Re: [PATCH mm-hotfixes v4 0/4] mm: fix UAF caused by race between ptdump and vmap pgtable freeing

[Andrew Morton]

On Thu, 16 Jul 2026 22:31:11 +0100 "Lorenzo Stoakes (ARM)" <ljs@xxxxxxxxxx> wrote:

> Kernel page table walkers fall into two broad categories - those ranges
> where no exclusion is required via walk_kernel_page_table_range_lockless()
> and those where exclusion is required via walk_kernel_page_table_range()
> or walk_page_range_debug().
> 
> ...
> 
> This series works around this by #ifndef CONFIG_ARM64'ing the mmap read
> lock in vmap logic, then partially reverting commit fa93b45fd397 ("arm64:
> Enable vmalloc-huge with ptdump"), keeping the enablement of huge vmap
> support, and removing the ifdeffery with the partial revert patch.

Thanks, I've updated mm.git's mm-hotfixes-unstable branch to this
version.

> v4:
> * Rebased on latest master of Linus's tree.
> * Accumulated tags, thanks everybody!
> * Updated commit messages further as per Kiryl.
> * Took the mmap write lock across the whole CPA operation as per Will.

Here's how v4 altered mm.git:


 arch/x86/mm/pat/set_memory.c |   25 +++++++++++++++----------
 include/linux/mmap_lock.h    |    2 ++
 mm/vmalloc.c                 |    8 ++++----
 3 files changed, 21 insertions(+), 14 deletions(-)

--- a/arch/x86/mm/pat/set_memory.c~b
+++ a/arch/x86/mm/pat/set_memory.c
@@ -411,7 +411,7 @@ static void __cpa_flush_tlb(void *data)
 
 static int collapse_large_pages(unsigned long addr, struct list_head *pgtables);
 
-static void cpa_collapse_large_pages(struct cpa_data *cpa)
+static void __cpa_collapse_large_pages(struct cpa_data *cpa)
 {
 	unsigned long start, addr, end;
 	struct ptdesc *ptdesc, *tmp;
@@ -437,17 +437,22 @@ static void cpa_collapse_large_pages(str
 
 	flush_tlb_all();
 
+	list_for_each_entry_safe(ptdesc, tmp, &pgtables, pt_list) {
+		list_del(&ptdesc->pt_list);
+		pagetable_free(ptdesc);
+	}
+}
+
+static void cpa_collapse_large_pages(struct cpa_data *cpa)
+{
 	/*
-	 * ptdump might read these page tables, so avoid a use-after-free by
-	 * acquiring the mmap read lock on init_mm (ptdump acquires the mmap
-	 * write lock).
+	 * Take the mmap write lock on init_mm to:
+	 * - Avoid a use-after-free if raced by ptdump (which takes its own
+	 *   write lock on init_mm).
+	 * - Serialise concurrent CPA walkers.
 	 */
-	scoped_guard(mmap_read_lock, &init_mm) {
-		list_for_each_entry_safe(ptdesc, tmp, &pgtables, pt_list) {
-			list_del(&ptdesc->pt_list);
-			pagetable_free(ptdesc);
-		}
-	}
+	scoped_guard(mmap_write_lock, &init_mm)
+		__cpa_collapse_large_pages(cpa);
 }
 
 static void cpa_flush(struct cpa_data *cpa, int cache)
--- a/include/linux/mmap_lock.h~b
+++ a/include/linux/mmap_lock.h
@@ -621,6 +621,8 @@ static inline void mmap_read_unlock(stru
 
 DEFINE_GUARD(mmap_read_lock, struct mm_struct *,
 	     mmap_read_lock(_T), mmap_read_unlock(_T))
+DEFINE_GUARD(mmap_write_lock, struct mm_struct *,
+	     mmap_write_lock(_T), mmap_write_unlock(_T))
 DEFINE_GUARD_COND(mmap_read_lock, _try, mmap_read_trylock(_T))
 
 static inline void mmap_read_unlock_non_owner(struct mm_struct *mm)
--- a/mm/vmalloc.c~b
+++ a/mm/vmalloc.c
@@ -163,11 +163,11 @@ static int vmap_try_huge_pmd(pmd_t *pmd,
 		return pmd_set_huge(pmd, phys_addr, prot);
 
 	/*
-	 * Acquire the mmap read lock to exclude ptdump, which walks kernel
-	 * page tables it does not own under the mmap write lock.
+	 * Acquire the mmap read lock to exclude ptdump, which walks
+	 * kernel page tables it does not own under the mmap write lock.
 	 *
-	 * Concurrent read lock holders are safe: each exclusively owns the
-	 * range it operates on and cannot reach this page table.
+	 * Concurrent read lock holders are safe: each exclusively owns
+	 * the range it operates on and cannot reach this page table.
 	 */
 	scoped_cond_guard(mmap_read_lock_try, return 0, &init_mm) {
 		if (!pmd_free_pte_page(pmd, addr))
_