[PATCH 5.10 46/80] mm: pagewalk: Fix race between unmap and page walker

Greg Kroah-Hartman
2022-09-06T13:37:06Z

From: Steven Price <steven.price@arm.com>

[ 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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
arch/riscv/mm/pageattr.c | 4 ++--
mm/pagewalk.c | 21 ++++++++++++---------
mm/ptdump.c | 4 ++--
3 files changed, 16 insertions(+), 13 deletions(-)

diff --git a/arch/riscv/mm/pageattr.c b/arch/riscv/mm/pageattr.c
index 19fecb362d815..09f6be19ba7b3 100644
--- 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 e81640d9f1770..371ec21a19899 100644
--- a/mm/pagewalk.c
+++ b/mm/pagewalk.c
@@ -71,7 +71,7 @@ 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)
@@ -129,7 +129,7 @@ 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)
@@ -318,19 +318,19 @@ 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;
@@ -402,9 +402,13 @@ 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);
@@ -422,9 +426,8 @@ 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);
@@ -453,9 +456,9 @@ 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 93f2f63dc52dc..a917bf55c61ea 100644
--- a/mm/ptdump.c
+++ b/mm/ptdump.c
@@ -141,13 +141,13 @@ 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);
--
2.35.1