ptdump walks the kernel page tables locklessly through walk_kernel_page_table_range_lockless(). It only holds the init_mm mmap lock and the memory hotplug lock, and neither excludes vmalloc/ioremap teardown from freeing kernel PTE pages via pmd_free_pte_page() -> pagetable_free_kernel(). syzbot hit a use-after-free in ptdump_pte_entry() reading a PTE page that was freed underneath the walk. Deferring the kernel page table free only batches the TLB flush; it does not wait for lockless walkers. Mirror the user page table walk, where pte_offset_map() already takes the RCU read lock: hold rcu_read_lock() across the lockless kernel walk and rcu-free the page tables in the kernel page table free worker, after the batched TLB flush. A walker then either observes the cleared PMD and skips the page, or keeps it alive until it drops the RCU read lock. Fixes: 5ba2f0a15564 ("mm: introduce deferred freeing for kernel page tables") Reported-by: syzbot+fd95a72470f5a44e464c@syzkaller.appspotmail.com Closes: https://lore.kernel.org/all/6a287988.39669fcc.33b062.00a0.GAE@google.com/T/ Assisted-by: Claude:claude-opus-4-8 Signed-off-by: David Carlier --- mm/pagewalk.c | 15 ++++++++++++++- mm/pgtable-generic.c | 16 +++++++++++++++- 2 files changed, 29 insertions(+), 2 deletions(-) diff --git a/mm/pagewalk.c b/mm/pagewalk.c index 3ae2586ff45b..6d9f14f86784 100644 --- a/mm/pagewalk.c +++ b/mm/pagewalk.c @@ -655,13 +655,26 @@ int walk_kernel_page_table_range_lockless(unsigned long start, unsigned long end .private = private, .no_vma = true }; + int err; if (start >= end) return -EINVAL; if (!check_ops_safe(ops)) return -EINVAL; - return walk_pgd_range(start, end, &walk); + /* + * Kernel intermediate page tables can be freed concurrently by + * vmalloc/ioremap teardown (e.g. pmd_free_pte_page()), which routes + * the freed pages through pagetable_free_kernel(). That path defers + * the free past an RCU grace period, so hold the RCU read lock across + * the lockless walk to prevent a page table from being freed while we + * are still dereferencing it. + */ + rcu_read_lock(); + err = walk_pgd_range(start, end, &walk); + rcu_read_unlock(); + + return err; } /** diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c index b91b1a98029c..5b53e9a5b7f8 100644 --- a/mm/pgtable-generic.c +++ b/mm/pgtable-generic.c @@ -424,6 +424,13 @@ static struct { .work = __WORK_INITIALIZER(kernel_pgtable_work.work, kernel_pgtable_work_func), }; +static void kernel_pgtable_free_rcu(struct rcu_head *head) +{ + struct ptdesc *pt = container_of(head, struct ptdesc, pt_rcu_head); + + __pagetable_free(pt); +} + static void kernel_pgtable_work_func(struct work_struct *work) { struct ptdesc *pt, *next; @@ -434,8 +441,15 @@ static void kernel_pgtable_work_func(struct work_struct *work) spin_unlock(&kernel_pgtable_work.lock); iommu_sva_invalidate_kva_range(PAGE_OFFSET, TLB_FLUSH_ALL); + + /* + * Lockless kernel page table walkers (ptdump, and any other user of + * walk_kernel_page_table_range_lockless()) dereference these pages + * under rcu_read_lock(). Free them after a grace period so a walker + * cannot still be reading a page we release. + */ list_for_each_entry_safe(pt, next, &page_list, pt_list) - __pagetable_free(pt); + call_rcu(&pt->pt_rcu_head, kernel_pgtable_free_rcu); } void pagetable_free_kernel(struct ptdesc *pt) -- 2.53.0