mm: pagewalk: Fix race between unmap and page walker

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>

diff --git a/arch/riscv/mm/pageattr.c b/arch/riscv/mm/pageattr.c
index 5e49e4b..86c5661 100644 (file)
--- a/arch/riscv/mm/pageattr.c
+++ b/arch/riscv/mm/pageattr.c
@@ -118,10 +118,10 @@ static int __set_memory(unsigned long addr, int numpages, pgprot_t set_mask,
        if (!numpages)
                return 0;
 
-       mmap_read_lock(&init_mm);
+       mmap_write_lock(&init_mm);
        ret =  walk_page_range_novma(&init_mm, start, end, &pageattr_ops, NULL,
                                     &masks);
-       mmap_read_unlock(&init_mm);
+       mmap_write_unlock(&init_mm);
 
        flush_tlb_kernel_range(start, end);
 

diff --git a/mm/pagewalk.c b/mm/pagewalk.c
index 9b3db11..fa7a3d2 100644 (file)
--- a/mm/pagewalk.c
+++ b/mm/pagewalk.c
@@ -110,7 +110,7 @@ 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 && !walk->no_vma)) {
+               if (pmd_none(*pmd)) {
                        if (ops->pte_hole)
                                err = ops->pte_hole(addr, next, depth, walk);
                        if (err)
@@ -171,7 +171,7 @@ 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 && !walk->no_vma)) {
+               if (pud_none(*pud)) {
                        if (ops->pte_hole)
                                err = ops->pte_hole(addr, next, depth, walk);
                        if (err)
@@ -366,19 +366,19 @@ static int __walk_page_range(unsigned long start, unsigned long end,
        struct vm_area_struct *vma = walk->vma;
        const struct mm_walk_ops *ops = walk->ops;
 
-       if (vma && ops->pre_vma) {
+       if (ops->pre_vma) {
                err = ops->pre_vma(start, end, walk);
                if (err)
                        return err;
        }
 
-       if (vma && is_vm_hugetlb_page(vma)) {
+       if (is_vm_hugetlb_page(vma)) {
                if (ops->hugetlb_entry)
                        err = walk_hugetlb_range(start, end, walk);
        } else
                err = walk_pgd_range(start, end, walk);
 
-       if (vma && ops->post_vma)
+       if (ops->post_vma)
                ops->post_vma(walk);
 
        return err;
@@ -450,9 +450,13 @@ 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, -1, &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, -1, &walk);
                } else { /* inside vma */
                        walk.vma = vma;
                        next = min(end, vma->vm_end);
@@ -470,9 +474,8 @@ 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);
@@ -501,9 +504,9 @@ int walk_page_range_novma(struct mm_struct *mm, unsigned long start,
        if (start >= end || !walk.mm)
                return -EINVAL;
 
-       mmap_assert_locked(walk.mm);
+       mmap_assert_write_locked(walk.mm);
 
-       return __walk_page_range(start, end, &walk);
+       return walk_pgd_range(start, end, &walk);
 }
 
 int walk_page_vma(struct vm_area_struct *vma, const struct mm_walk_ops *ops,

diff --git a/mm/ptdump.c b/mm/ptdump.c
index eea3d28..8adab45 100644 (file)
--- a/mm/ptdump.c
+++ b/mm/ptdump.c
@@ -152,13 +152,13 @@ void ptdump_walk_pgd(struct ptdump_state *st, struct mm_struct *mm, pgd_t *pgd)
 {
        const struct ptdump_range *range = st->range;
 
-       mmap_read_lock(mm);
+       mmap_write_lock(mm);
        while (range->start != range->end) {
                walk_page_range_novma(mm, range->start, range->end,
                                      &ptdump_ops, pgd, st);
                range++;
        }
-       mmap_read_unlock(mm);
+       mmap_write_unlock(mm);
 
        /* Flush out the last page */
        st->note_page(st, 0, -1, 0);