mm/mempolicy: Take VMA lock before replacing policy

mbind() calls down into vma_replace_policy() without taking the per-VMA
locks, replaces the VMA's vma->vm_policy pointer, and frees the old
policy.  That's bad; a concurrent page fault might still be using the
old policy (in vma_alloc_folio()), resulting in use-after-free.

Normally this will manifest as a use-after-free read first, but it can
result in memory corruption, including because vma_alloc_folio() can
call mpol_cond_put() on the freed policy, which conditionally changes
the policy's refcount member.

This bug is specific to CONFIG_NUMA, but it does also affect non-NUMA
systems as long as the kernel was built with CONFIG_NUMA.
Signed-off-by: Jann Horn <jannh@google.com>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Fixes: 5e31275c ("mm: add per-VMA lock and helper functions to control it")
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mempolicy.c

@@ -384,8 +384,10 @@ void mpol_rebind_mm(struct mm_struct *mm, nodemask_t *new)
 	VMA_ITERATOR(vmi, mm, 0);
 
 	mmap_write_lock(mm);
-	for_each_vma(vmi, vma)
+	for_each_vma(vmi, vma) {
+		vma_start_write(vma);
 		mpol_rebind_policy(vma->vm_policy, new);
+	}
 	mmap_write_unlock(mm);
 }
 
@@ -768,6 +770,8 @@ static int vma_replace_policy(struct vm_area_struct *vma,
 	struct mempolicy *old;
 	struct mempolicy *new;
 
+	vma_assert_write_locked(vma);
+
 	pr_debug("vma %lx-%lx/%lx vm_ops %p vm_file %p set_policy %p\n",
 		 vma->vm_start, vma->vm_end, vma->vm_pgoff,
 		 vma->vm_ops, vma->vm_file,
@@ -1313,6 +1317,14 @@ static long do_mbind(unsigned long start, unsigned long len,
 	if (err)
 		goto mpol_out;
 
+	/*
+	 * Lock the VMAs before scanning for pages to migrate, to ensure we don't
+	 * miss a concurrently inserted page.
+	 */
+	vma_iter_init(&vmi, mm, start);
+	for_each_vma_range(vmi, vma, end)
+		vma_start_write(vma);
+
 	ret = queue_pages_range(mm, start, end, nmask,
 			  flags | MPOL_MF_INVERT, &pagelist);
 
@@ -1538,6 +1550,7 @@ SYSCALL_DEFINE4(set_mempolicy_home_node, unsigned long, start, unsigned long, le
 			break;
 		}
 
+		vma_start_write(vma);
 		new->home_node = home_node;
 		err = mbind_range(&vmi, vma, &prev, start, end, new);
 		mpol_put(new);