm.com Cc: Vlastimil Babka vbabka@suse.cz Cc: Thomas Hellström thomas.hellstrom@linux.intel.com Cc: Konstantin Khlebnikov koct9i@gmail.com Cc: Andrew Morton akpm@linux-foundation.org Signed-off-by: Linus Torvalds torvalds@linux-foundation.org [manually backported. backport note: walk_page_range_novma() does not exist in 5.4, so I'm omitting it from the backport] Signed-off-by: Jann Horn jannh@google.com --- mm/pagewalk.c | 13 ++++++++----- 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/mm/pagewalk.c b/mm/pagewalk.c index 4eb09e0898817..ec41e7552f37c 100644 --- a/mm/pagewalk.c +++ b/mm/pagewalk.c @@ -38,7 +38,7 @@ static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end, do { again: next = pmd_addr_end(addr, end); - if (pmd_none(*pmd) || !walk->vma) { + if (pmd_none(*pmd)) { if (ops->pte_hole) err = ops->pte_hole(addr, next, walk); if (err) @@ -84,7 +84,7 @@ static int walk_pud_range(p4d_t *p4d, unsigned long addr, unsigned long end, do { again: next = pud_addr_end(addr, end); - if (pud_none(*pud) || !walk->vma) { + if (pud_none(*pud)) { if (ops->pte_hole) err = ops->pte_hole(addr, next, walk); if (err) @@ -254,7 +254,7 @@ static int __walk_page_range(unsigned long start, unsigned long end, int err = 0; struct vm_area_struct *vma = walk->vma;

- if (vma && is_vm_hugetlb_page(vma)) { + if (is_vm_hugetlb_page(vma)) { if (walk->ops->hugetlb_entry) err = walk_hugetlb_range(start, end, walk); } else @@ -324,9 +324,13 @@ int walk_page_range(struct mm_struct *mm, unsigned long start, if (!vma) { /* after the last vma */ walk.vma = NULL; next = end; + if (ops->pte_hole) + err = ops->pte_hole(start, next, &walk); } else if (start < vma->vm_start) { /* outside vma */ walk.vma = NULL; next = min(end, vma->vm_start); + if (ops->pte_hole) + err = ops->pte_hole(start, next, &walk); } else { /* inside vma */ walk.vma = vma; next = min(end, vma->vm_end); @@ -344,9 +348,8 @@ int walk_page_range(struct mm_struct *mm, unsigned long start, } if (err < 0) break; - } - if (walk.vma || walk.ops->pte_hole) err = __walk_page_range(start, next, &walk); + } if (err) break; } while (start = next, start < end);

base-commit: f28b7414ab715e6069e72a7bbe2f1354b2524beb